9V Converter Using Two AA Cells

The power supply is designed using a boost converter with fixed ‘on’ time and variable ‘off’ time. The variable ‘off’ time regulates power to the load. The converter consists of transistor T2, inductor L1 and capacitor C2. The conductance of transistor T1 controls ‘off’ time of the oscillator in conjunction with capacitor C2. IC TL431 (IC1) monitors the voltage across capacitor C4. When the voltage exceeds 2.5V at the reference pin (Ref) of IC1, the opto-coupler conducts more and reduces the conduction of transistor T1.

The frequency of oscillations mainly depends on the time constant (R-C) of feedback capacitor C3 and the input stage impedance (R1 plus VR1). Adjust preset VR1 to tweak the circuit for efficiency. The converter works with a single cell also. In that case, keep the output current-drain minimal.

The power supply is designed using a boost converter with fixed ‘on’ time and variable ‘off’ time. The variable ‘off’ time regulates power to the load. The converter consists of transistor T2, inductor L1 and capacitor C2. The conductance of transistor T1 controls ‘off’ time of the oscillator in conjunction with capacitor C2. IC TL431 (IC1) monitors the voltage across capacitor C4. When the voltage exceeds 2.5V at the reference pin (Ref) of IC1, the opto-coupler conducts more and reduces the conduction of transistor T1.

The frequency of oscillations mainly depends on the time constant (R-C) of feedback capacitor C3 and the input stage impedance (R1 plus VR1). Adjust preset VR1 to tweak the circuit for efficiency. The converter works with a single cell also. In that case, keep the output current-drain minimal.

Note:We have measured maximum output of 8.7V at 28mA current. Above this current, the output becomes zero.

Electronic Water Alarm

Aburst water-supply hose of the washing machine, a bathroom tap that you forgot to close, or a broken aquarium wall may turn your house into a pond. You can avoid this mess by using an electronic water alarm that warns you of the water leakage as soon as possible.

The acoustic water alarm circuit presented here takes advantage of the fact that the tap water is always slightly contaminated (or has salts and minerals) and thus conducts electricity to a certain extent. It is built around IC LMC555 (IC1), which is a CMOS version of the bipolar 555 timer chip. IC1 is followed by a complementary pair of emitter followers (T1 and T2) to drive a standard 8-ohm speaker (LS1). Power is supplied by a compact 9V PP3 battery.

Power is applied when power switch S1 is closed. The reset input (pin 4) of IC1 is held low by resistor R1 (2.2-kilo-ohm). The astable oscillator wired around IC1 is in disabled mode. When probes P1 and P2 become wet, these conduct to reverse the state of IC1’s reset terminal. As a result, the astable multivibrator starts oscillating at a frequency determined by resistor R2 and capacitor C3. The output of IC1 drives the complementary pair of transistors T1 and T2.

Although this combination causes significant crossover distortion, it doesn’t have any adverse effect on the square-wave audio signal processing. A 10-kilo-ohm potentiometer (VRI) is inserted between output pin 3 of IC1 and the bases of transistors T1 and T2 for volume control.

The probes can be made using two suitable copper needles or small pieces of circuit board with the copper surface coated with solder. Fit these at the lowest point where water will accumulate. After construction, place the alarm circuit well away from the point of possible leakage. Use a pair of thin twisted flexible wires to connect the probes to the circuit.

Capacitor C1 connected across IC1 input (pin 4 and GND) keeps the alarm circuit from responding to stray electrostatic fields. Similarly, twisting the wires together makes the relatively long connection between the probes and the circuit less sensitive to false alarms due to external electromagnetic interference. Finally, if you want to lower the probe sensitivity, reduce the value of grounding resistor R1.

Mobile Electronic Workbench

Typically, implementing and testing even a small circuit requires an elaborate set-up that includes breadboards, a dual DC power supply, hookup wires, ICs and resistors of different values. This setup can be quite messy and difficult to clean up at the end of the experiment. Also, the power supply can make the set-up non-portable. Here we present a mobile electronic workbench that makes it easier for you to assemble and test circuits.

This mobile workbench is useful for students in schools, colleges, research institutions and industries alike. It can be used conveniently wherever you want. It is also cost-effective and very useful for giving demo';s. As the power is supplied by the batteries, the voltage is noise-free. 

Fig. 1 shows the circuit of the mobile electronic workbench. Two low-drop-out (LDO) regulators (one positive and the other negative) are used here to provide regulated +5V and -5V for digital ICs. 

Fig. 1: Circuit for mobile electronic workbench

When switch S1 is pushed to 'on' position, LEDs indicate the availability of voltages on the breadboard. When it is in 'off' position, the battery terminals connect to the sockets for charging the batteries. Apart from +6V and +5V supplies, you can also have a 12V source between +6V and -6V terminals. 

As shown in Fig. 2, the mobile workbench consists of a big melamine tray. At the centre of this tray, mount the breadboard. On the sides of the breadboard, stick two 6V, 4.5Ah maintenance-free lead-acid batteries (Batt.1 and Batt.2). On a wooden batten, mount two-pole, two-way toggle switch S1 and two fuses and two sockets symmetrically. Mount LED1 and LED2 on the sides of S1.

Fig. 2: Photograph of electronic workbench 

If you do not want this mobile workbench on a breadboard, you can assemble it on a general-purpose PCB and enclose in a suitable cabinet. Fix LEDs and switch S1 on the front panel of the cabinet and the fuses at the back side of the box. 

In place of LM2990-5, you can use a 5.1V, 2W zener diode with 100-ohm, 2W series limiting resistor.

Bipolar Transistor Tester

This tester is primarily meant to test bipolar transistors. It can indicate the type of the transistor as well as identify its base, collector and emitter pins. The circuit is very simple. The direction of current flow from the terminals of the transistor under test (TUT) is indicated by a pair of LEDs (green-red). An npn transistor produces a red-green-red glow, while a pnp transistor produces a green-red-green glow, depending on the test point that connects to the terminal of the transistor. Emitter and collector are differentiated by pressing push button switch S1 that actually increases the supply voltage of the circuit by about 5.1V. 

At the heart of the circuit is IC CD4069 (IC3), which oscillates and produces pulses required to test a pair of transistor leads for conduction in both the directions. Different combinations are selected by an arrangement of counter CD4040 (IC1) and bilateral switch CD4016 (IC2).

Fig. 1 shows the circuit of the bipolar transistor tester. A pair of LEDs is connected to each test point through which current flows in both the directions. Each LED corresponds to a particular direction. In this manner, both junctions of the transistor can be tested. The LEDs are arranged to indicate the type of the semiconductor across the p-n junction. The counter is clocked by the AC generator built around gates N5 and N6. This makes the LEDs glow continuously for easy observation, revealing the direction of current flow between different test points. So if the red LED connected to certain point glows, it means that n-type of the junction is connected to that test point, and vice versa. Thus a red-green-red glow indicates npn type of the transistor, while a green-red-green glow indicates a pnp transistor. From this observation, you can easily detect the base. 

Fig. 1: Circuit of bipolar transistor tester

Collector and emitter are differentiated based on the principle that the base-emitter junction breaks down under reverse bias much more easily than the base-collector junction. Thus under increased AC voltage, you can easily see that the emitter conducts more in the reverse direction (associated LED glows significantly) than the collector. Use of transparent or semi-transparent LEDs is recommended.

Adjust preset VR1 (2-mega-ohm) to get equal glow when any two test points are shorted. Unregulated 15V-18V is regulated by the zener-transistor combination to power the circuit.

The testing procedure is simple. Normally, the transistors can be plugged in any orientation as they come in a variety of possible arrangements of base, collector and emitter pins, such as CEB, BEC and CBE. Simply plug the TUT in the possible combinations of three points. A red-green-red glow means that it is npn transistor and the pin associated with green LED is base. To identify the emitter and collector, simply press switch S1 and observe green LEDs adjacent to already glowing red LEDs. The green LED glowing with a high intensity indicates the emitter side, while the low-intensity LED indicates the collector side.

Similarly, a green-red-green glow means that the transistor is pnp type and the pin associated with the red LED is the base. To identify the emitter and collector, simply press switch S1 and observe red LEDs associated with the already glowing green LEDs on the sides. The LED glowing with a high intensity indicates the emitter side, while the low-intensity LED indicates the collector side.

Assemble the circuit on a general-purpose PCB and enclose in a small box. Keep the preset knob in the middle. In order to make it easy to plug the TUT, you can increase the number of test points

Electronic Metronome

Metronome is used by musicians for practice in maintaining a consistent tempo, or rubato, around a fixed beat. This circuit produces a regular beat at the rate of 40 to 200 beats per minute. It accentuates every second, third, fourth, fifth, sixth or eighth beat, which is adjustable as per your liking and requirement. Every beat is indicated by the glowing of an LED. The accented beat is indicated by another LED.

The beat is derived from an astable multivibrator (IC1) running between 0.67 Hz (40 beats per minute) and 3.47 Hz (208 beats per minute), and a pulse generator built around NOR gates N1 and N3, resistor R3 and capacitor C2. The beat covers all the musical tempi from adagio to presto. The results are a very short burst of sound, reminiscent of the 'tick'of a mechanical metronome. If you prefer a beep rather than a tick sound, the pulses should be lengthened by reducing the value of R3 to, say, 5.6 or 6.8 kilo-ohms.

IC1 drives the pulse generator. The length of the pulse is about 10 ms, and it appears at pin 1 of IC3 (NOR gate N3). At each pulse, the red LED (LED1) flashes to indicate occurrence of the beat. The pulse passes through NAND gates N6 and N7 of IC4. The pulse output from pin 6 of N7 is fed to NAND gate N8. The audio signal output generated by another multivibrator (IC6) is also fed to gate N8. The audio signal can be adjusted to obtain a note of suitable pitch. 

The output from IC1 also goes to IC2 (CD4022), which is a divide-by-eight counter/divider with eight decoded outputs. Rotary switch S1 allows the counter to be reset every two, three, four, five or six counts, or cycle through eight counts without resetting. 

Output Q0 of IC2 drives the second pulse generator built around NOR gates N2 and N4, resistor R4 and capacitor C3. The output is an accented beat pulse, which is fed to NAND gates N5 and N9 and the base of transistor T2. Since C3 has a higher capacitance than C2, this pulse is longer (about 40ms) and is used to mark the accented beat. The result is a 'tick'sound lasting about 40 ms, which sounds every second, third, fourth, fifth, sixth or eighth beat, depending on the setting of S1. The accent pulse makes the yellow LED (LED2) flash.It is important that the base 'tick' note or beat is not heard on the accented beat. This is achieved by gates N5 through N7 of IC4. 

The final audio signal appears at pin 3 of IC5 (NAND gate N10). This The beat is derived from an astable multivibrator (IC1) running between 0.67 Hz (40 beats per minute) and 3.47 Hz (208 beats per minute), and a pulse generator built around NOR gates N1 and N3, resistor R3 and capacitor C2. The beat covers all the musical tempi from adagio to presto. The results are a very short burst of sound, reminiscent of the 'tick' of a mechanical metronome. If you prefer a beep rather than a tick sound, the pulses should be lengthened by reducing the value of R3 to, say, 5.6 or 6.8 kilo-ohms.

IC1 drives the pulse generator. The length of the pulse is about 10 ms, and it appears at pin 1 of IC3 (NOR gate N3). At each pulse, the red LED (LED1) flashes to indicate occurrence of the beat. The pulse passes through NAND gates N6 and N7 of IC4. The pulse output from pin 6 of N7 is fed to NAND gate N8. The audio signal output generated by another multivibrator (IC6) is also fed to gate N8. The audio signal can be adjusted to obtain a note of suitable pitch. 

The output from IC1 also goes to IC2 (CD4022),which is a divide-by-eight counter/divider with eight decoded outputs. Rotary switch S1 allows the counter to be reset every two, three, four, five or six counts, or cycle through eight counts without resetting. 

Output Q0 of IC2 drives the second pulse generator built around NOR gates N2 and N4, resistor R4 and capacitor C3. The output is an accented beat pulse, which is fed to NAND gates N5 and N9 and the base of transistor T2. Since C3 has a higher capacitance than C2, this pulse is longer (about 40ms) and is used to mark the accented beat. The result is a 'tick'sound lasting about 40 ms, which sounds every second, third, fourth, fifth, sixth or eighth beat, depending on the setting of S1. The accent pulse makes the yellow LED (LED2) flash. It is important that the base 'tick' note or beat is not heard on the accented beat. This is achieved by gates N5 through N7 of IC4. 

The final audio signal appears at pin 3 of IC5 (NAND gate N10). This signal can be fed to the audio power amplifier stage. When you supply 6V DC to the circuit, you can hear the base or tempo beats and accented beats from the speaker of your power amplifier. The red LED (LED1) flashes with the beat and the yellow LED (LED2) flashes on the accented beat.

Construction and testing is simple. Assemble the circuit on a breadboard or general-purpose PCB. Mount all the components, except S1, and temporarily connect pin 15 of IC2 to ground rail. IC1 produces an audible tick sound (tempo beat) at a fixed rate that varies as VR1 is adjusted. IC6 produces a tone that varies in pitch from about 250 Hz (about an octave below middle C) to about 2 kHz (about two octaves above middle C) as VR2 is adjusted. The counter goes through its normal eight-stage cycle and the yellow LED (LED2) flashes once for every eight flashes of the red LED (LED1). 

Now connect a loudspeaker to pin 3 of NAND gate N10 through a 10 F capacitor. The circuit should produce a series of tick sound with a double-tick sound at every eighth tick sound. If this works well, remove pin 15 of IC2 from the ground rail and connect to six-way rotary switch S1. Remove the speaker and 10µF capacitor from pin 3 of N10 and connect pin 3 to an audio power amplifier. Use presets VR1 and VR2 such that turning their knobs clockwise increases the tempo and the pitch, respectively.

LED Flasher for Festivals

The circuit for a portable electric lamp-cum-LED flasher. It uses a 25W, 230V AC bulb and nine LEDs. When the bulb glows all the LEDs remain 'off, 'and when the LEDs glow the bulb remains 'off.'

The circuit is built around timer IC 555 (IC1), which is wired as an astable multi vibrator generating square wave. The output of IC1 drives transistor T1. 

Working of the circuit is simple. When output pin 3 of IC1 goes high, transistor T1 conducts to fire triac1 and the bulb glows. Bulb L1 turns off when output pin 3 of IC1 goes low.

The collector of transistor T1 is connected to anodes of all the LEDs (LED1 through LED9). So when T1 is cut-off the LEDs glow, and when T1 conducts the LEDs go off. Current-limiting resistor R4 protects the LEDs from higher currents.

In brief, the bulb and the LEDs flash alternately depending on the frequency of IC1. Flashing rates of the bulb as well as LEDs can be varied by adjusting potmeter VR1. Connect the power supply line (L) of mains to bulb L1 via switch S1 and neutral (N) to MT1 terminal of triac1. 

A 12V, 200mA AC adaptor is used to power the circuit. Using switch S1,you can switch off the bulb permanently if you do not want it to flash.

Assemble the circuit on a general-purpose PCB and enclose in a circular plastic cabinet keeping the bulb at the centre and LEDs at the circumference. Drill holes for mounting the 'on' /'off' Switch .Use a bulb holder for bulb L1 and LED holders for the LEDs. Also use an IC socket for timer IC 555.

Warning. While assembling, testing or repairing, take care to avoid the lethal electric shock.

Guitar Effect Pedal Power

A small box is fitted to the rear of the amplifier providing a 9V output for the effect pedal. The amplifier section gets 9V through a pedal switch. This power output and guitar signal input lines are combined into a single unit with multi-way cable connecting points as shown in the following figure.

The circuit can be divided into two sections: power supply and signal handling. The power supply section is built around transformer X1, regulators 7805 and 7905, bridge rectifier comprising diodes D1 through D4, and a few discrete components. The signal-handling circuit is built around two OP27 op-amps (IC3 and IC4).

The power supply of about 9V for the effect pedals is derived from step-down transformer X1. MOV1 is a metal-oxide varistor that absorbs any large spike in mains power. 

IC 7905 (IC1) is a -5V low-power regulator. By using a 3.9V zener diode (ZD1) at its ground terminal, you get -8.9V output. The same technique is also applied to IC 7805 (IC2)-a +5V regulator to get 8.9V. Use good-quality components and heat-sinks for the regulators. This supply is more than enough for the five effect pedals.

The greater the voltage drop across the regulator, the lower the output current potential. Resistors R1 and R2 provide a constant load to ensure that the regulators keep regulating. Capacitors C3 through C8 ensure that the supplies are as clean as possible. It is very important to use proper heat-sinks for IC1 and IC2. Otherwise, these could heat up.

Working of the circuit is simple. The input signal stage uses a basic differentiation amplifier to accept the incoming signal and a voltage follower to buffer the output to the power amplifier. The differential amplifier is built around IC3. It works by effectively looking at the signals presented to its inputs. If the input signals are of different amplitudes, IC3 amplifies the difference by a factor determined by R4/R3 (where R4=R6 and R3=R5). If the input signals have same amplitudes, these are attenuated by the common-mode rejection ratio (CMRR) of the circuit. The value of CMRR is determined by the choice of the op-amp the auxiliary components used and circuit topology. You can use standard resistors. With the values shown, you get an overall gain of unity. 

The combination of resistor R7 and C13 serves as a passive low-pass filter, progressively attenuating unwanted high-frequency signals. The second op-amp (IC4) forms a simple voltage follower (its output follows its input), providing a low output impedance to drive into the standard power amplifier. 

Assemble the circuit on a general-purpose PCB and fit it to the rear of an amplifier. The unit must be compact, yet robust. So use a very sturdy aluminium extrusion for the cabinet in order to neatly house the assembled PCB. 

To ensure simple operation, there are only three connections to the unit. First, mains power is tapped from the transformer. The second lead carries the 9V output to the amplifier. The third is the guitar signal input at the five-way socket for connection to the effect pedal.

SAFE CIGARETTE LIGHTER POWER SOCKET

Cigarette lighter sockets available on the dashboards of some vehicles act as a heat source to light cigarettes. Besides, these can power electronic gadgets like tablets, laptops, portable video players, MP3 players and cellphones. Some of these devices can be plugged directly into the cigarette lighter socket, while others may need an inverter (DC-AC converter). 

However, using the cigarette lighter socket to power electronic gadgets while the car engine is not running may drain the car’s battery. This circuit lets you use the cigarette lighter socket to power your electronic gadgets without the fear of draining the car’s battery.

The circuit supplies power to electronic devices with low-voltage protection mechanism. A moulded cigarette lighter socket extension cable (refer Fig.3) is necessary for the circuit. Cut the extension cable and solder its leads to the input of the circuit (J1). The circuit receives 12V DC at J1 through polarity protection diode D1.

As shown in Fig.1, the heart of the circuit is IC TL431 (IC1)—a voltage regulator wired as a comparator. When the voltage at the reference terminal (Ref) of IC1 exceeds 2.5 V , its cathode (K) goes low, which provides base emitter biasing of transistor T1. Transistor T1 provides regenerative feedback via the combination of resistor R6 and diode D2 to turn on transistor T2, which is an n-channel power MOSFET. As a result, the DC supply from input pin J1 is routed to the output load connected at socket J2.

 

Fig.1: Safe cigarette lighter socket

However, if the battery voltage drops below 10V, the reference terminal voltage of IC1 falls below 2.5V and its cathode goes high. Transistor T1 unbiases to turn off transistor T2 and the output load disconnects to prevent deep discharge of the storage battery. The red LED (LED1) is used as a simple output-power status indicator. SPST toggle switch S1 is the power-on/ off-cum-reset switch. For testing the circuit, apply 12V DC to input point J1 and adjust trimmer VR1 (22-kilo-ohm) such that LED1 lights up and the output voltage is available at J2. Recheck the calibration to ensure that the output supply is disabled when the input voltage falls to a value of about 10V DC.

 

Fig.2: Pin configurations of  IRF540, SK100 and TL431

IC1 is a three-terminal adjustable shunt regulator. Its output voltage may be set at any level greater than 2.5V (VRef) and up to 3.6V by selecting resistors R1 and R2 that act as a voltage-divider network. T2 is a general-purpose n-channel power MOSFET. Any MOSFET with similar characteristics can also be used in place of T2. Use a heat sink for T2 to allow heat dissipation.

After construction, enclose the circuit in a small metallic or plastic box with holes to mount the power switch (S1) and indicator (LED1). Output jack J2 should match with the appliance to be used.

ULTRASONIC TRANSMITTER AND RECEIVER

Most ultrasonic transmitters and receivers are built around timer IC 555 or complementary metal-oxide semiconductor (CMOS) devices. These devices are preset-controlled variable oscillators. The preset value of the working frequency is likely to drift due to mechanical vibrations or variations in temperature. This drift in frequency affects the range of transmission from the ultrasonic transducer.

The ultrasonic transmitter and receiver circuits described here use CD4017 decade counter ICs.

The transmitter circuit (Fig.1) is built around two CD4017 decade counter ICs (IC1 and IC2), D-type flip-flop IC CD4013 (IC3) and a few discrete components. The arrangement generates stable 40kHz signals, which are transmitted by transducer TX.

The crystal-controlled radio-frequency (RF) oscillator built around transistor T 1 (BC549) generates an 8MHz signal, which serves as input to the first decade counter built around IC1. The decade counter divides the oscillator frequency to 800 kHz. The output of IC1 is fed to the second CD4017 decade counter (IC2), which further divides the frequency to 80 kHz.

The flip-flop (IC3) divides 80kHz signal by 2 to give 40kHz signal, which is transmitted by ultrasonic transducer TX.

Coil L is made with 36SWG enamelled copper wire that is wound 15 times around an 8mm-diameter plastic former as used for radio oscillators, which has a ferrite bead.

The transmitter circuit works off 9-12V DC.

The receiver circuit (Fig.2) is built around a single decade counter CD4017 (IC4) and a few discrete components. To check the working of the transmitter, it is necessary to down-convert the 40kHz signal into 4kHz to bring it in the audible range. By using the receiver, the 40kHz ultrasonic transmitter can be tested quickly. The receiver’s transducer unit (RX) is kept near the ultrasonic transmitter under test. It detects the transmitted 40kHz signal, which is amplified by the amplifier built around transistor BC549 (T2). The amplified signal is fed to decade counter IC4, which divides the frequency to 4 kHz. Transistor T3 (SL100) amplifies the 4kHz signal to drive the speaker.

Use a 9V PP3 battery to power the receiver circuit.

House the transmitter and receiver circuits in separate small cabinets. If the 40kHz transducer under test is working, the receiver circuit produces audible whistling sound.

DUAL- COLOUR STROBOSCOPE

Stroboscope is a device used to make a cyclically moving object appear slow-moving or stationary. This is realised by illuminating the object intermittently with short pulses of light. Stroboscope is used in the study of insect flight. It can also be used for experiments with simple pendulum, studying details of rapidly moving objects and strobo-animation.

Here is the circuit of a stroboscope that produces dual-coloured light pulses (refer Fig.1). The circuit uses red and green LEDs as light sources to illuminate the object. You can choose the frequency of the stroboscope’s light pulses from a wide range of 5 Hz to 5 kHz as desired. The range of frequency (5-50 Hz, 50-500 Hz, 500 Hz-5 kHz, etc) is selected through capacitors Ca (C2, C3 and C4), Cb (C5, C6 and C7) and Cc (C8, C9 and C10). 

Smooth variation of the frequency range is achieved by varying VR1. The length of the light pulses (both red and green pulses are of equal duration) is adjusted by VR4. A flash of red and green light makes one cycle of the stroboscope. The frequency meter reads this frequency.

The circuit comprises a free-running oscillator formed by IC NE555 (IC1), IC CD4069 (IC2), dual-timer IC NE556 (IC3) and a few discrete components. The pulse output at pin 3 of NE555 triggers one-shot monostable IC3(A), which outputs a positive-going pulse that appears at the anodes of the LEDs (LED1 through LED4) through transistor T1. 

However, gates G3 and G4 cause only one of the transistors T2 and T3 to turn on. So the positive-going pulse from monostable IC3 (A) causes either the red LEDs (LED1 and LED2) or the green LEDs (LED3 and LED4) to turn on, depending on the phase of the oscillator.

The astable multivibrator configured around IC1 is made to produce a symmetric square waveform by using the combination of low-value fixed resistor R1 (2.2-kilo-ohm) and high-value potmeter VR1 (2-mega-ohm).

One-shot monostable IC3 (B) configured as a frequency meter triggers along with IC3(A). Strobe-phase delay or advance facility introduces a shift in the stationary orientation either clockwise or anti-clockwise when S2 is pressed momentarily. The 100µA ammeter is calibrated to read the frequency from 5 Hz to 5 kHz.

Presets VR5 and VR6 are used to set the maximum brightness of the LEDs. Potmeter VR7 is used to calibrate the frequency scale on the ammeter. The supply voltage should be regulated (7.0V) and there should be no voltage drop across the circuit when the LEDs glow. Any voltage drop affects the meter readings.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Fix the LEDs on the top or front side of the cabinet such that the LEDs’ light falls on the moving object.

For testing, construct a paper disk with two white patches as shown in Fig. 2. Fit the disk onto a direct-current motor spindle.

Fig.2 (L) A paper disk with a white patch, fitted onto a DC motor spindle, and (R) stationary patches of red and green color seen when the frequency of the stroboscope equals that of rotation of the disk.

Using switches S1 and S2, change the position of the patches in advance and delay motion, respectively. Pressing S1 slightly increases the frequency and pressing S2 slightly reduces the frequency. The change in frequency will make the patches to move in either clockwise or anti-clockwise direction.

Switch S3 provides a direct way to toggle between continuous and pulsed variable time periods. When switch S3 is thrown to continuous mode, the lights (LED1 through LED4) never go off—green and red LEDs glow alternately. For 50 per cent duty cycle of NE555, both red and green light go on and off alternately for the same duration. Use bright red and green LEDs for a good stroboscope output.

To measure the revolutions per minute (RPM) of the disk, tune the frequency using VR1 until stationary red and green patches are seen on the rotating disk. Next, note the reading of the ammeter to get revolutions per second (RPS) of the disk or motor. Multiply the result by 60 to get the RPM.

The stroboscope can be used to measure the RPM of a ceiling fan or table fan. For good visibility, make sure that the fan is clean and the light in the room switched off. Focus the red and green LED lights on the wings of the rotating fan. Vary VR1 until stationary red and green lights are seen on the wings of the rotating fan. Read the ammeter and divide the reading by 3 (for three wings of the fan) to get the rotation speed in RPS.

LAPTOP AUDIO AMPLIFIER

Usually, the audio output from a laptop’s built-in speakers is low. A power amplifier is required to get a high volume. Here is a simple circuit to amplify the laptop’s audio output.

The circuit is built around power amplifier IC LA 4440 (IC1) and a few other components. LA4440 is a dual channel audio power amplifier. It has low distortion over a wide range of low to high frequencies with good channel separation. Inbuilt dual channels enable it for stereo and bridge amplifier applications.

In dual mode LA4440 gives 6 watts per channel and in bridge mode 19- watt output. It has ripple rejection of 46 dB. The audio effect can be realised by using two 6-watt speakers. Connect pins 2, 6 and ground of IC1 to the stereo jack which is to be used with the laptop.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. The circuit works off regulated 12V power supply. It is recommended to use audio input socket in the circuit board. Use a proper heat-sink for LA4440.

SIMPLE SOLDERING IRON TEMPERATURE REGULATOR

Soldering irons are available in different wattage and usually run at 230V AC mains. However, these have no temperature control. Low-voltage soldering irons (e.g., 12V) generally form part of a soldering station and are designed to be used with a temperature controller. A proper temperature-controlled soldering iron or station is expensive. Here is a simple circuit that provides manual control of the temperature of an ordinary 12V AC soldering iron. 

The circuit consists of power switch S1, TRIAC1, DIAC1, potentiometer VR1, resistors R1 to R3, capacitors C1 and C2, and step-down transformer X1. Adjusting the resistance of VR1 changes the charging rate of C1 to regulate the conduction angle of TRIAC1, and hence the output power (heat) of the low-voltage soldering iron connected to X1. The red LED (LED1) indicates the power status.

Assemble the circuit on a general-purpose PCB and enclose in a suitable plastic box. Since the front end of the circuit is directly connected to 230V AC mains supply, never attempt to operate the circuit without the cabinet. Use a heavy-duty potentiometer with plastic shaft and a knob for temperature control.

PORTABLE SIGNAL WAND

Flashing lights have varied applications. For example, travellers can use these as warning beacons on highways to catch the attention of the public in case of an emergency. Described here is the circuit of a battery-operated, LED-based portable signal wand.

The signal wand is in fact a three channel, sequential dual-colour LED flasher built around two popular and inexpensive chips—CD4093 and CD4017. CD4093 (IC1) consists of four in built Schmitt-trigger circuits. Each circuit functions as a two-input NAND gate with Schmitt-trigger action on both the inputs. Here one gate (N1) of IC1 is wired as a gated astable multivibrator with its control pin 1 permanently connected to the positive supply as shown in Fig. 1. The frequency of the astable multivibrator can be varied with the help of a 1-mega-ohm preset (VR1). CD4017 (IC2) is a five-stage divide-by-ten Johnson counter with ten decoded outputs and a carry bit.

Counter IC2 is cleared to zero count by a logic high on its reset pin. IC2 advances by one count on the positive edge of the clock signal when the clock-enable signal is in the logic-low state. Ten decoded outputs of IC2 (Q0 through Q9) are normally low and reach high state only at their respective time-slot. Each decoded output remains high for a full clock cycle.

The clock output from gated astable N1 is fed to clock-enable pin (pin 13) of IC2. The clock input (pin 14) of IC2 is permanently high. Three outputs of IC2 (Q0 through Q2) are connected to three independent LED strings. Each LED string consists of a red and a green LED in series with a common current limiting resistor R2. The fourth output (Q3) of IC2 is returned to its reset terminal (pin 15).

Assemble the circuit on a general-purpose PCB. After assembling and testing, enclose the circuit along with a 9V battery in a transparent tube as shown in Fig.2. Use of low-current, high brightness 5-millimetre LEDs is recommended.

PEAK LEVEL MONITOR

Measurement of peak level is useful while testing an amplifier or a similar device. It is handy while testing a logic circuit, as it can pick up a high transient pulse (or show that there is none) and measure an output that is swinging strongly in both directions.

The peak level monitor presented here is useful for testing amplifiers and digital circuits, or in fact anything that produces an output of about +7V to – 7V peak.

The heart of the circuit is op-amp IC TL072 (IC2(A)), which is wired as a unity-gain voltage follower with a slight difference—diode D2 on its output terminal. In a normal voltage follower, the output is directly fed back to the inverting input (–), as seen in the second voltage follower IC TL072 (IC2(B)) of this circuit. 

The output of IC2(A) stabilises when the voltage at its two inputs is equal. This is when the input voltage at pin 3 equals the voltage at the cathode of D2. There is, of course, the usual voltage drop of about 0.7V across D2, but the op-amp compensates this by increasing its output voltage by 0.7V. As a result, capacitor C3 is charged to a voltage that is equal to the input voltage.

If the input voltages rise to a higher level, the voltage at the cathode of D2 rises equally, and C3 is charged to the higher level. But the reverse does not happen if the input falls. If the voltage across C3 falls, the charge has nowhere to go. It cannot pass through S2 to the 0V line because S2 is open. Neither it can pass into op-amp IC2 (B) because it is a biFET op-amp with an input resistance of 10 to 20 ohms, nor it can pass through diode D2 as it is reverse biased.

Capacitor C3 remains charged even though the input voltage has fallen. Each time the input voltage increases above its previous maximum level, the charge across C3 increases. So at any instant, C3 is charged to the maximum or the peak voltage reached. The circuit’s high input resistance means that the monitor can be used to measure the peak voltage from a source with high output resistance.

Another reason for choosing IC TL072 is its high slew rate of 13V/μs. Such a high rate means that its output is able to swing rapidly to catch sharp voltage peaks. The input offset voltage is 3 mV, which is typical of a biFET op-amp. However, this is not large enough to be of concern in this application.

IC2(B) allows measuring the voltage across C3 without letting the charge escape. It is wired as a unity gain voltage follower. The amplifier is stable with both its input voltages equal to the voltage across C3. This means that the output of IC2(B) is equal to the voltage across C3, which, in turn, is equal to the peak input voltage.

Although IC2(B) has an exceedingly high input resistance, it has a low output resistance of the order of 75 ohms like all op-amps. So it provides sufficient current to drive the multimeter without any appreciable fall in the voltage reading.

Peak-level circuits often have a high-value resistor wired across C3. This allows the charge to leak away slowly, so the output voltage eventually falls to zero. This should take five time constants, where one time constant equals RC.

With a 1μF capacitor and 1-mega ohm resistor (not shown in the circuit), RC=1, so the charge leaks away in five seconds.

This is rather too short a time for most applications, especially if the meter is digital with a relatively slow refresh rate. A 10-mega-ohm resistor (not shown in the circuit) would be better. However, reverse leakage through D2 discharges the capacitor in a reasonable time. So the resistor has been omitted. Instead, there is push button switch S2 for discharging C3 instantly whenever a new peak reading is required.

The op-amp runs off ±9V supply generated by a pair of 9V PP3 batteries. Instead of the pair of batteries, a single battery is used to provide 9V supply and a voltage inverter circuit to provide -9V supply. This costs less than the second battery and avoids the problem of one battery running out before the other.

IC 7660 (IC1) is a switched-capacitor voltage converter that produces a negative output voltage equal to the inverse of its positive supply voltage. The amount of current that it is able to supply is limited, so the negative voltage does not match the positive supply. In this circuit, it is about -8V, which is adequate for peak input voltage of up to about 7V.

Assemble the circuit on a general-purpose PCB and enclose in a suitable case. Use crocodile clips as input connectors. These can be clipped to an appropriate point in the test circuit. The output leads are terminated in 4mm banana plugs that are to be plugged into the terminals of a multimeter in place of the casual test probe.

To test the circuit, connect a 10-kilo-ohm or 100-kilo-ohm potentiometer across the 9V supply. Connect the wiper of the potentiometer to the input of the circuit. By turning the knob of the potentiometer, you can deliver to the input a voltage varying from 0V to 9V. Start with the wiper at 0V end of the track so that the circuit receives no voltage.

Connect a meter to the output and set it to the 10V or 20V scale. If you have a second meter, you can connect it to the input to monitor the input voltage. Press S2 to reset the circuit and vary the potentiometer to increase the input voltage to, say, 2V. The output should read 2V. Now decrease the input voltage to 1V. The output voltage should still read nearly 2V, though it can be seen falling slowly. Press S2 to reset the input. The output will fall to 0V but instantly rise to 1V when the button is released. Repeat for a few other voltage levels ranging from 1V to 7V to confirm that everything is working correctly.

SERVO MOTOR TESTER

When using a servo motor in a project, if the servo motor does not respond as per the input, how to make sure that the fault is not in the servo motor but the circuit or logic? One way is to isolate the servo motor from the circuit and check its proper working by feeding it pulses of varying width and checking the angle that the servo motor turns to. For example, a 1.5ms pulse should make the motor turn to a 90-degree position (neutral position).

The circuit presented here generates pulses of varying widths. It is built around two NE555 timer ICs (IC1 and IC2) and a few discrete components. Timer IC1 is configured as an astable multivibrator with a time period of 20 ms. Every 20 ms, the astable provides a very sharp negative pulse to trigger IC2. Timer IC2 is configured as a monostable multivibrator that produces 1ms, 1.5ms and 2ms long pulses to rotate the servo motor (M1). 

Pin 4 of IC1 is pulled down by resistor R2. When switch S1 is pressed, the astable multivibrator triggers the monostable to produce a pulse as per the position of switch S2. Switch S2 can select resistors R4, R5 and R6 together, and R7 to produce monostable pulse output of 1 ms, 1.5 ms and 2 ms, respectively. Preset VR1 is used to set the time period of IC1 to 20 ms.

Using switch S2, select the monostable time period as 1 ms, 1.5 ms or 2 ms and press switch S1. The servo motor should rotate to extreme left, middle or extreme right, respectively.

Intercom Using LM386

Keep in touch with your family members from one room to another and also from outside areas such as the garage, using this intercom circuit for bidirectional communication. The advantage of this circuit is that there is no talk/listen switch as many intercoms have. The circuit is built around two low-power LM386 audio amplifiers. 

Fig. 1 shows the block diagram of the intercom system. Fig. 2 shows the circuit. It has two simple and identical channels—unit 1 and unit 2. As shown in Fig. 2, the gain of both the amplifiers (built around IC1 and IC2) is about 200, which is usually enough to work with the condenser microphones. The circuit also works with carbon microphones or other high-level and low-impedance microphones. 

As both units are identical, working of only the first unit is described here. When you speak in front of the microphone (MIC1), the low-level signal is amplified by the amplifier built around transistor T1. Transistor T1 (BC547) is low-noise and high-gain type. 

The value of resistor R11 is selected such that the voltage between the collector and emitter of T1 is approximately half the power supply voltage. Resistor R12 should have a minimal value. For example, if the emitter of T1 is connected to the ground, the value of R12 could be as low as 1 kilo-ohm (usually, the range is 680 ohms to 4.3 kilo-ohms). Use of capacitor C18 is optional. The pre amplifier may exhibit instability at very high frequencies. So minimal appropriate value is chosen for resistor R1 in order to make the input circuit less vulnerable to electromagnetic noises. 

Signal from the microphone is filtered by the combination of resistor R2 and capacitor C1. Capacitor C2 blocks the DC component but allows AC signal to pass. The volume is adjusted by potentiometer VR1. The potentiometer can be replaced with a preset of the same value because the volume need not be adjusted frequently. The signal is amplified by IC1 and fed to the loudspeaker (LSP2). 

The second unit works in the same way as the first unit. 

Assemble the circuits for units 1 and 2 on separate general-purpose PCBs and enclose in suitable cabinets. As shown in the block diagram, place microphone MIC1 and loudspeaker LSP1 in the first room, and microphone MIC2, loudspeaker LSP2 and the entire electronics block plus the power supply in the second room. Keep the interconnection cables as short as possible—preferably shorter than 10 metres. These should be kept away from the cables of the mains power supply and other sources of electromagnetic interference. Use a shielded cable for connecting the microphone to the circuit. 

It is important that there is no acoustical feedback between the microphone and the loudspeaker on both the sides. So the microphones and the loudspeakers should not face each other. As the gain of each channel is not very high, the probability of acoustical feedback is low. In case of acoustical feedback, lower the volume of the amplifier and change the positions of the microphones and loudspeakers. 

The preferred power supply for the circuit is 6V or above but the circuit also works with regulated 5V from IC 7805 (not shown in the diagram). If a higher dynamic range is needed, the power supply should be 9V or even 12V. For power supply, you can use an AC-DC wall adaptor, dry batteries or rechargeable batteries.

Versatile DC-DC Converter

Here is a versatile power coupler that connects a device to 5V-19V DC generated from AC mains by a power adaptor. Power adaptors come in different voltage outputs like 5V (for mobile phones), 12V (for external hard drives) and 19V (for laptops). Sometimes the power adaptor may have a voltage rating higher than the required voltage. With the converter circuit given here, the adaptor can be used to power any device at a lower voltage. 

For instance, by using a 19V laptop adaptor, you can power a TTL circuit at 5V. There can also be other instances when one needs a 3V or 6V supply. All these and many other intermediate voltages are easily possible with this versatile converter circuit when used together with any off-hand power adaptor.

Fig. 1 shows the circuit of the DC-DC converter. Smooth reduction in the voltage is achieved using the LM317 regulator IC. The complete unit can fit inside a piece of a glue stick tube. 

Adjusting variable resistor VR1 gives the desired output voltage. The output voltage is read using a 0-100µA ammeter, whose series resistance R* is chosen such that the maximum desired voltage could be covered. For instance, if full-scale deflection (FSD) current of the meter is 100 µA and you need an output voltage of up to 15V, then R* = 15/0.0001 = 150 kΩ. The desired value of R* is obtained by using 150-kilo-ohm preset VR2. 

Use of a variable resistor which also has an on/off switch like the one in old radios is recommended. It will cut off the coupler from the input power supply without having to accomodate an additional switch. Also, use a heat-sink with LM317 to handle the desired amount of power. 

Assemble the circuit on a small general-purpose PCB and enclose in a suitable case. Fit the entire PCB inside a glue stick tube as shown in Fig. 2. Affix the female and male connectors on the opposite ends and place the ammeter in between the stick tube. You can directly read the output voltage on the ammeter after due calibration.

Note. You can use a suitable VU meter instead of 0-100µA ammeter and calibrate accordingly.