Universal IR Remote Control

Here is a circuit which can switch on and switch off any appliance with the help of a common type of infra-red remote control (transmitter). 

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This circuit senses the IR pulse and then controls the appliance accordingly. The circuit functions such that a short pulse from the remote tran- smitter switches on the triac (and the load) while a longer pulse switches off the triac as also the load. The circuit is built around hex inverter IC CD 4049. When the infra-red pulse is received by the sensor, its output (Note: Here VR1 denotes the in-circuit resistance of preset VR1.) The output of multivibrator is fed to the base of current amplifier 2N3055 via resistor R2 (1kilo-ohm). The brakelight bulb is connected in series with the collector of 2N3055. The flashing rate of this bulb is adjusted by 100k preset (VR1). 

Transistor 2N3055 may get heated due to high current switching action; hence a small heatsink, similar to the type used in television power supply, is recommended. The category of 2-wheelers which do not have a battery, can use the bridge rectifier circuit shown here. Several designs of round, square and rectangular reflectors are available which may be used in conjunction with any suitable 12V bulb with proper rating (around 20 watts). However, if flashing of the brake- light affects intensity of headlight bulb, reduce the rating of brakelight bulb to 10 watts.

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Phone Amplifier

While talking to a distant sub- scriber on telephone, quite often we feel frustrated when the voice of the distant subscriber is so faint that it is barely intelligible. 

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To overcome the problem, circuit of an inexpensive amplifier is presented here. It can be assembled and tested easily. There is no extra power source needed to power up the circuit, as it draws power from the telephone line itself. The amplifier will provide fairly good volume for the telephone conversation to be properly heard in a living room. A volume control is included to adjust the volume as desired. The circuit is built around IC LM386. Diodes D6 and D7 are used to limit the input signal strength. Transformer X1 is a transistor radio’s output transformer used in reverse. As original secondary (output) winding is connected in series with the telephone lines, the speech signals passing through the lines cause change in the magnetic flux in the core of transformer and thereby induce signal voltage across the primary winding. This audio signal is used as input for IC LM386. Diodes D2 through D5 connected in bridge configuration constitute a polarity guard so that the amplifier is powered with correct polarity, irrespective of the line polarity, Zener diode D1 may have any breakdown voltage between 6 and 12 volts range. There is no need of a separate power switch as the circuit energises (via the normally open contacts of the cradle switch) when one lifts the handset. The circuit may be wired on a general-purpose PCB or by etching a PCB for this circuit. The circuit can be easily tested by connecting a 6 volts supply to line terminals 1 and 2. A hissing sound will be heard from the loudspeaker. Now connect 6V AC from a transformer to terminals 1 and 2 and observe hum in the loudspeaker. The volume of the hum can be changed through potentiometer VR1. Diodes D6 and D7 limit the input below ± 700 mV. The circuit is to be connected to the telephone lines in series with the telephone instrument, as shown in the figure.

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Water Tank Overflow Preventer

There is practically no house without an overhead tank (OHT). People who use electrically-operated water pumps for filling the OHT find it very inconvenient to switch off the pump when their overhead tank starts overflowing, specially when they are busy. So there is plenty of water wastage as well as wastage of power (consumed by the pump). However, there is a solution to get rid of this headache. 

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The circuit given here will switch off the pump and also generate a melodious tune when the overhead tank gets filled up to the maximum desired level. All you have to do is switch off the power supply to the circuit when you are relatively free. The heart of the circuit is the CMOS latch CD4001. Usually the latch can be operated in two modes, namely, set and reset mode, i.e. the latch output can be set to logic 1 or reset to logic 0 by applying appropriate active low level input signal to pins 1 and 13, respectively. Here, in the given circuit, the set point is pin 1 and the reset point is pin 13. The inverted output of the latches are obtained at pins 3 and 11, respectively. When the circuit is powered there is a voltage drop at pin 1 due to the resistor-capacitor R1-C1 combination. The values of resistor R1 and C1 are chosen in such a way that pin 1 is low for about two seconds which is sufficient to energise the relay through transistor T1 and thus the pump starts running. When sufficient water gets filled in the overhead tank, switch S1 in the sensing unit, in the overhead tank as shown in Fig. 2, sends an active low signal to pin 13 which resets latch gate N1 output to logic 0. This causes transistor T1 to stop conducting, thereby de-energising the relay and shutting down the pump. At the same time, the output at pin 11 of gate N2 will be logic 1. This results in conduction of transistor T2 and melodious buzzer sounds. The green LED also lights up while the red LED, which remains on as long as the relay remains energised, gets switched off when water reaches the specified level in the overhead tank. The circuit possesses the following advantages:

1.    Special sensing mechanism (easy to build) is used to sense the water level in the overhead tank.

2.    One can replace CD4011 with IC 7400. In that case, a 5.1V zener may be connected additionally between pin 14 of IC1 and the ground.

The circuit can be easily fabricated on a general-purpose PCB.

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Automatic Room power Controler

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. 

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 The circuit described here overcomes the above-mentioned problem. It has a small memory which enables it to  utomatically 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 istance 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 coming into the room and the other will count as -1,-2, -3 etc when persons are going 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 istriggered. 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 bicolour 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) and 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 to handle 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.

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Security Alarm for Audio Player

This small circuit, based on popular CMOS NAND chip CD4093, can be effectively used for protecting your expensive car audio system against theft.

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 When 12V DC from the car battery is applied to the gadget (as indicated by LED1) through switch S1, the circuit goes into standby mode. LED inside optocoupler IC1 is lit as its cathode terminal is grounded via the car audio (amplifier) body. As a result, the output at pin 3 of gate N1 goes low and disables the rest of the circuit.

 Whenever an attempt is made to remove the car audio from its mounting by cutting its connecting wires, the optocoupler immediately turns off, as its LED cathode terminal is hanging. As a result, the oscillator circuit built around gates N2 and N3 is enabled and it controls the ‘on’/‘off’ timings of the relay via transistor T2. (Relay contacts can be used to energise an emergency beeper, indicator, car horns, etc, as desired.) Different values of capacitor C2 give

different ‘on’/‘off’ timings for relay RL1 to be ‘on’/‘off’. With 100μF we get approximately 5 seconds as ‘on’ and 5 seconds as ‘off’ time.

 Gate N4, with its associated components, forms a self-testing circuit. Normally, both of its inputs are in ‘high’ state. However, when one switches off the ignition key, the supply to the car audio is also disconnected. Thus the output of gate N4 jumps to a ‘high’ state and it provides a differentiated short pulse to forward bias transistor T1 for a short duration. (The combination of capacitor C1 and resistor R5 acts as the differentiating circuit.) s a result, buzzer in the collector terminal of T1 beeps for a short duration to announce that the security circuit is intact. This ‘on’ period of buzzer can be varied by changing the values of capacitor C1 and/or resistor R5.

  After construction, fix the LED and buzzer in dashboard as per your requirement and hide switch S1 in a suitable location. Then connect lead A to the body of car stereo (not to the body of vehicle) and lead B to its positive lead terminal. Take power supply for the circuit from the car battery directly.

Caution. This design is meant for car audios with negative ground only.

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Signal Generator

This circuit is very useful while checking/operating counters, stepping relays, etc. It avoids the procedure of setting a switch for the required number of pulses. By pressing appropriate switches S1 to S9, one can get 1 to 9 negative- going clock pulses, respectively. 

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 Schmitt trigger NAND gate N1 of IC2, resistor R1, and capacitor C1 are wired to produce clock pulses. These pulses are taken out through NAND gate N3 that is controlled by decade counter CD4017 (IC1). Initially no switch from S1 to S9 is depressed and the LED is glowing. As pins 5 and 6 of NAND gate N2 are pulled up by resistor R3, its output pin 4 goes low. This disables NAND gate N3 to take its output pin 10 to high state, and no pulse is available.

 IC1 is a decade counter whose Q outputs normally remain low. When clock pulses are applied, its Q outputs go high successively, i.e. Q0 shifts to Q1, Q1 shifts to Q2, Q3 shifts to Q4, and so on. If any one of switches S1 through S9, say, S5 (for five pulses), is momentarily depressed, pins 5 and 6 of NAND gate N2 go low, making its output pin 4 high, which fully charges capacitor C2 via diode D. At the same time, this high output of N2 enables NAND gate N3 and clock pulses come out through pin 10. These are the required number of pulses used to check our device.

 The clock pulses are fed to clock-enable pin 13 of IC1, which starts counting. As soon as output pin 1 (Q5) of IC1 turns high, input pins 5 and 6 of NAND gate N2 will also become high via switch S5 because high-frequency clock allowed five pulses during momentary pressing. This high input of N2 provides low output at pin 4 to disable NAND gate N3 and finally no pulse will be available to advance counter IC1. Before the next usage, counter IC1 must be in the standby state, i.e. Q0 output must be in the high state. To do this, a time-delay pulse generator wired around NAND gate N4, resister R4, diode D, capacitor C2, and differentiator circuit comprising C3 and R5 is used.

 When output pin 4 of NAND gate N2 is low, it discharges capacitor C2 slowly through resistor R4. When the voltage across capacitor C2 goes below the lower trip point, output pin 11 of NAND gate N4 turns high and a high-going sharp pulse is produced at the junction of capacitor C3 and resistor R5. This sharp pulse resets counter IC1 and its Q0 output (pin 3) goes high. This is represented by the glowing of LED.

 Ensure the red LED is glowing before proceeding to get the next pulse. Press any of the switches momentarily and the LED will glow. If the switch is kept pressed, the counter counts continuously and you cannot get the exact number of pulses.

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Digital Dice

The digital dice presented here acts just like a normal dice. It has six faces (refer Fig. 2) like the normal dice and uses four different logic gate combinations to bring out the six faces of the dice. At the heart of the circuit is a 14- stage ripple-carry binary counter IC CD4060BC (IC1) with built-in oscillator. The logic section is designed around CMOS quad 2-input NOR gate IC CD4001BC (IC2) and quad 2-input NAND gate IC CD4011BC (IC3). The display section is formed by a group of seven LEDs. 

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The circuit is divided into three sections: counter, logic and display. The counter section is built around binary counter IC CD4060BC (IC1). The counter frequency (f) is decided by the in-built oscillator formed by resistor R1 and capacitor C1 as follows: f=1/2.2R1C1. Here, the frequency is fixed at around 2056 Hz. Only the first three outputs of the counter (designated as A, B and C, respectively) have been used in the circuit.

 The counter is designed to reset at the sixth count (110) as only six counts are required for operation. This is done with the help of diodes D1 and D2 and resistor R3, which are connected such that they generate an AND logic. From the table it can be noticed that at the sixth count, the counter outputs A and B hold logic 1 simultaneously for the first time,so by ANDing A and B outputs you can give logic 1 to the reset terminal of the counter at the sixth count, thereby resetting the counter.

 LED2 and LED5 always glow at the same count, as do LED1 and LED6, and LED3 and LED4. Using these three pairs of LEDs and LED7, four logical combinations have been made in the circuit. LED1 and LED6 glow at all counts, except ‘0’ and ‘1.’ Further, it can be noticed that they glow when ‘A’ or ‘B’ is high, hence a NOR gate whose output is A+B according to Boolean algebra will perform the job of operating these LEDs.

 LED3 and LED4 glow at all counts, except for the first three counts, i.e., they glow when either A, or B and C are high. This logic function can be obtained by using an OR gate and an AND gate, but since we are using only NAND and NOR gates in the circuit we make use of two NAND gates and a NOR gate (with A+BC output) to perform this function. LED2 and LED5 glow only at the first and fifth counts. In other words, they glow only when the complement of B and C outputs goes high. This function can be obtained by using two NAND gates such that their output corresponds to the Boolean expression BC or B+C according to De Morgan’s theorem.

  LED7 glows at even counts like 0, 2 and 4. In other words, it glows when the C output is low. This function can be achieved easily by inverting the C output twice using the remaining two NAND gates. The output will also be buffered by these two inverter gates.

 The display section comprises seven LEDs. LED1 and LED6 have common cathodes, as do LED2 and LED5, and LED3 and LED4. The anodes of all the LEDs are tied together to the positive terminal of the battery via resistors R4 through R10, respectively.

 When you place your finger on the touch pad, the oscillator starts oscillating. The counter will start counting at the rate of 2056 Hz and all the LEDs of the display section will appear to glow simultaneously due to the high counter frequency. This high-frequency counting will make the dice foolproof. When you remove your finger from the touch pad, the counter will stop counting and the display section will show any one of the six possible faces with a probability of 1/6. The entire circuit can be powered by a 9V battery as the inbuilt oscillator of the counter IC will not work properly below 7V. Use of CMOS ICs means less power consumption.

 The circuit can be constructed on a general-purpose PCB and housed inside a plastic case with the LEDs arraymounted on the top as shown in Fig. 2. The touch pad can be mounted beside the array.

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Digital Thermometer

This digital thermometer can measure temperatures up to 150°C with an accuracy of ±1°C. The temperature is read on a 1V full scale-deflection (FSD) moving-coil voltmeter or digital voltmeter. 

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 Operational amplifier IC 741 (IC3) provides a constant flow of current through the base-emitter junction of npn transistor BC108 (T1). The voltage across the base-emitter junction

of the transistor is proportional to its temperature. The transistor used this way makes a low-cost sensor. You can use silicon diode instead of transistor. The small variation in voltage across the base-emitter junction is amplified by second operational amplifier (IC4), before the temperature is displayed on the meter. Preset VR1 is used to set the zero-reading on the meter and preset VR2 is used to set the range of temperature measurement.

 Operational amplifiers IC3 and IC4 operate off regulated ±5V power supply, which is derived from 3-terminal positive voltage regulator IC 7805 (IC1) and negative low-dropout regulator IC 7660 (IC2). The entire circuit works off a 9V battery.

 Assemble the circuit on a general-purpose PCB and enclose in a small plastic box. Calibrate the thermometer using presets VR1 and VR2. After calibration, keep the box in the vicinity of the object whose temperature is to be measured.

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Laser Torch-Based Voice Transmitter and Receiver

Using this circuit you can communicate with your neighbours wirelessly. Instead of RF signals, light from a laser torch is used as the carrier in the circuit. The laser torch can transmit light up to a distance of about 500 metres. The phototransistor of the receiver must be accurately oriented towards the laser beam from the torch. If there is any obstruction in the path of the laser beam, no sound will be heard from the receiver.  

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 The transmitter circuit (Fig. 1) comprises condenser microphone transistor amplifier BC548 (T1) followed by an opamp stage built around μA741 (IC1). The gain of the op-amp can be controlled with the help of 1-mega-ohm potmeter VR1. The AF output from IC1 is coupled to the base of transistor BD139 (T2), which, in turn, modulates the laser beam.

 The transmitter uses 9V power supply. However, the 3-volt laser torch (after removal of its battery) can be directly connected to the circuit—with the body of the torch connected to the emitter of BD139 and the spring-loaded lead protruding from inside the torch to circuit ground.

 The receiver circuit (Fig. 2) uses an npn phototransistor as the light sensor that is followed by a two-stage transistor preamplifier and LM386-based audio power amplifier. The receiver does not need any complicated alignment. Just keep the phototransistor oriented towards the remote transmitter’s laser point and adjust the volume control for a clear sound.

  To avoid 50Hz hum noise in the speaker, keep the phototransistor away from AC light sources such as bulbs. The reflected sunlight, however, does not cause any problem. But the sensor should not directly face the sun.

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Invisible Broken Wire Detector

Portable loads such as video cameras, halogen flood lights, electrical irons, hand drillers,  drinders, and cutters are powered by connecting long 2- or 3-core cables to the mains plug. Due to prolonged usage, the power cord wires are subjected to mechanical strain and stress, which can lead to internal snapping of wires at any point. In such a case most people go for replacing the core/cable, as finding the exact location of a broken wire is difficult. In 3-core cables, it appears almost impossible to detect a broken wire and the point of break without physically disturbing all the three wires that are concealed in a PVC jacket. 

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 The circuit presented here can easily and quickly detect a broken/faulty wire and its breakage point in 1-core, 2-core, and 3-core cables without physically disturbing wires. It is built using hex inverter CMOS CD4069. Gates N3 and N4 are used as a pulse generator that oscillates at around 1000 Hz in audio range. The frequency is determined by timing components comprising resistors R3 and R4, and capacitor C1. Gates N1 and N2 are used to sense the presence of 230V AC field around the live wire and buffer weak AC voltage picked from the test probe. The voltage at output pin 10 of gate N2 can enable or inhibit the oscillator circuit. When the test probe is away from any high-voltage AC field, output pin 10 of gate N2 remains low. As a result, diode D3 conducts and inhibits the oscillator circuit from oscillating. Simultaneously, the output of gate N3 at pin 6 goes ‘low’ to cut off transistor T1. As a result, LED1 goes off.

  When the test probe is moved closer to 230V AC, 50Hz mains live wire, during every positive halfcycle, output pin 10 of gate N2 goes high.

 Thus during every positive half-cycle of the mains frequency, the oscillator circuit is allowed to oscillate at around 1 kHz, making red LED (LED1) to blink. (Due to the persistence of vision, the LED appears to be glowing continuously.) This type of blinking reduces consumption of the current from button cells used for power supply.

 A 3V DC supply is sufficient for powering the whole circuit. AG13 or LR44 type button cells, which are also used inside laser pointers or in LED-based continuity testers, can be used for the circuit. The circuit consumes 3 mA during the sensing of AC mains voltage.

 For audio-visual indication, one may use a small buzzer (usually built inside quartz alarm time pieces) in parallel with one small (3mm) LCD in place of LED1 and resistor R5. In such a case, the current consumption of the circuit will be around 7 mA. Alternatively, one may use two 1.5V R6- or AA-type batteries. Using this gadget, one can also quickly detect fused small filament bulbs in serial loops powered by 230V AC mains.

 The whole circuit can be accommodated in a small PVC pipe and used as a handy broken-wire detector. Before detecting broken faulty wires, take out any connected load and find out the faulty wire first by continuity method using any multimeter or continuity tester. Then connect 230V AC mains live wire at one end of the faulty wire, leaving the other end free. Connect neutral terminal of the mains AC to the remaining wires at one end. However, if any of the remaining wires is also found to be faulty, then both ends of these wires are connected to neutral. For single-wire testing, connecting neutral only to the live wire at one end is sufficient to detect the breakage point.

 In this circuit, a 5cm (2-inch) long, thick, single-strand wire is used as the test probe. To detect the breakage point, turn on switch S1 and slowly move the test probe closer to the faulty wire, beginning with the input point of the live wire and proceeding towards its other end. LED1 starts glowing during the presence of AC voltage in faulty wire. When the breakage point is reached, LED1 immediately extinguishes due to the non-availability of mains AC voltage. The point where LED1 is turned off is the exact broken-wire point.

 While testing a broken 3-core rounded cable wire, bend the probe’s edge in the form of ‘J’ to increase its sensitivity and move the bent edge of the test probe closer over the cable. During testing avoid any strong electric field close to the circuit to avoid false detection.

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