VISUAL AC MAINS VOLTAGE INDICATOR

You should not be surprised if someone tells you that the mains voltage fluctuation could be anywhere from 160 volts to 270 volts. Although majority of our electrical and electronics appliances have some kind of voltage stabilisation internally built-in, more than 90 per cent of the faults in these appliances occur due to these power fluctuations.

This simple test gadget gives visual indication of AC mains voltage from 160 volts to 270 volts in steps of 10 volts. There are twelve LEDs numbered LED1 to LED12 to indicate the voltage level. For input AC mains voltage of less than 160 volts, all the LEDs remain off. LED1 glows when the voltage reaches 160 volts, LED2 glows when the voltage reaches 170 volts and so on. The number of LEDs that glow keeps increasing with every additional 10 volts. When the input voltage reaches 270 volts, all the LEDs glow.

The circuit basically comprises three LM339 comparators (IC1, IC2 and IC3) and a 12V regulator (IC4). It is powered by regulated 12V DC. For power supply, mains 230V AC is stepped down to 15V AC by step-down transformer X1, rectified by a bridge rectifier comprising diodes D1 through D4, filtered by capacitor C4 and regulated by IC4. The input voltage of the regulator is also fed to the inverting inputs of gates N1 through N12 for controlling the level of the AC.

The LED-based display circuit is built around quad op-amp comparators IC1 through IC3. The inverting input of all the comparators is fed with the unregulated DC voltage, which is proportional to mains input, whereas the non-inverting inputs are derived from regulated output of IC4 through a series network of precision resistors to serve as reference DC voltages.

Resistors R13 to R25 are chosen such that the reference voltage at points 1 to 12 is 0.93V, 1.87V, 2.80V, 3.73V, 4.67V, 5.60V, 6.53V, 7.46V, 8.40V, 9.33V, 10.27V and 11.20V, respectively. When the input voltage varies from 160V AC to 270V AC, the DC voltage at the anode of ZD1 also varies accordingly. With input voltage varying from 160V to 270V, the output across filter capacitors C1 and C2 varies from 14.3V to 24.1V approximately. Zener ZD1 is used to drop fixed 12V and apply proportional voltages to all comparator stages (inverting pins). Whenever the voltage at the non-inverting input of the comparators goes high, the LED connected at the output glows.

Assemble the circuit on a general-purpose PCB such that all the LEDs make a bargraph. In the bargraph, mark LED1 for minimum level of 160V, then LED2 for 170V and so on. Finally, mark LED12 for maximum level of 270V.

Now your test gadget is ready to use. For measuring the AC voltage, simply plug the gadget into the mains AC measuring point, press switch S1 and observe the bargraph built around LEDs. Let’s assume that LED1 through LED6 glow. The measured voltage in this case is 220V. Similarly, if all the LEDs glow, it means that the voltage is more than 270V.

Touch Alarm

This is a new type of touch alarm that uses an RF oscillator at its input. One special feature of this touch alarm is that it can use a big-size touch plate. Also, no shielded wire is required between the touch plate and the circuit.

Circuit and working
Fig. 1 shows the circuit of the touch alarm, which is built around CD4060 (IC1), timer 555 (IC2) and a few discrete components. IC1 contains an easy-to-use astable or oscillator stage, followed by a 14-stage binary counter/divider. 

Fig. 1: Circuit of the touch alarm

Pulses from the oscillator stage are fed to the 14-bit binary counter stage. Not all the outputs of the binary counter are available externally. The frequency at pin 7 (Q3) of IC1 is actually divided by 24=16 compared to the initial astable frequency. Similarly, Q4 will have half the frequency of Q3. Reset pin (pin 12) is connected to ground to enable the pulses.
IC1 generates an astable frequency of around 4 MHz using low-cost inductor L1. The frequency available at pin 1 (Q11) of IC1 is around 1 kHz, which is actually astable frequency divided by 2¹². This frequency keeps transistor T2 conducting. Pin 4 of IC2 is held low as long as transistor T2 conducts.

When you touch the touch plate, the oscillator stops working because of loading. No output signals are available at pin 1 of IC1, due to which transistor T2 stops conducting. When transistor T2 is non-conducting, pin 4 of IC2 is high, activating the alarm. The frequency of sound can be changed by changing the value of timing components R6, R7 and C7.
Construction  and testing
An actual-size, single-side PCB for the touch alarm is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on a PCB, enclose it in a suitable case.

Fig. 2: An actual-size, single-side PCB for the touch alarm

Fig. 3: Component layout for the PCB

To test the circuit, check correct power supply at TP1 with respect to TP0. Do not touch the touch plate and verify 1kHz (approx.) frequency at TP2. This frequency will die if you touch the touch plate. At TP3 and TP4, verify voltage levels as indicated in the table.

Liquid-Level Alarm

In water-level controllers for tanks, a DC current is passed through the metallic probes fitted in the water tank to sense the water level. This causes electrolysis and corrosion of probes, inhibiting the conduction of current and degrading its performance. As a consequence, probes have to be replaced regularly to maintain proper current flow.

The liquid-level alarm given here overcomes this problem. A 1kHz AC signal is passed through the probes, so there will be no electrolysis and therefore the probes last longer.

Fig. 1: Block diagram of liquid level alarm

The block diagram for the liquid-level alarm is shown in Fig. 1. The signal generator sends the generated signal to the first metallic probe. The second metallic probe is connected to the sensing circuit followed by the alarm circuit.

The complete circuit for the liquid-level alarm is shown in Fig. 2. The astable multibrator built around IC 555 (IC1) generates 1kHz square wave signal, which is fed to one of the probes via a DC blocking capacitor.

Fig. 2: Liquid level alarm

Fig. 3: Pin configuration of UM66

When the water tank is empty, pnp transistor T1 does not get negative base bias. But as water fills up in the tank, it receives 1kHz signal from IC1 via the probes immersed in water and conducts during the negative half cycle of 1kHz signals. Due to the presence of capacitor C7 (2.2µF), npn transistor T2 continues to get base bias and conducts to provide 3.3V DC to melody generator IC UM66 (IC2). Pin configuration of IC UM66 is shown in Fig. 3. Preset VR1 acts as the output loudness controller. It can be varied to set the alarm sound from the speaker at the desired level.

The circuit works off 12V unregulated power and can be used to detect any conductive liquid.

Battery Charger with Automatic Switch-off

This smart charger automatically switches off when your rechargeable batteries reach the full charge.

The circuit comprises a bistable multivibrator wired around timer IC 555. The bistable output is fed to an ammeter (via diode D1) and potmeter VR1 before it goes to three Ni-Cd batteries that are to be charged.
Normally, the full charge potential of an Ni-Cd cell is 1.2V. Trigger the bistable by pressing switch S1 and adjust potmeter VR1 for 60mA current through the ammeter.

Now remove the ammeter and connect a jumper wire between its points ‘a’ and ‘b.’ Connect the positive output terminal of the batteries to the emitter of pnp transistor T1. The base of transistor T1 is held at 2.9V by adjusting potmeter VR2. The output of transistor T1 is inverted twice by npn transistors T2 and T3.

Thus when the batteries are fully charged to 3×1.2V=3.6V, a voltage higher than this makes transistor T1 to conduct. Transistor T2 also conducts and transistor T3 goes off. The threshold level of timer 555 reaches 6V, which is more than 2/3×VCC = 2/ 3×6=4V, to turn off the timer.

During charging, the threshold level of the timer is held low. The green LED (LED1) glows during charging of the batteries and goes off at the attainment of full charge.

Note that this circuit can be used only for 1.2V, 600mAH Ni-Cd rechargeable batteries that require 60 mA of current for 15 hours to charge fully.

FM Bug

FM Bug

This FM bug transmitter circuit will let you spy on people. The transmitter can be placed in the desired room and the conversation heard from a place far away just using a regular FM radio set.

The circuit is designed around a single transistor 2N3904 (T1), a custom-made coil (L1), three capacitors (C1 through C3), a trimmer (VC1), two resistors (R1 and R2) and, of course, a condenser microphone (MIC1). The circuit transmits in the frequency range of 88-105 MHz. Transmission range is 100 metres.

Working of the circuit is simple and based on analogue modulation in which a carrier signal is varied corresponding to the message signal.
The microphone picks up the sounds in its vicinity to produce corresponding electrical signal. This is the message signal that needs to be transmitted over FM band. The message signal is fed to the base of transistor T1. The tank circuit made using trimmer VC1 and coil L1 generates the carrier frequency. This frequency can be tuned using the trimmer. Transistor T1 modulates the audio signal from condenser microphone over the carrier signal produced by tank circuit. This modulated signal is transmitted through the antenna (ANT.).

Using trimmer VC1, tune the carrier frequency in FM band and confirm it with an oscilloscope. You will hear the conversation picked up by MIC1 when you tune frequency of the FM radio set to match frequency of the carrier.

Prepare the coil L1 using about 25cm length of 25SWG wire. Wrap the wire around a cylindrical object of 6mm diameter and take it out after eight turns.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. After proper soldering, fix switch S1 on the front side of the cabinet. Ensure that the oscillator is properly tuned. Addition of a dipole antenna will extend range of the FM bug transmitter. The circuit works off a 3V battery.

MEDIUM-POWER FM TRANSMITTER

The range of this FM transmitter is around 100 metres at 9V DC supply. 

Fig. 1: FM transmitter

The circuit comprises three stages. The first stage is a microphone preamplifier built around BC548 transistor. The next stage is a VHF oscillator wired around another BC548. (BC series transistors are generally used in low-frequency stages. But these also work fine in RF stages as oscillator.) The third stage is a class-A tuned amplifier that boosts signals from the oscillator. Use of the additional RF amplifier increases the range of the transmitter.

Coil L1 comprises four turns of 20SWG enamelled copper wire wound to 1.5cm length of a 4mm dia. air core. Coil L2 comprises six turns of 20SWG enamelled copper wire wound on a 4mm dia. air core.

Fig. 2: Pin configurations of transistors BC548 and C2570

Use a 75cm long wire as the antenna. For the maximum range, use a sensitive receiver. VC1 is a frequency-adjusting trimpot. VC2 should be adjusted for the maximum range. The transmitter unit is powered by a 9V PP3 battery. It can be combined with a readily available FM receiver kit to make a walkie-talkie set as shown in Fig. 3

Fig. 3: Walkie-talkie arrangement