117.
Clock Tick-Tock Sound Generator & LED Pendulum
Wooden-case, battery-operated wall clocks with pendulums are available in the market. Some
even have chimes. What is missing is the tick-tock sound of old mechanical pendulum clocks
[116].
118. Battery Charger with Automatic Switch-off
This smart charger automatically switches off when your rechargeable batteries reach the full
charge [117].
119. Earth Leakage Tester
Earth leakage from electrical wiring is a serious problem, especially during rainy season.
Pipelines are more vulnerable to earth leakage and may cause an unexpected electric shock.
Electrical appliances or faulty wiring may be the source of leakage current through the wet wall
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to the earth. An ordinary AC tester cannot detect the earth leakage if current is not high enough to
switch on a neon lamp [118].
120. Controllable Electronic Load Circuit for DC Power Supply
If you are interested in testing voltage-regulated power supplies under loaded conditions, here is a
simple electronic load circuit controllable from a single 2-12V, 200mA power supply. The
variable power supply is generated from regulator LM317 [119].
121. 16-Way Clap-Operated Switch
Control your home appliances without getting out of your bed. You just have to clap in the
vicinity of the microphone used in this circuit, which you can keep by the bedside. You can
switch on/off up to four different electrical equipment (TV, fan, light, etc) in 16 different ways
[120].
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122. Bedroom Light
This circuit allows you enough time to reach your bed and lie down before the bedroom lamp
switches off automatically. You can find a number of applications for this circuit. The circuit
draws almost no power when it is inactive [121].
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123. Inexpensive car Protection Unit
For car protection, custom-made units are available but they are costly. Here’s a circuit to protect
car stereo, etc from pilferage that costs less and requires no adjustments in the car but a good car
cover [122].
124. White LED-Based Emergency Lamp and Turning Indicator
White LEDs are replacing the conventional incandescent and fluorescent bulbs due to their high
power efficiency and low operating voltage. These can be utilised optimally for emergency lamp
and vehicle turning indication. The circuits for the purpose are given here [123].
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125. Mains-Operated Christmas Star
Here is a low-cost circuit of Christmas star that can be easily constructed even by a novice. The
main advantage of this circuit is that it doesn’t require any step-down transformer or ICs [125]
126. LED Lighting For Christmas
Using light effects for decoration on festive occasions is a normal practice. Designers are coming
up with varieties of electronic circuits to fill the imagination of users [126].
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127. Timer for Geyser
This timer circuit for geyser sounds an alarm after the set timing of 22 minutes when the water is
heated up [127].
128. Multicell Charger
Using this charger, you can safely charge up to two pieces of Ni-Cd cells or Ni-MH cells. The
circuit is compact, inexpensive and easy-to-use [128].
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129. Light Dimmer that Doubles as
Measure AC mains voltage without using a multimeter. All you need to do is to slightly modify
the light dimmer fitted at the base of a table lamp for use as a voltmeter. When the dimmer is
turned anticlockwise to a point where the filament glow is just visible, that point can be used as
the reference point for measuring the voltage [129].
130. 220V Live Wire Scanner
This simple circuit lets you scan a 220V live wire. The clock input of the IC is connected to a
wire, which acts as the sensor. Here, we have used 10cm length of 22SWG wire as the sensor
[130].
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131. Smart Switch
To switch on the mains voltage, either a mechanical switch or a relay offers a simple solution.
However, the relay and its associated components occupy a lot of space and cannot be
accommodated in a standard switch box. The smart switch circuit, shown here, offers a better
alternative. It is nothing but an on/off controller and uses an electronic circuit that behaves like a
normal switch. A flat pushbutton control provides an aesthetic look to your switch panel [131].
132. Power Failure and Resumption Alarm
This circuit gives audio-visual indication of the failure and resumption of mains power. The
circuit is built around dual timer IC LM556. When mains is present the bicolour LED glows in
green colour, and when mains fails it turns red [131].
133. Doorbell-Cum-Visitor Indicator
This doorbell circuit can also give identification of the visitor to your home in your absence.
When you’re home, you can use it simply as a normal doorbell [133].
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134. Zener Value Evaluator
Using this simple circuit and a known-value zener diode, you can find the breakdown voltage
value of any zener diode. The circuit is divided into two sections: zener evaluator and display
unit. Regulated 12V and 5V are required to power the zener evaluator section, while the display
section works off only 5V. Connect +5V, point A and ground of the zener evaluator section to the
respective terminals of the display section [134].
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135. 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 [135].
136. Electronic Fuse
An absolute necessity of every electronics lab is a workbench power supply. The power supply
should be regulated and protected against short circuit [136].
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137. Bicycle Guard
This antitheft device for bicycles is inexpensive and can be constructed easily using a few
components [137]. This antitheft device for bicycles is inexpensive and can be constructed easily
using a few components.
At the heart of the circuit is a wheel rotation detector, realised using a DC micro motor. For the
purpose, you can use the micromotor (spindle motor) of a discarded local CD deck mechanism.
With a little skill and patience, you can easily attach a small metallic pulley covered with a rubber
washer to the motor spindle. Thereafter, fix the unit in the back wheel of the cycle, like the
existing dynamo assembly.
Power supply switch S1 should be kept ‘on’ when you are using this bicycle guard. When it is
flipped towards ‘on’ position, the circuit gets power from the miniature 12V battery. Now LED1
lights up and resistor R4 limits the LED current. Next, the monostable built around IC1, which is
CMOS version of timer LM555, is powered through a low-current, fixed-voltage regulator IC2
(78L05).
Initially, when the bicycle is standing still, the monostable output at pin 3 of IC1 is low and the
circuit is in idle state. In the event of a theft attempt, forward or reverse rotation of the DC motor
induces a small voltage at its DC input terminals and the internal LED of 4-pin DIP AC input
isolator optocoupler IC3 (PS2505-1 or PC814) glows. As a result, the internal transistor of IC3
conducts and pin 2 of IC1 is pulled low by the optocoupler and the monostable built around IC1
is triggered.
The output at pin 3 of IC1 now drives piezobuzzer-driver transistor T1 via resistor R3 and the
buzzer starts sounding to alert you. In this circuit, the buzzer remains ‘on’ for around two
minutes. You can change this time by changing the values of resistor R2 and capacitor C1.
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