Wednesday, July 29, 2009

Simple Car Battery Charger

This very simple circuit uses a transformer ,two diodes , a capacitor and an ammeter.
To charge a battery just connect the + and - terminals of the circuit to the corresponding terminals of the battery.
When the battery is not charged, the ammeter reading shows 1-3 amps.
When the battery is fully charged the ammeter reads Zero or nearly zero, after which the battery should be removed from the
The circuit is a full wave rectifier using 2 diodes for rectification. The capacitor is used for smoothing.
I think the circuit works fine without the capacitor since the battery itself acts a BIG capacitor. But when you are using the
circuit to supply 12V (as a battery eliminator) the capacitor needs to be present.
Care should be taken NOT to reverse the + and - terminals while connecting it to the battery.

Automatic Speed Controller for fans & Coolers

During summer nights, the temperature is initially quite high. As time passes, the temperature starts dropping. Also, after a person falls asleep, the metabolic rate of one’s body decreases. Thus, initially the fan/cooler needs to be run at full speed. As time passes, one has to get up again and again to adjust the speed of the fan or the cooler.The device presented here makes the fan run at full speed for a predetermined time. The speed is decreased to medium after some time, and to slow later on. After a period of about eight hours, the fan/cooler is switched off.Fig. 1 shows the circuit diagram of the system. IC1 (555) is used as an astable multivibrator to generate clock pulses. The pulses are fed to decade dividers/counters formed by IC2 and IC3. These ICs act as divide-by-10 and divide-by-9 counters, respectively. The values of capacitor C1 and resistors R1 and R2 are so adjusted that the final output of IC3 goes high after about eight hours.The first two outputs of IC3 (Q0 and Q1) are connected (ORed) via diodes D1 and D2 to the base of transistor T1. Initially output Q0 is high and therefore relay RL1 is energised. It remains energised when Q1 becomes high. The method of connecting the gadget to the fan/cooler is given in Figs 3 and 4.

It can be seen that initially the fan shall get AC supply directly, and so it shall run at top speed. When output Q2 becomes high and Q1 becomes low, relay RL1 is turned ‘off’ and relay RL2 is switched ‘on’. The fan gets AC through a resistance and its speed drops to medium. This continues until output Q4 is high. When Q4 goes low and Q5 goes high, relay RL2 is switched ‘off’ and relay RL3 is activated. The fan now runs at low speed.Throughout the process, pin 11 of the IC is low, so T4 is cut off, thus keeping T5 in saturation and RL4 ‘on’. At the end of the cycle, when pin 11 (Q9) becomes high, T4 gets saturated and T5 is cut off. RL4 is switched ‘off’, thus switching ‘off’ the fan/cooler.Using the circuit described above, the fan shall run at high speed for a comparatively lesser time when either of Q0 or Q1 output is high. At medium speed, it will run for a moderate time period when any of three outputs Q2 through Q4 is high, while at low speed, it will run for a much longer time period when any of the four outputs Q5 through Q8 is high.If one wishes, one can make the fan run at the three speeds for an equal amount of time by connecting three decimal decoded outputs of IC3 to each of the transistors T1 to T3. One can also get more than three speeds by using an additional relay, transistor, and associated components, and connecting one or more outputs of IC3 to it.
In the motors used in certain coolers there are separate windings for separate speeds. Such coolers do not use a rheostat type speed regulator. The method of connection of this device to such coolers is given in Fig. 4.
The resistors in Figs 2 and 3 are the tapped resistors, similar to those used in manually controlled fan-speed regulators. Alternatively, wire-wound resistors of suitable wattage and resistance can be used.

Alternating Flasher

This circuit uses three easily available 555 timer ICs. All three work as astable multivibrators. The first 555 has an on period and off period equal to 1 sec. This IC controls the on/ off periods of the other 2 555s which are used to flash two bulbs through the relay contacts.
The flashing occurs at a rate of 4 flashes per second.
The diodes are used to protect the 555 ICs from peaks. The relays should have an impedance greater than 50ohms i.e, they should not draw a current more than 200mA.
The flashing sequence is as follows:
The bulb(s) connected to the first relay flashes for about 1 sec at a rate of 4 flashes per second. Then the bulb(s) connected to the second relay flashes for 1 sec at a rate of 4 flashes per second. Then the cycle repeats.
The flashing rates can be varied by changing the capacitors C3 and C5. A higher value gives a lower flashing rate.
Note that the values of C3 and C5 should be equal and should be less than that of C1.
The value of C1 controls the change-over rate ( default 1sec). A higher value gives a lower change-over rate.
If you use the normally open contacts of the relay, on bulb will be OFF while other is flashing,and vice versa.
If normally closed contacts are used, one bulb will be ON while the other is flashing

Audio Light Modulator

Audio light modulations add to the enjoyment of music during functions organised at home or outdoors. Presented here is one such simple circuit in which light is modulated using a small fraction of the audio output from the speaker terminals of the audio amplifier. The output from the speaker terminals of audio amplifier is connected to a transformer (output transformer used in transistor radios) through a non-polarised capacitor. The use of transformer is essential for isolating the audio source from the circuit in The sensitivity control potentiometer VR1 provided in the input to transistor T1 may be adjusted to ensure that conduction takes place only after the AF exceeds certain amplitude. This control has to be adjusted as per audio source level. The audio signal Proper earthing of the circuit is quite essential. The diode bridge provides pulsating DC output and acts as a guard circuit between the mains input and pulsating DC output. Extreme care is necessary to avoid any electric shock

Automatic Room Lights

An ordinary automatic room power control circuit has only one light sensor. So when a person enters the room it gets one pulse and the lights come ‘on.’ When the person goes out it gets another pulse and the lights go ‘off.’ But what happens when two persons enter the room, one after the other? It gets two pulses and the lights remain in ‘off’ state. The circuit described here overcomes the above-mentioned problem. It has a small memory which enables it to automatically switch ‘on’ and switch ‘off’ the lights in a desired fashion. The circuit uses two LDRs which are placed one after another (separated by a distance of say half a metre) so that they may separately sense a person going into the room or coming out of the room. Outputs of the two LDR sensors, after processing, are used in conjunction with a bicolour LED in such a fashion that when a person gets into the room it emits green light and when a person goes out of the room it emits red light, and vice versa. These outputs are simultaneously applied to two counters. One of the counters will count as +1, +2, +3 etc when persons are getting into the room and the other will count as -1, -2, -3 etc when persons are getting out of the room. These counters make use of Johnson decade counter CD4017 ICs. The next stage comprises two logic ICs which can combine the outputs of the two counters and determine if there is any person still left in the room or not. Since in the circuit LDRs have been used, care should be taken to protect them from ambient light. If desired, one may use readily available IR sensor modules to replace the LDRs. The sensors are installed in such a way that when a person enters or leaves the room, he intercepts the light falling on them sequentially—one after the other. When a person enters the room, first he would obstruct the light falling on LDR1, followed by that falling on LDR2. When a person leaves the room it will be the other way round. In the normal case light keeps falling on both the LDRs, and as such their resistance is low (about 5 kilo-ohms). As a result, pin 2 of both timers (IC1 and IC2), which have been configured as monostable flip-flops, are held near the supply voltage (+9V). When the light falling on the LDRs is obstructed, their resistance becomes very high and pin 2 voltages drop to near ground potential, thereby triggering the flip-flops. Capacitors across pin 2 and ground have been added to avoid false triggering due to electrical noise. When a person enters the room, LDR1 is triggered first and it results in triggering of monostable IC1. The short output pulse immediately charges up capacitor C5, forward biasing transistor pair T1-T2. But at this instant the collectors of transistors T1 and T2 are in high impedance state as IC2 pin 3 is at low potential and diode D4 is not conducting. But when the same person passes LDR2, IC2 monostable flip-flop is triggered. Its pin 3 goes high and this potential is coupled to transistor pair T1-T2 via diode D4. As a result transistor pair T1-T2 conducts because capacitor C5 retains the charge for some time as its discharge time is controlled by resistor R5 (and R7 to an extent). Thus green LED portion of bi-colour LED is lit momentarily. The same output is also coupled to IC3 for which it acts as a clock. With entry of each person IC3 output (high state) keeps advancing. At this stage transistor pair T3-T4 cannot conduct because output pin 3 of IC1 is no longer positive as its output pulse duration is quite short and hence transistor collectors are in high impedance state. When persons leave the room, LDR2 is triggered first followed by LDR1. Since the bottom half portion of circuit is identical to top half, this time with the departure of each person red portion of bi-colour LED is lit momentarily and output of IC4 advances in the same fashion as in case of IC3. The outputs of IC3 and those of IC4 (after inversion by inverter gates N1 through N4) are ANDed by AND gates (A1 through A4) are then wire ORed (using diodes D5 through D8). The net effect is that when persons are entering, the output of at least one of the AND gates is high, causing transistor T5 to conduct and energise relay RL1. The bulb connected to the supply via N/O contact of relay RL1 also lights up. When persons are leaving the room, and till all the persons who entered the room have left, the wired OR output continues to remain high, i.e. the bulb continues to remains ‘on,’ until all persons who entered the room have left. The maximum number of persons that this circuit can handle is limited to four since on receipt of fifth clock pulse the counters are reset. The capacity of the circuit can be easily extended for up to nine persons by removing the connection of pin 1 from reset pin (15) and utilising Q1 to Q9 outputs of CD4017 counters. Additional inverters, AND gates and diodes will, however, be required

Emergency Light

The circuit of automatic emergency light presented here has the following features: 1. When the mains supply (230V AC) is available, it charges a 12V battery up to 13.5V and then the battery is disconnected from the charging section. 2. When the battery discharges up to 10.2V, it is disconnected from the load and the charging process is resumed. 3. If the mains voltage is available and there is darkness in the room, load (bulb or tube) is turned on by taking power from the mains; otherwise the battery is connected to the load. 4. When the battery discharges up to 10.2V and if the mains is not yet available, the battery is completely disconnected from the circuit to avoid its further discharge. The mains supply of 230V AC is stepped down to 18V AC (RMS) using a 230V AC primary to 0-18V AC, 2A secondary transformer (X1), generally used in 36cm B&W TVs. Diodes D1 through D4 form bridge rectifier and capacitor C5 filters the voltage, providing about 25V DC at the output. Charging section includes 33-ohm, 10-watt resistor R2 which limits the charging current to about 425 mA when battery voltage is about 10.2V, or to 325 mA when battery voltage is about 13.5V. When the battery charges to 13.5V (as set by VR2), zener diode D17 goes into breakdown region, thereby triggering triac TR1. Now, since DC is passing through the triac, it remains continuously ‘on’ even if the gate current is reduced to zero (by disconnecting the gate terminal). Once the battery is fully charged, charging section is cut-off from the battery due to energisation of relay RL2. This relay remains ‘on’ even if the power fails because of connection to the battery via diode D10. S4, a normally closed switch, is included to manually restart the charging process if required. Battery disconnect and charging restart section comprises an NE555 timer (IC2) wired in monostable mode. When the battery voltage is above 10.2V (as indicated by red LED D15), zener diode (D16) remains in the breakdown region, making the trigger pin 2 of IC2 high, thereby maintaining output pin 3 in low voltage state. Thus, relay RL3 is ‘on’ and relay RL4 is ‘off.’ But as soon as the battery voltage falls to about 10.2V (as set by preset VR1), zener diode D16 comes out of conduction, making pin 2 low and pin 3 high to turn ‘on’ relay RL4 and orange LED D13. This also switches off relay RL3 and LED D15. Now, if the mains is available, charging restarts due to de-energisation of relay RL2 because when relay RL4 is ‘on,’ it breaks the circuit of relay RL2 and triac TR1. But if the mains supply is not present, both relays RL3 and RL1 de-energise, disconnecting the battery from the remaining circuit. Thus when battery voltage falls to 10.2 volts, its further discharge is eliminated. But as soon as the mains supply resumes, it energises relay RL1, thereby connecting the battery again to the circuit. Light sensor section also makes use of a 555 timer IC in the monostable mode. As long as normal light is falling on LDR1, its resistance is comparatively low. As a result pin 2 of IC3 is held near Vcc and its output at pin 3 is at low level. In darkness, LDR resistance is very high, which causes pin 2 of IC3 to fall to near ground potential and thus trigger it. As a consequence, output pin 3 goes high during the monostable pulse period, forward biasing transistor T3 which goes into saturation, energising relay RL5. With auto/bypass switch S2 off (in auto mode), the load gets connected to supply via switch S3. If desired, the load may be switched during the day-time by flipping switch S2 to ‘on’ position (manual). Preset VR3 is the sensitivity control used for setting threshold light level at which the load is to be automatically switched on/off. Capacitors with the relays ensure that there is no chattering of the relays. When the mains is present, diode D8 couples the input voltage to regulator IC1 whereas diode D10 feeds the input voltage to it (from battery) in absense of mains supply. Diode D5 connects the load to the power supply section via resistor R5 when mains is available (diode D18 does not conduct). However, when mains power fails, the situation reverses and diode D18 conducts while diode D5 does not conduct. . The load can be any bulb of 12 volts with a maximum current rating of 2 amperes (24 watts). Resistor R5 is supposed to drop approximately 12 volts when the load current flows through it during mains availability . Hence power dissipated in it would almost be equal to the load power. It is therefore desirable to replace R5 with a bulb of similar voltage and wattage as the load so that during mains availability we have more (double) light than when the load is fed from the battery. For setting presets VR1 and VR2, just take out (desolder one end) diodes D7, D10 and D18. Connect a variable source of power supply in place of battery. Set preset VR1 so that battery-high LED D15 is just off at 10.2V of the variable source. Increase the potential of the variable source and observe the shift from LO BAT LED D13 to D15. Now make the voltage of the source 13.5V and set preset VR2 so that relay RL2 just energises. Then decrease the voltage slowly and observe that relay RL2 does not de-energise above 10.2V. At 10.2V, LED D15 should be off and relay RL2 should de-energise while LED D13 should light up. Preset VR3 can be adjusted during evening hours so that the load is ‘on’ during the desired light conditions

Automatic Dual output Display

This circuit lights up ten bulbs sequentially, first in one direc- tion and then in the opposite direction, thus presenting a nice visual effect. In this circuit, gates N1 and N2 form an oscillator. The output of this oscillator is used as a clock for BCD up/down counter CD4510 (IC2). Depending on the logic state at its pin 10, the counter counts up or down. During count up operation, pin 7 of IC2 outputs an active low pulse on reaching the ninth count. Similarly, during count-down operation, you again get a low-going pulse at pin 7. This terminal count output from pin 7, after inversion by gate N3, is connected to clock pin 14 of decade counter IC3 (CD4017) which is configured here as a toggle flip-flop by returning its Q2 output at pin 4 to reset pin 15. Thus output at pin 3 of IC3 goes to logic 1 and logic 0 state alternately at each terminal count of IC2. Initially, pin 3 (Q0) of IC3 is high and the counter is in count-up state. On reaching ninth count, pin 3 of IC3 goes low and as a result IC2 starts counting down. When the counter reaches 0 count, Q2 output of IC3 momentarily goes high to reset it, thus taking pin 3 to logic 1 state, and the cycle repeats. The BCD output of IC2 is connected to 1-of-10 decoder CD4028 (IC4). During count-up operation of IC2, the outputs of IC4 go logic high sequentially from Q0 to Q9 and thus trigger the triacs and lighting bulbs 1 through 10, one after the other. Thereafter, during count-down operation of IC2, the bulbs light in the reverse order, presenting a wonderful visual effect

Light Flasher

This is a very basic circuit for flashing one or more LEDS and also to alternately flash one or more LEDs.
It uses a 555 timer setup as an astable multivibrator with a variable frequency.
With the preset at its max. the flashing rate of the LED is about 1/2 a second. It can be increased by increasing the value of the capacitor from 10uF to a higher value. For example if it is increased to 22uF the flashing rate becomes 1 second.

There is also provision to convert it into an alternating flasher. You just have to connect a LED and a 330ohm as shown in Fig.2 to the points X and Y of Fig.1. Then both the LEDs flash alternately.

Since the 555 can supply or sink in upto 200mA of current, you can connect upto about 18 LEDS in parallel both for the flasher and alternating flasher (that makes a total of 36 LEDs for alternating flasher).

Programmable Digital Code Lock

A programmable code lock can be used for numerous applications in which access to an article/gadget is to be restricted to a limited number of persons. Here is yet another circuit of a code lock employing mainly the CMOS ICs and thumbwheel switches (TWS) besides a few other components. It is rugged and capable of operation on voltages ranging between 6 and 15 volts. The supply current drain of CMOS ICs being quite low, the circuit may be operated even on battery.
The circuit uses two types of thumbwheel switches. switch numbers TWS1 through TWS8 are decimal-to-BCD converter type while switch numbers TWS9 through TWS16 are 10-input multiplexer type in which only one of the ten inputs may be connected to the output (pole). One thumbwheel switch of each of the two types is used in combination with IC CD4028B (BCD to decimal decoder) to provide one digital output.Eight such identical combinations of thumbwheel switches and IC CD4028 are used. The eight digital outputs obtained from these combinations are connected to the input of 8-input NAND gate CD4068.For getting a logic high output, say at pole-1, it is essential that decimal numbers selected by switch pair TWS1 and TWS9 are identical, as only then the logic high output available at the Specific output pin of IC1 will get transferred to pole-1. Accordingly, when the thumbwheel pair of switches in each combination is individually matched, the outputs at pole-1 to pole-8 will be logic high.This will cause output of 8-input NAND gate IC CD4068b to change over from logic high to logic low, thereby providing a high-to-low going clock pulse at clock input pin of 7-stage counter CD4024B, which is used here as a flip-flop (only Q0 output is used here).The output (Q0) of the flip-flop is connected to a relay driver circuit consisting of transistors T1 and T2. The relay will operate when Q0 output of flip-flop goes low. As a result transistor T1 cuts off and T2 gets forward biased to operate the relay.Switch S1 is provided to enable switching off (locking) and switching on (unlocking) of the relay as desired, once the correct code has been set.
With the code set correctly, the NAND gate output will stay low and flip-flop can be toggled any number of times, making it possible to switch on or switch off the relay, as desired. Suppose we are using the system for switching-on of a deck for which the power supply is routed via the contacts of the relay. The authorised person would select correct code which would cause the supply to become available to the deck. After use he will operate switch S1 and then shuffle the thumbwheel switches TWS1 through TWS8 such that none of the switches produces a correct code. Once the code does not match, pressing of switch S1 has no effect on the output of the flip-flop.Switches TWS9 through TWS16 are concealed after setting the desired code. In place of thumbwheel switches TWS1 through TWS8 DIP switches can also be used

Ultrasonic Pest Repeller

It is well know that pests like rats, mice etc are repelled by ultrasonic frequency in the range of 30 kHz to 50 kHz. Human beings can’t hear these high-frequency sounds. Unfortunately, all pests do not react at the same ultrasonic frequency. While some pests get repelled at 35 kHz, some others get repelled at 38 to 40 kHz. Thus to increase the effectiveness, freque- ncy of ultrasonic oscillator has to be continuously varied between certain limits. By using this circuit design, frequency of emission of ultrasonic sound is continuously varied step-by-step automatically. Here five steps of variation are used but the same can be extended up to 10 steps, if desired. For each clock pulse output from op-amp IC1 CA3130 (which is wired here as a low-frequency square wave oscillator), the logic 1 output of IC2 CD4017 (which is a well-known decade counter) shifts from Q0 to Q4 (or Q0 to Q9). Five presets VR2 through VR6 (one each connected at Q0 to Q4 output pins) are set for different values and connected to pin 7 of IC3 (NE555) electronically. VR1 is used to change clock pulse rate. IC3 is wired as an astable multivibrator operating at a frequency of nearly 80 kHz. Its output is not symmetrical. IC4 is CD4013, a D-type flip-flop which delivers symmetrical 40kHz signals at its Q and Q outputs which are amplified in push-pull mode by transistors T1, T2, T3 and T4 to drive a low-cost, high-frequency piezo tweeter. For frequency adjustments, you may use an oscilloscope. It can be done by trial and error also if you do not have an oscilloscope. This pest repeller would prove to be much more effective than those published earlier because here ultrasonic frequency is automatically changed to cover different pests and the power output is also sufficiently high. If you want low-power output in 30-50 kHz ultrasonic frequency range then the crystal transducer may be directly connected across Q and Q outputs of IC4 (transistor amplifier is not necessary

Simple IF Signal Generator

Here is a versatile circuit of IF signal generator which may be of interest to radio hobbyists and professionals alike.Transistors T1 and T2 form an astable multivibrator oscillating in the audio frequency range of 1 to 2 kHz. RF oscillator is built around transistor T3. Here again a 455kHz ceramic filter/resonator is employed for obtaining stable IF. The AF from multivibrator is coupled from collector of transistor T2 to emitter of transistor T3 through capacitor C3. The tank circuit at collector of transistor T3 is formed using medium wave oscillator coil of transistor radio, a fixed 100pF capacitor C5 and half section of a gang capacitor (C6).
The oscillator section may be easily modified for any other intermediate frequency by using ceramic filter or resonator of that frequency and by making appropriate changes in the tank circuit at collector of transistor T3. Slight adjustment of bias can be affected by varying values of resistors R6 and R7, if required

Audio Light Modulator

Audio light modulations add to the enjoyment of music during functions organised at home or outdoors. Presented here is one such simple circuit in which light is modulated using a small fraction of the audio output from the speaker terminals of the audio amplifier. The output from the speaker terminals of audio amplifier is connected to a transformer (output transformer used in transistor radios) through a non-polarised capacitor. The use of transformer is essential for isolating the audio source from the circuit in The sensitivity control potentiometer VR1 provided in the input to transistor T1 may be adjusted to ensure that conduction takes place only after the AF exceeds certain amplitude. This control has to be adjusted as per audio source level. The audio signal Proper earthing of the circuit is quite essential. The diode bridge provides pulsating DC output and acts as a guard circuit between the mains input and pulsating DC output. Extreme care is necessary to avoid any electric shock.

Tuesday, July 28, 2009

PC based Frequency Meter

Here is a simple technique for measuring frequencies over quite a wide frequency range and with acceptable accuracy limits using a PC. It follows the basic technique of measuring low frequencies, i.e. at low frequency, period is measured for a complete wave and frequency is calculated from the measured time-period. Cascaded binary counters are used for converting the high-frequency signals into low-frequency signals. The parallel port of a computer is used for data input from binary counters. This data is used for measuring time and calculating the frequency of the signal. The block diagram shows the basic connections of the counters and parallel port pin numbers on 25-pin ‘D’ connector of a PC (control register 379 Hex is used for input). External hardware is used only for converting the higher frequency signals into low frequency signals. Thus, the major role in frequency-measurement is played by the software. The PC generates a time-interrupt at a frequency of 18.21 Hz, i.e. after every 54.92 millisecond. Software uses this time-interrupt as a time-reference. The control register of the PC’s parallel port is read and the data is stored continuously in an array for approximately 54.9 ms using a loop. This stored data is then analysed bit-wise. Initially, the higher-order bit (MSB or the seventh-bit) of every array element is scanned for the presence of a complete square wave. If it is found, its time period is measured and if not then the second-highest order bit (sixth bit) is scanned. This operation is performed till the third bit and if no full square wave is still found, an error message is generated which indicates that either there is an error in reading or the frequency signal is lower than 19 Hz. Lower three bits of the control register are not used. When a wave is found, along with its time-period and frequency components, its measurement precision in percentage is also calculated and displayed. Number of data taken in 54.9 ms is also displayed. As stated above, the lower starting range is about 19 Hz. Data is read for approximately 54.9 ms. Thus, the lowest possible frequency that can be measured is 1/.0549 Hz. Lower range depends only on the sampling time and is practically fixed at 19 Hz (18.2 Hz, to be precise). Upper range depends on factors such as value of the MOD counter used and the operating frequency range of the counter IC. If MOD-N counter is used (where N is an integer), upper limit (UL) of frequency is given by UL=19xN5 Hz. Thus for MOD 16 counters UL@20 MHz, and for MOD 10 counters UL@1.9 MHz. Care should be taken to ensure that this upper limit is within the operating frequency range of counter IC used. Precision of measurement is a machine-dependent parameter. High-speed machines will have better precision compared to others. Basically, precision depends directly upon the number of data read in a standard time. Precision of measurement varies inversely as the value of MOD counter used. Precision is high when MOD 10 counters are used in place of MOD 16 counters, but this will restrict the upper limit of frequency measurement and vice-versa.

Simple Analog to Digital Converter

Normally analogue-to-digital con-verter (ADC) needs interfacing through a microprocessor to convert analogue data into digital format. This requires hardware and necessary software, resulting in increased complexity and hence the total cost.
The circuit of A-to-D converter shown here is configured around ADC 0808, avoiding the use of a microprocessor. The ADC 0808 is an 8-bit A-to-D converter, having data lines D0-D7. It works on the principle of successive approximation. It has a total of eight analogue input channels, out of which any one can be selected using address lines A, B and C. Here, in this case, input channel IN0 is selected by grounding A, B and C address lines.
Usually the control signals EOC (end of conversion), SC (start conversion), ALE (address latch enable) and OE (output enable) are interfaced by means of a microprocessor. However, the circuit shown here is built to operate in its continuous mode without using any microprocessor. Therefore the input control signals ALE and OE, being active-high, are tied to Vcc (+5 volts). The input control signal SC, being active-low, initiates start of conversion at falling edge of the pulse, whereas the output signal EOC becomes high after completion of digitisation. This EOC output is coupled to SC input, where falling edge of EOC output acts as SC input to direct the ADC to start the conversion.
As the conversion starts, EOC signal goes high. At next clock pulse EOC output again goes low, and hence SC is enabled to start the next conversion. Thus, it provides continuous 8-bit digital output corresponding to instantaneous value of analogue input. The maximum level of analogue input voltage should be appropriately scaled down below positive reference (+5V) level.
The ADC 0808 IC requires clock signal of typically 550 kHz, which can be easily derived from an astable multivibrator constructed using 7404 inverter gates. In order to visualise the digital output, the row of eight LEDs (LED1 through LED8) have been used, wherein each LED is connected to respective data lines D0 through D7. Since ADC works in the continuous mode, it displays digital output as soon as analogue input is applied. The decimal equivalent digital output value D for a given analogue input voltage Vin can be calculated from the relationship

Control electrical appliances using PC

Here is a circuit for using the printer port of a PC, for control application using software and some interface hardware. The interface circuit along with the given software can be used with the printer port of any PC for controlling up to eight equipment .
The interface circuit shown in the figure is drawn for only one device, being controlled by D0 bit at pin 2 of the 25-pin parallel port. Identical circuits for the remaining data bits D1 through D7 (available at pins 3 through 9) have to be similarly wired. The use of opto-coupler ensures complete isolation of the PC from the relay driver circuitry.
Lots of ways to control the hardware can be implemented using software. In C/C++ one can use the outportb(portno,value) function where portno is the parallel port address (usually 378hex for LPT1) and 'value' is the data that is to be sent to the port. For a value=0 all the outputs (D0-D7) are off. For value=1 D0 is ON, value=2 D1 is ON, value=4, D2 is ON and so on. eg. If value=29(decimal) = 00011101(binary) ->D0,D2,D3,D4 are ON and the rest are OFF.

/*program to control devices using PC parallel port
The devices are controlled by pressing the keys 1-8
that corresponds to each of the 8 possible devices

#define PORT 0x378 /* This is the parallel port address */

char val=0,key=0;
char str1[]="ON ";
char str2[]="OFF";
char *str;
printf("Press the approriate number key to turn on/off devices:\n\n");
printf("Here Device1 is connected to D0 of parallel port and so on\n\n");
printf("Press \"x\" to quit\n\n");
printf("Device1:OFF Device2:OFF Device3:OFF Device4:OFF\n");
printf("Device5:OFF Device6:OFF Device7:OFF Device8:OFF");

while(key!='x' && key!='X')
printf("Value in hex sent to the port:");

case '1':


case '2':


case '3':


case '4':


case '5':


case '6':


case '7':


case '8':
printf("%x",(unsigned char)val);




Infrared beam barrier/ proximity sensor

This circuit can be used as an Infrared beam barrier as well as a proximity detector.
The circuit uses the very popular Sharp IR module (Vishay module can also be used). The pin nos. shown in the circuit are for the Sharp & VIshay modules. For other modules please refer to their respective datasheets.
The receiver consists of a 555 timer IC working as an oscillator at about 38Khz (also works from 36kHz to 40kHz) which has to be adjusted using the 10K preset. The duty cycle of the IR beam is about 10%. This allows us to pass more current through the LEDS thus achieving a longer range.
The receiver uses a sharp IR module. When the IR beam from the transmitter falls on the IR module, the output is activated which activates the relay and de-activated when the beam is obstructed. The relay contacts can be used to turn ON/OFF alarms, lights etc. The 10K preset should be adjusted until the receiver detects the IR beam.

The circuit can also be used as a proximity sensor, i.e to detect objects in front of the device without obstructing a IR beam. For this the LEDs should be pointed in the same direction as the IR module and at the same level. The suggested arrangement is shown in the circuit diagram. The LEDs should be properly covered with a reflective material like glass or aluminum foils on the sides to avoid the spreading of the IR beam and to get a sharp focus of the beam.
When there is nothing in front of them, the IR beam is not reflected onto the module and hence the circuit is not activated. When an object comes near the device, the IR light from the LEDs is reflected by the object onto the module and hence the circuit gets activated.

If there still a lot of mis-triggering, use a 1uF or higher capacitor instead of the 0.47uF.

Car anti theft wireless alarm.

This FM radio-controlled anti- theft alarm can be used with any vehicle having 6- to 12-volt DC supply system. The mini VHF, FM transmitter is fitted in the vehicle at night when it is parked in the car porch or car park. The receiver unit with CXA1019, a single IC-based FM radio module, which is freely available in the market at reasonable rate, is kept inside. Receiver is tuned to the transmitter's frequency. When the transmitter is on and the signals are being received by FM radio receiver, no hissing noise is available at the output of receiver. Thus transistor T2 (BC548) does not conduct. This results in the relay driver transistor T3 getting its forward base bias via 10k resistor R5 and the relay gets energised. When an intruder tries to drive the car and takes it a few metres away from the car porch, the radio link between the car (transmitter) and alarm (receiver) is broken. As a result FM radio module gene-rates hissing noise. Hissing AC signals are coupled to relay switching circ- uit via audio transformer. These AC signals are rectified and filtered by diode D1 and capacitor C8, and the resulting positive DC voltage provides a forward bias to transistor T2. Thus transistor T2 conducts, and it pulls the base of relay driver transistor T3 to ground level. The relay thus gets de-activated and the alarm connected via N/C contacts of relay is switched on. If, by chance, the intruder finds out about the wireless alarm and disconnects the transmitter from battery, still remote alarm remains activated because in the absence of signal, the receiver continues to produce hissing noise at its output. So the burglar alarm is fool-proof and highly reliable.

4 in 1 Burglar Alarm

I n this circuit, the alarm will be switched on under the following four different conditions: 1. When light falls on LDR1 (at the entry to the premises). 2. When light falling on LDR2 is obstructed. 3. When door switches are opened or a wire is broken. 4. When a handle is touched. The light dependent resistor LDR1 should be placed in darkness near the door lock or handle etc. If an intruder flashes his torch, its light will fall on LDR1, reducing the voltage drop across it and so also the voltage applied to trigger 1 (pin 6) of IC1. Thus transistor T2 will get forward biased and relay RL1 energise and operate the alarm. Sensitivity of LDR1 can be adjusted by varying preset VR1. LDR2 may be placed on one side of a corridor such that the beam of light from a light source always falls on it. When an intruder passes through the corridor, his shadow falls on LDR2. As a result voltage drop across LDR2 increases and pin 8 of IC1 goes low while output pin 9 of IC1 goes high. Transistor T2 gets switched on and the relay operates to set the alarm. The sensitivity of LDR2 can be adjusted by varying potentiometer VR2. A long but very thin wire may be connected between the points A and B or C and D across a window or a door. This long wire may even be used to lock or tie something. If anyone cuts or breaks this wire, the alarm will be switched on as pin 8 or 6 will go low. In place of the wire between points A and B or C and D door switches can be connected. These switches should be fixed on the door in such a way that when the door is closed the switch gets closed and when the door is open the switch remains open. If the switches or wire, are not used between these points, the points should be shorted. With the help of a wire, connect the touch point (P) with the handle of a door or some other suitable object made of conducting material. When one touches this handle or the other connected object, pin 6 of IC1 goes ‘low’. So the alarm and the relay gets switched on. Remember that the object connected to this touch point should be well insulated from ground. For good touch action, potentiometer VR3 should be properly adjusted. If potentiometer VR3 tapping is held more towards ground, the alarm will get switched on even without touching. In such a situation, the tapping should be raised. But the tapping point should not be raised too much as the touch action would then vanish. When you vary potentiometer VR1, re-adjust the sensitivity of the touch point with the help of potentiometer VR3 properly. If the alarm has a voltage rating of other than 6V (more than 6V), or if it draws a high current (more than 150 mA), connect it through the relay points as shown by the dotted lines. As a burglar alarm, battery backup is necessary for this circuit. Note: Electric sparking in the vicinity of this circuit may cause false triggering of the circuit. To avoid this adjust potentiometer VR3 properly.

Melody generator for greeting cards

This tiny circuit comprising of a single 3 terminal IC UM66 can be built small enough to be placed inside a greeting card and operated off a single 3V flat button cell.
There is not much to the circuit. The UM66 is connected to its supply and its output fed to a transistor for amplification. You can either use a 4ohm speaker or a " flat" piezoelectric tweeter like the one found in alarm wrist watches.
If you use the piezo, then it can be connected directly between the output pin 1 and ground pin 3 without the transistor.
The UM66 looks like a transistor with 3 terminals. It is a complete miniature tone generator with a ROM of 64 notes, oscillator and a preamplifier. When it first came into market, it was programmed for the "Jingle bells" tune. Now they come with a wide variety of different tunes.

Water Level Indicator with alarm

This circuit not only indicates the amount of water present in the overhead tank but also gives an alarm when the tank is full.
The circuit uses the widely available CD4066, bilateral switch CMOS IC to indicate the water level through LEDs.
When the water is empty the wires in the tank are open circuited and the 180K resistors pulls the switch low hence opening the switch and LEDs are OFF. As the water starts filling up, first the wire in the tank connected to S1 and the + supply are shorted by water. This closes the switch S1 and turns the LED1 ON. As the water continues to fill the tank, the LEDs2 , 3 and 4 light up gradually.
The no. of levels of indication can be increased to 8 if 2 CD4066 ICs are used in a similar fashion.

When the water is full, the base of the transistor BC148 is pulled high by the water and this saturates the transistor, turning the buzzer ON. The SPST switch has to be opened to turn the buzzer OFF.
Remember to turn the switch ON while pumping water otherwise the buzzer will not sound!

Rain Alarm

This circuit gives out an alarm when its sensor is wetted by water.
A 555 astable multivibrator is used here which gives a tone of about 1kHz upon detecting water.
The sensor when wetted by water completes the circuit and makes the 555 oscillate at about 1kHz.

The sensor is also shown in the circuit diagram.
It has to placed making an angle of about 30 - 45 degrees to the ground. This makes the rain water to flow through it to the ground and prevents the alarm from going on due to the stored water on the sensor.
The metal used to make the sensor has to be aluminium and not copper. This is because copper forms a blue oxide on its layer on prolonged exposure to moisture and has to be cleaned regularly.
The aluminium foils may be secured to the wooden / plastic board via epoxy adhesive or small screws.
The contact X and Y from the sensor may be obtained by small crocodile clips or you may use screws.

Theft preventer alarm

This circuit utilising a 555 timer IC can be used as an alarm system to prevent the theft of your luggage, burglars breaking into your house etc. The alarms goes ON when a thin wire, usually as thin as a hair is broken.
The circuit is straightforward. It uses a 555 IC wired as an astable multivibrator to produce a tone of frequency of about 1kHz which gives out a shrill noise to scare away the burglar.
The wire used to set off the alarm can be made of a thin copper wire like SWG 36 or higher.
You can even use single strands of copper form a power cable.

The circuit operates on a wide range of voltages from 5V to 15V.
The speaker and the circuit could be housed inside a tin can with holes drilled on the speaker side for the sound to come out.

Power supply failure alarm

Most of the power supply failure indicator circuits need a separate power supply for themselves. But the alarm circuit presented here needs no additional supply source. It employs an electrolytic capacitor to store adequate charge, to feed power to the alarm circuit which sounds an alarm for a reasonable duration when the supply fails.
This circuit can be used as an alarm for power supplies in the range of 5V to 15V.
To calibrate the circuit, first connect the power supply (5 to 15V) then vary the potentiometer VR1 until the buzzer goes from on to off.
Whenever the supply fails, resistor R2 pulls the base of transistor low and saturates it, turning the buzzer ON.

TV remote control Blocker

Just point this small device at the TV and the remote gets jammed . The circuit is self explanatory . 555 is wired as an astable multivibrator for a frequency of nearly 38 kHz. This is the frequency at which most of the modern TVs receive the IR beam . The transistor acts as a current source supplying roughly 25mA to the infra red LEDs. To increase the range of the circuit simply decrease the value of the 180 ohm resistor to not less than 100 ohm.

It is required to adjust the 10K potentiometer while pointing the device at your TV to block the IR rays from the remote. This can be done by trial and error until the remote no longer responds.

JAM(Just A Minute) Circuit

This jam circuit can be used in quiz contests wherein any par- ticipant who presses his button (switch) before the other contestants, gets the first chance to answer a question. The circuit given here permits up to eight contestants with each one allotted a distinct number (1 to 8). The display will show the number of the contestant pressing his button before the others. Simultaneously, a buzzer will also sound. Both, the display as well as the buzzer have to be reset manually using a common reset switch. Initially, when reset switch S9 is momentarily pressed and released, all outputs of 74LS373 (IC1) transparent latch go ‘high’ since all the input data lines are returned to Vcc via resistors R1 through R8. All eight outputs of IC1 are connected to inputs of priority encoder 74LS147 (IC2) as well as 8-input NAND gate 74LS30 (IC3). The output of IC3 thus becomes logic 0 which, after inversion by NAND gate N2, is applied to latch-enable pin 11 of IC1. With all input pins of IC2 being logic 1, its BCD output is 0000, which is applied to 7-segment decoder/driver 74LS47 (IC6) after inversion by hex inverter gates inside 74LS04 (IC5). Thus, on reset the display shows 0. When any one of the push-to-on switches—S1 through S8—is pressed, the corresponding output line of IC1 is latched at logic 0 level and the display indicates the number associated with the specific switch. At the same time, output pin 8 of IC3 becomes high, which causes outputs of both gates N1 and N2 to go to logic 0 state. Logic 0 output of gate N2 inhibits IC1, and thus pressing of any other switch S1 through S8 has no effect. Thus, the contestant who presses his switch first, jams the display to show only his number. In the unlikely event of simultaneous pressing (within few nano-seconds difference) of more than one switch, the higher priority number (switch no.) will be displayed. Simultaneously, the logic 0 output of gate N1 drives the buzzer via pnp transistor BC158 (T1). The buzzer as well the display can be reset (to show 0) by momentary pressing of reset switch S9 so that next round may start. Lab Note: The original circuit sent by the author has been modified as it did not jam the display, and a higher number switch (higher priority), even when pressed later, was able to change the displayed number.

Electronic Scoring Game

You can play this game alone or with your friends. The circuit comprises a timer IC, two decade counters and a display driver along with a 7-segment display. The game is simple. As stated above, it is a scoring game and the competitor who scores 100 points rapidly (in short steps) is the winner. For scoring, one has the option of pressing either switch S2 or S3. Switch S2, when pressed, makes the counter count in the forward direction, while switch S3 helps to count downwards. Before starting a fresh game, and for that matter even a fresh move, you must press switch S1 to reset the circuit. Thereafter, press any of the two switches, i.e. S2 or S3. On pressing switch S2 or S3, the counter’s BCD outputs change very rapidly and when you release the switch, the last number remains latched at the output of IC2. The latched BCD number is input to BCD to 7-segment decoder/driver IC3 which drives a common-anode display DIS1. However, you can read this number only when you press switch S4. The sequence of operations for playing the game between, say two players ‘X’ and ‘Y’, is summarised below:
1. Player ‘X’ starts by momentary pressing of reset switch S1 followed by pressing and releasing of either switch S2 or S3. Thereafter he presses switch S4 to read the display (score) and notes down this number (say X1) manually.
2. Player ‘Y’ also starts by momentary pressing of switch S1 followed by pressing of switch S2 or S3 and then notes down his score (say Y1), after pressing switch S4, exactly in the same fashion as done by the first player.
3. Player ‘X’ again presses switch S1 and repeats the steps shown in step 1 above and notes down his new score (say, X2). He adds up this score to his previous score. The same procedure is repeated by player ‘Y’ in his turn.
4. The game carries on until the score attained by one of the two players totals up to or exceeds 100, to be declared as the winner.
Several players can participate in this game, with each getting a chance to score during his own turn. The assembly can be done using a multipurpose board. Fix the display (LEDs and 7-segment display) on top of the cabinet along with the three switches. The supply voltage for the circuit is 5V

DC Motor-Driver H-Bridge Circuit

DC Motor-Driver H-Bridge Circuit

Physical motion of some form helps differentiate a robot from a computer. It would be nice if a motor could be attached directly to a chip that controlled the movement. But, most chips can't pass enough current or voltage to spin a motor. Also, motors tend to be electrically noisy (spikes) and can slam power back into the control lines when the motor direction or speed is changed.

Specialized circuits (motor drivers) have been developed to supply motors with power and to isolate the other ICs from electrical problems. These circuits can be designed such that they can be completely separate boards, reusable from project to project.

A very popular circuit for driving DC motors (ordinary or gearhead) is called an H-bridge. It's called that because it looks like the capital letter 'H' on classic schematics. The great ability of an H-bridge circuit is that the motor can be driven forward or backward at any speed, optionally using a completely independent power source.

An H-bridge design can be really simple for prototyping or really extravagant for added protection and isolation. An H-bridge can be implemented with various kinds of components (common bipolar transistors, FET transistors, MOSFET transistors, power MOSFETs, or even chips).

The example provided on this page features:
  • TTL/CMOS compatible Microchip or Maxim 4427A or 4424 MOSFET driver chips that protect the logic chips, isolate electrical noise, and prevent potential short-circuits inherently possible in a discrete H-bridge.
  • Schottky diodes to protect against overvoltage or undervoltage from the motor.
  • Capacitors to reduce electrical noise and provide spike power to the driver chips.
  • Pull-up resistors that prevent unwanted motor movement while the microcontroller powers up or powers down.
A diode-less version of this circuit successfully drove Bugdozer to mini-sumo victory. The more robust (diode protected) version actually illustrated below is from Sweet, the line-following robot.

Hmm. It doesn't look like the letter 'H'.
(Note: An improved version of this circuit appears on Page 197 - Figure 10-18 of Intermediate Robot Building.)

R1 and R2:
Two pull-up resistors (any value from 10 kilohm to 100 kilohm).

These make sure the inputs are both on unless a signal from the microcontroller tells one or the other to turn off. With both on or both off, the motor doesn't spin because there's no voltage drop between them.

Think of these as default values. Unless a different value is specified, the lines are pulled up. This means the circuit can come loose or be disconnected completely and the motor won't spin or stutter.

Technically, R1 and R2 could be eliminated, although then the motors are likely to jerk when the microcontroller powers up or powers down.

TC4424 dual MOSFET transistor driver chip. (The MAX4427 and TC4427A is the same but with a lower amperage rating.) The DIP part can be purchased at DigiKey as part #TC4424CPA.

Direct motor driving with this chip is only possible for motors that draw less than 100 mA (4427) to 150 mA (4424) under load. To determine if your motors qualify, use a multimeter to measure how much current your motor uses under load (for example, when actually driving your robot around) when the motors are connected directly to the battery (not through these chips).

This chip is not really supposed to drive a motor by itself. If you find the chip gets very hot and the motor doesn't spin (or barely spins or stalls when loaded) then you need to have the chip drive some real power MOSFETs like it is supposed to. Check out Figure 10-13 and Figure 10-15 on pages 186 and 187 of Intermediate Robot Building. It's not that much more difficult and it really makes a huge difference in performance.
This chip provides two independent inputs that are compatible with CMOS or TTL chips. This circuit design uses IN A to vary power (on, off, or pulsed in-between) and IN B to determine direction.

OUT A follows the IN A signal but uses the full voltage from the power source, not the tiny voltage from the input signal itself. OUT B follows IN B in the same way.

For example, if IN A is turned on completely (2.4 volts or better) and IN B is turned off completely (0.8 volts or less) then OUT A turns on completely (up to 22 volts) and OUT B turns off completely (GND). The motor gets 22 volts.

This chip is constructed to protect the static sensitive MOSFETs, but also to protect the input sources from current being jammed back by the motors. Optoisolator ICs could be used at the inputs if greater protection and freedom from noise is desired.

Normally four transistors are needed in an H-bridge. Each transistor forms a corner in the letter 'H', with the motor being the bar in the middle. (See Figure 9-14 on page 158 of Intermediate Robot Building.) In this design, each output of the chip forms a complete vertical side of the letter 'H', with the motor still being in the middle. Because a side is now a single output, short-circuits can't form from the top of a side to the bottom of a side. No matter what the inputs, all power must travel from one side to the other -- through the motor.

A mechanical switch, relay, or logical gate could be used to turn the inputs on and off. It would work just fine at providing no movement (on/on or off/off), forward movement (on/off), or reverse movement (off/on). To provide power levels in between (like 50%), rapid pulses of on or off can be provided by pulse-width modulation using a chip or timer.

An important note regarding current rating: The plastic DIP package can only dissipate enough heat when the power usage is below 730 milliwatts. Therefore, it isn't possible to continuously run the chip at both the maximum voltage (22 V) and maximum amperage (3 A) rating. That would result in 66 watts of power usage. (That's 100x the maximum allowed.)

From: Paul Jurczak
Sent: Monday, March 12, 2001 10:59 AM
Subject: DC Motor-Driver H-Bridge Circuit

The actual DC power losses in the H-bridge would be:
I2 * (Rl + Rh)
= (3 A * 3 A) * (2.8 ohm + 2.5 ohm) = 47.7 W typical
= (3 A * 3 A) * (5 ohm + 5 ohm) = 90 W maximum

Which still is more than enough to melt this IC.


I'm sincerely grateful for the feedback.

For current to flow, the chip must have one gate high and one gate low. Therefore, Paul is adding the typical high and low resistance (from the 4427 datasheets) together to calculate the total amount of resistance the chip causes.

When a moving motor is added to the circuit, the motor uses up some (hopefully most) of the power. Just dive for that robot if the motors stall!

In summary, the chip can't run at maximum volts and maximum amps because most of the 66 watts (47 watts typical) would need to be dissipated by the chip.

D1 and D3:
Schottky small-signal diodes.

I couldn't find any! So, I used 5817 Schottky diodes instead.

The key factors in substitution are:
  • Are the diodes rated to turn on with less voltage than the TC4424's internal transistor base voltage? (600 millivolts)
  • Are the diodes rated to handle the maximum reverse voltage? (22 volts)
  • Are the diodes rated to handle the maximum current? (3 amps)
In the case of the 5817s, the datasheets answers are:
  • Yes. (400 millivolts or less)
  • Nearly. (20 volts -- so this is the circuit's new voltage maximum)
  • Yes, peak (25 amps)
When a motor accelerates or decelerates for any reason (signal, load, or friction), there is reluctance for the electric field present in the motor coils to change. More properly, the changing field induces power. This "refunded" power can jam back into the chips.

D1 and D3 protect the chips from overvoltage by turning on when more voltage is coming from the motor than is coming from the batteries. The batteries absorb the power.

The turn-on rating of the diode must be lower than the turn-on rating of the chip, or else the diode won't turn on early enough to protect the chip.

Because the diode is installed in "reverse", the power can't flow from the batteries to the motors. If the diode was installed differently, power would immediately flow to the motors, bypassing the chip outputs (or worse, short-circuiting through the chip).

By the way, this arrangement is why the reverse or breakdown voltage of the diodes is important. If the reverse voltage rating was less than the full battery voltage, the battery would break down the reversed diode and just shoot through.

D2 and D4:
Schottky small-signal diodes.

D2 and D4 protect the chips from undervoltage (less than ground) by turning on when the voltage in the motor is below GND. Once again, the batteries take care of the problem, rather than power flowing backwards from the chip.

D1 through D4 could be eliminated. In fact, Bugdozer runs without the diodes. However, parasitic voltages can and do temporarily short power supplies (reset!) and can even destroy the driver chips.

Actual implementation of motor driver

Despite what may seem complicated at first, the above photograph includes added features such as an LP2954 5V voltage regulator, a bicolor LED, and two switches for testing.

One H-bridge drives one motor. For a common two-wheeled robot, obviously two copies of the H-bridge circuit are needed.

Click for a movie of the H-bridge in action
(Click the picture above for a movie)

  • Pressing the right-side button makes the motor turn counter-clockwise and lights the LED green.
  • Pressing the left-side button makes the motor turn clockwise and lights the LED red.
  • Pressing both the buttons turns on the brakes (stopping the motor) and turns off the LED.
  • Pressing the brakes quickly enough provides variable speed (between 0% and 100%).


Balun -

"A transmission line transformer for converting balanced input to unbalanced output or vice versa. It may or may not provide wide frequency range impedance transformation depending upon the configuration used."

An old typical usage of the balun was (and still is) with TV antennas. The folded dipole is part of a yagi antenna which looks something like a pole with rods set across it at right angles. The second last one is folded into an oblong or rectangular shape. A folded dipole exhibits two important characteristics (a) its bandwidth is good for over an octave (e.g. 50 Mhz to 100 Mhz or say 120 Mhz to 240 Mhz) AND its characteristic impedance is a more or less a constant 300 ohms.

In earlier days extensive use was made of 300 ohm twin lead ribbon cable to feed the signals to the TV receiver. BTW you can use just a length of 300 ohm ribbon cable to make a folded dipole.

When colour (or color if you prefer) TV was introduced the ribbon cable often created problems which could be rectified by the use of co-axial cable. This is not strictly correct because coax had earlier uses in TV because of other problems such as ghosting which became intolerable with the introduction of colour.

Now 300 ohm coax can be and is made. However 50 and 75 ohm cable is preferred for a variety of reasons. Our folded dipole also exhibits a "balanced" feed characteristic whilst coaxial cable has an "unbalanced" characteristic. Two problems. Each solved by the use of the balun.

Perhaps we should have some clarification about this balanced versus unbalanced jazz. Mentally visualise it this way. If we have a plus 12V D.C. supply with ground return. That could be regarded for our illustrative purposes as the unbalanced 75 ohm input. On the other hand we could a symmetrical power supply referenced to ground which provides a + 12V D.C. AND a - 12V D.C. voltage. The only difference is we are dealing with R.F. which of course is very high A.C.

A practical 300 / 75 ohm balun would consist of four pieces of 0.4mm wire wound 2 1/2 turns through a balun core (look a bit like binoculars). In so doing we have achieved our two goals, (a) the impedance transformation and (b) gone from unbalanced to balanced.


First off some rough definitions for the mixer and balun:

1. Double Balanced Mixer

- A passive mixer where F1 is introduced at one port whilst F2 is introduced at the second port. These frequencies are then mixed with the result that F1 + F2 and F2 - F1appear at the output port. We select the one we are looking for and discard the other. The important feature is the mixer, because of the balance of the hot carrier diodes and transmission line transformers cancels even harmonics of both the R.F. input (port 1) and L.O. frequencies (port 2) and provides isolation among ports.

2. Balun -

A transmission line transformer for converting balanced input to unbalanced output or vice versa. It may or may not provide wide frequency range impedance transformation depending upon the configuration used.

A typical double balanced mixer is depicted in figure 1 below.

This image is copyrighted © by Ian C. Purdie VK2TIP - double balanced mixer using baluns

Figure 1 - double balanced mixer using baluns

Now first off you can see one transmission line transformer on the left and there is also a mirror reverse one on the right. In the middle are four diodes arranged in a bridge configuration. If you think it looks a bit like a bridge rectifier you would almost be right.

In this application the diodes operate as high speed switches where the incoming F1 signal is "chopped" at the rate of the F2 signal. It can be mathematically proven that "sum" and "difference" signals result, now unless you are into sado-masochism you don't want to particularly see those figures.

Now let's list the advantages and disadvantages and then see if the disadvantages can be overcome:


  • reasonable conversion loss on signal F1. This is usually about 7dB. Many would consider this a disadvantage but I'm quite happy with it. I never look for conversion gain, that's what a quality I.F. Amplifier is all about.
  • balance of the output. You should expect to see "sum" and "difference" signals only. No F1 or F2 signals should appear at the output. Similarly because of this balance the F1 and F2 signals do not appear at each others port.
  • consumes no power except for the losses incurred in conversion.
  • broadband in nature and therefore suited to multiband designs.
  • high intercept points.


  • a relatively high noise figure, about the same as the conversion loss.
  • fairly high local oscillator drive requirements. Typical values are +13dBm (that's 20 mW into 50 ohms or 1V rms or nearly 3V pk to pk). Some mixers require even higher levels.
  • each port is highly sensitive to reactive terminations. A pure 50 ohms would be ideal. Proper termination is absolutely critical, particularly the "sum" and "difference" or I.F. port.
  • hot carrier diodes are relatively expensive. High quality, high speed diodes which will take the necessary saturating current and large reverse voltages across the non conducting diodes are an absolute must where performance counts.
  • diodes need to be well "matched".
  • the transmission line transformers require great care in design and construction. The actual construction will determine the bandwidth.
  • somewhat prone to harmonic mixing. Diodes make ideal harmonic generators.


1. relative high noise figure -

in some applications you can use a singly balanced mixer where a noise figure of about 5dB might be expected. Noise figure is rarely an issue in H.F. sets. At V.H.F. and beyond it is likely you would employ a low noise r.f. preamp anyway. At H.F. don't even think of using an R.F. amplifier ahead of a mixer unless you know precisely why you are doing it. No! No! No!

2. high local oscillator drive -

if you are running portable and using battery power then don't worry about it. A well designed and constructed local oscillator should have been well "buffered" and filtered anyway and I would expect you would have power to burn if you are operating a mains powered set. A few extra transistors and components won't send you broke and you are after performance after all. Why else would you be reading this?.

3. reactive ports -

ahah! now were getting down. The most critical port is the I.F. port which MUST see 50 ohms at all frequencies. The simple solution here is to place a diplexer at this port together with a post mixer amplifier. Don't fret, more on this later. The local oscillator port should have a low pass filter in line designed to work into a 50 ohm load. You have done my tutorials on LC filters haven't you. If not save yourself some grief and do it!.

This low pass filter should have a 6dB attenuator after it. Bang your L.O. drive has just gone up from +13dB to +19dB now - so there. You could use 3dB I suppose but something higher is better.

The R.F. input port should have some filtering ahead of it leading to the 50 ohm mixer load. Now you have to do the LC filter tutorial.

4 diodes are expensive -

well first off you could save yourself a lot of angst by buying a commercially manufactured mixer. Minicircuits SBL-1 would be an example. It comes in a can with a pin out the same as an 8 pin DIP and can be bought for only a few dollars. You could shop around for suitable diodes cheaply available e.g. Hewlett Packard's HP5082-2800 or similar. In North America Dan's Small Parts seem to have a variety and I think the mixer is also available. Links to all sites mentioned appear at the end of this tutorial.

5. diodes need to be well matched -

don't even consider using 1N914 types if you want results. The HP 2800 types are pretty well matched but I'd still check a bunch for foward and back resistance.

6. transmission line transformers need care in design and construction -

well may be this is a further reason for buying an SBL-1 or similar. Considering the cost of four quality diodes, two ferrite cores and suitable winding wire etc. The commercial unit looks dirt cheap. But so you understand I will continue.

The transmission line transformers in figure 1 are tri-filar wound i.e. there are three windings. The large dots indicate phasing which is most important. It might be considered the start of each winding.

Take a typical ferrite toroid used for this purpose. An example might be 15 turns of # 26 wire on an Amidon T-50-61 ferrite toroid. Firstly estimate the length of wire required to complete 15 turns, leaving a bit left over to connect to the board or whatever and for knots etc. (see next para). Now these wires should exhibit an impedance of 50 ohms and at home this is difficult to obtain. I sometimes use this method.

Take three suitable lengths of wire. With the first one tie one little knot at about 4" or 100mm from the extreme both ends. The next piece tie two knots similarly at both ends. Now we have three wires with (1) no knot, (2) one knot and (3) two knots - easily identifiable o.k. - you could try marker pens etc. but I find that rubs off.

Now for this you need a hand drill with a fish hook or similar tightened in the chuck. You might have to improvise once you get the gist of what I am telling you.

Tie the three wires together at one end in such a fashion it can be readily undone later and clamp this securely in a bench vise. Pull the other end of the wires straight and reasonably taut and tie to the fish hook. Once secure, gently stretch the wires a bit - this is good. Then commence slowly winding the hand drill and you will note the wires twisting together. After a while the twists become closer together and at about 2.5 twists per inch or one per centimetre (10mm) you should have something suitable.



This tutorial on LC filter design should allow you to incorporate into your radio design low pass, high pass and bandpass filters. Your electronic project should then hopefully perform the way you want it to.

Low pass and high pass filters are easy to design and implement. A high pass filter is really just the complement of a low pass filter. On the other hand bandpass filters can get quite complex. Also whilst a LC bandpass filter is particularly helpful in reducing image interference it is not the panacea for all ills. A properly designed, carefully constructed and aligned lc bandpass filter will give you a fractional bandwidth of about 2%. It is likely the percentage bandwidth might be somewhat worse than that.

Bandwiths of LC bandpass filters are more a function of the loaded Q of the circuit which in turn is determined by the terminating resistances. The shape factor i.e. - 6dB to - 60 dB points for LC bandpass filters is more a function of the number of resonators.

1. How this electronic tutorial is organised:

I would suggest you create a separate directory called lc-filters, perhaps with a sub-directory called tutorials.

Then I would advise you to save each LC filter lesson as you go. In this way if you wish to refer back later then you can. At times I will recommend you print directly from your online browser. This is because I have graphics files embedded throughout the lc filter lessons and these will not be saved with File|Save As|. The graphic files are to give you some schematics and also because some mathematical formula can't be readily inserted into the text.

Alternatively save the file and then right click to save any graphic you encounter to your new directory also.

2. Basis of the Tutorial

Clearly I must assume my reader will range from those who know nothing at all about LC filters - why else would you be here - to those who know just this and only that and then on up to those people who possibly know more than I do.

3. Tutorial Layout

I have endeavoued to provide sufficient hyperlinks to filter basics, low pass, high pass and bandpass filters so you can jump out if necessary and then come back to me. I have done some layout revision because it was pretty scatty - thanks to all those who continue to email constructive comments and suggestions.

Just follow the links forward and/or backward. Hopefully it will work. If it doesn't let me know because it is quite common for links to become corrupted over time..

4. Tutorial Quiz

I must assume my reader will be enthusiastic enough to want to do a few self-tests. You can look up the answers when you're finished.

5. L.C. Filter Tutorials

1. Basics you must learn to proceed further.

2. How low pass filters are designed - the basis of many other filters.

3. Again you could need a high pass filter in your application.

4. Down to a simple band pass filter for many purposes.

5. Next we have multi-pole narrow band pass filters.

6. How did they design I.F. Amplifier filters in the olden days anyway?



Section 1 - Essential Basics of the Tutorial:-

1 (a). L/C - basics:-

1. What is the L/C for the following frequencies;
(a) 455Khz (b) 7224 Khz and (c) 9 Mhz.

Answers:(a) 122,354 (b) 485.38 and (c) 312.72

2. What capacitor will resonate at the above respective frequencies with the following respective inductors ;
(a) 679 uH (b) 22uH and (c) 3.81 uH.

Answers: (a) 180pF (b) 22pF and (c) 82pF

3. What is the resonant frequency for the following L/C (inductor - capacitor) combinations to the nearest Khz;
(a) 2,533,030 (b) 116,716 (c) 312.72 and (d) 2.53303.

Answers: (a) 100 Khz (b) 466 Khz (c) 9 Mhz and (d) 100 Mhz

Back to L/C .

1 (b). reactance - basics:-

1. What is the reactance of a 100pF capacitor at;
(a) 100Khz (b) 1.5 Mhz and (c) 22 Mhz.

Answers:(a) 15K915 ohms (b) 1K061 ohms and (c) 72R34ohms

Where K and R indicate the place of the comma or decimal point. Wait for a later tutorial to see my rabid tirade about the necessity for this method.

2. What is the reactance of a 22uH inductor at;
(a) 100Khz (b) 1.5 Mhz and (c) 22 Mhz.

Answers:(a) 13R823 ohms (b) 207R35 ohms and (c) 3K041 ohms

3. This is a revision of a previous tutorial:- What capacitor will resonate with the above inductor at;
(a) 7234Khz (b) 1224 Khz and (c) 3.5 Mhz.

Answers: (a) 22 pF (b) 769 pF and (c) 94 pF

Back to Reactance

1 (c). Q - basics:-

1. In our previous tutorial we calculated the reactance of a 22uH inductor at;
(a) 100Khz (b) 1.5 Mhz and (c) 22 Mhz.
What are the impedances if the unloaded Q's are respectively (a) 32, (b) 170, and (c) 110

Answers:(a) 442 ohms (b) 35K249 ohms and (c) 334K518 ohms

2. What is the effective r.f. resistance of the inductor above at the respective frequencies/Q.
(a) 100Khz/32, (b) 1.5 Mhz/170, and (c) 22 Mhz/110

Answers:(a) 0.432 or R432 ohms (b) 1R22 ohms and (c) 27R65 ohms

3. What is the correct method of describing the following resistances and capacitances.

(a) 0.68 ohms, (b) 10 ohms, (c) 220 ohms, (d) 1,800 ohms, (e) 82,000 ohms, (f) 470,000 ohms, (g) 5,600,000 ohms, and;

(h) 6.8 Pf, (i) 180 Pf, and (j) 4.7 uF

Answers:(a) R68 ohms (b) 10R ohms (c) 220R ohms (d) 1K8 ohms (e) 82K ohms (f) 470K ohms (g) 5M6 ohms (h) 6P8 (i) 180 Pf (j) 4U7