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May 2013

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C I R C U I T L A B

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Crystal-Based 50Hz Generator (New)

Here is a simple oscillator circuit that generates 50Hz frequency using a crystal. It produces alternating 50Hz pulses with 50 per cent duty cycle, which can be used in inverter circuits. It comprises a 14-stage counter and oscillator (CD4060), dual J-K flip-flop (CD4027), operational amplifier (LM324) and a few discrete components.

Computerised Static Circuit Breaker (New)

There are many circuits available to minimise interferences in electronic circuits. But these circuits may not do much on a device that is operating through a PC. The radio frequency interference (RFI)generated from switching on and off the device will generate disturbance on the screen of the computer monitor.

Pressure-Sensitive Alarm (New)

Here is a low-cost, pressure-sensitive burglar alarm. The alarm uses a home-made pressure sensor, which works as a variable capacitor using two copper-clad boards and a piece of sponge in between them.

Cordless Multidoor Alarm

Thwart the attempt of burglary by detecting intrusion with this alarm circuit. Each door is protected by a separate circuit built around an independent 555 timer IC in conjunction with reed switch magnet. All the three units are powered from a single power source. The buzzer can be plugged into the earth line of a socket in any room of the same building having proper earth line connection. There is no need of laying external wires up to the buzzer unit from different rooms.

Wire-Break Alarm With Delay

Here is a simple circuit of wire-break alarm that activates after a delay of 15 to 30 seconds. When the thin-wire loop running across the entrance door is broken, the alarm sounds after a delay of 15 to 30 seconds, the time period set through VR1. Thus the occupants get sufficient time to lock the room from the outside and catch the thief.

Sunset Lamp

LDR-based automatic lights flicker due to the change in light intensity at dawn and dusk. So compact fluorescent lamps (CFLs) are unsuitable in such circuits as flickering may damage the electronic circuits within these lamps. The circuit described here can solve the problem and switch on the lamp instantly when the light intensity decreases below a preset level.

Ultrasonic Sound Beam Burglar Alarm

This unique burglar alarm makes use of the invisible, inaudible ultrasonic sound beam to detect movements. Ultrasonic transducers operate at maximum efficiency when driven at 40kHz frequency. So an ultrasonic transmitter and receiver pair operating at 40 kHz is used to control the buzzer or the relay.

Electronic Heart

With this electronic heart glowing on and off, you are sure to steal the heart of that special someone. The circuit uses an NE555 timer wired in astable multivibrator mode. Its frequency of oscillations is determined by resistors R1 and R2 and capacitor C2. Here the frequency is approximately 0.2 Hz. Each period lasts for just a little over 4 seconds.

Heat Control Unit

This circuit will turn the heater ‘on’ when the temperature of water falls below the lower limit set by you and turn it ‘off’ when the temperature increases above the higher limit.

Panic Alarm

If you feel threatened or need emergency assistance, simply activate this alarm. It will catch the attention of others for immediate help. The alarm will sound for three minutes and then cease. Especially useful for women travelling alone, it is small enough to pocket or carry in a handbag.

Peak Hour Timer

Electrical appliances like refrigerators and air-conditioners consume heavy current if the line voltage drops during the peak hours between 6 pm and 9 pm. If there is no low-voltage cut-off in these appliances, it will cause wastage of current and heating of the appliances. Over-heating may, in turn, reduce the efficiency of the compressors of these appliances.

Automatic Parking Light For Cars

At night, parking lights make your parked car visible to motorists so they don’t smash into your car. However, these lights drain considerable power out of your car’s battery. Here is a simple, automatic parking light system that works with zero standby current. The circuit is designed to turn on the parking lights automatically for 30 seconds when an approaching vehicle’s light is detected from the rear or front side. This automatic feature provides safety at night for a parked vehicle.

LONG-RANGE IR TRANSMITTER

Most of the IR remotes work reliably within a range of 5 metres. The circuit complexity increases if you design the IR transmitter for reliable operation over a longer range, say, 10 metres. To double the range from 5 metres to 10 metres, you need to increase the transmitted power four times.

Electronic Combination Lock

This 7-digit combination lock can be easily hard-wired for any combination that you choose. The circuit uses a 4-bit, divide-by-8 Johnson counter (IC1), ten pushbutton switches and npn transistor T1.

Hot-Water-Ready Alarm

Electric kettles turn off automatically when water has boiled. What if the boiler beeps to alert you when your water has boiled? The tripping sound of the thermal switch may not register as an alarm in your mind. Here is such an add-on unit that gives intermittent beeps at the end of boiling. It has the advantages of extremely low component count, low cost, small size and light weight.

Continuity Tester With A Chirping Sound

Today, there’s a party at your home. And you are busy detecting faults in the decorative lights hanging over the boundaries of your house. You want to finish the job before evening. But daylight adds to your frustration by making it difficult to observe whether the neon bulb inside the tester is glowing or not.

Power Pulser

The idea behind this multipurpose power pulser is very simple. As shown in the circuit (Fig. 1), it uses a low-frequency oscillator to drive a voltage regulator. Timer chip LM555 (IC1) is wired as an astable multivibrator. Components R1 and R2, VR1 and C1 produce the free-running frequency. You can adjust it to some extent by varying potentiometer VR1. The output of IC1 at pin 3 controls the switching on/off of adjustable voltage regulator LM317T (IC2) through npn transistor SL100B (T1).

PC Temperature Controller

Here is a simple temperature controller that turns your personnel computer (PC) off when the temperature of the PC increases beyond the optimal temperature value. Some of the larger integrated circuits become quite hot and if the temperature inside the PC becomes too high, these devices may not be able to dissipate heat fast enough. This, in turn, could lead to failure of devices and eventually of the PC.

Ethernet Interface for Automation Systems

Communication with remotely located automation systems is possible via the Internet. A physical communication port with a network is required for this communication. An Ethernet interface can be used for that purpose.

EEPROM Interface for Beginners

Electronic systems today incorporate a number of peripheral ICs that have to communicate with each other and the outside world. To maximise hardware efficiencyand simplify circuit design, Philips developed a simple bidirectional two-wire communication interface between components residing on the same circuit board. This interface is referred to as two-wire interface (TWI) or I2C (inter-integrated circuit).

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