13-05-2011, 11:55 AM
Voltage regulator ICs (78xx series)provide a steady outputvoltage, as against a widelyfluctuating input supply, when thecommon terminal is grounded. Anyvoltage about zero volt (ground) connectedin the common terminal is addedto the output voltage. That meansthe increase in the common terminalvoltage is reflected at the output. Onthe other hand, if the common terminalis disconnected from the ground, thefull input voltage is available at theoutput.This characteristic is utilised in thepresent circuit. When the commonterminal is connected to the ground,the regulator output is equivalent tothe rated voltage, and as soon as theterminal is disconnected from theground, the output increases up to theinput voltage.The common terminal is controlledby a transistor, which works as aswitch on the terminal. For automaticcontrol of light, a light-dependent resistor(LDR1) is connected to the baseof the transistor. In this way, the voltageregulator is able to operate a lightbulb automatically as per the ambientlight.To derive the power supply forthe circuit, the 50Hz, 230V AC mainsis stepped down by transformer X1to deliver a secondary output of 12V,250 mA. The secondary output of thetransformer is applied to a bridge rectifiercomprising diodes D1 throughD4, filtered by capacitor C1 and fedto the input terminal of the regulator(IC1).The common terminal (pin 2) of IC1is connected to the ground line of thecircuit through transistor BC557 (T1).The transistor is biased by R2, R3, VR1and LDR1. The grounding of IC1 iscontrolled by transistor T1, while lightis sensed by LDR1. Using preset VR1,you can adjust the light-sensing levelof transistor T1.The output of IC1 is fed to the baseof transistor T2 (through resistor R4and zener diode ZD1) and relay RL1.LED1 connected across the positiveand ground supply lines acts as apower-‘on’ indicator.Normally, the resistance of LDR1is low during daytime and high duringnighttime. During daytime, whenlight falls on LDR1, pnp transistor T1conducts. The common terminal of IC1connects to the ground and IC1 outputs6V. As a result, transistor T2 does notconduct and the relay remains de-energised.The light bulb remains ‘off’ asthe mains connection is not completedthrough the relay contacts.During nighttime, when no lightfalls on LDR1, it offers a high resistanceat the base junction of transistorT1. So the bias is greatly reduced andT1 doesn’t conduct. Effectively, thisremoves the common terminal of IC1from ground and it directs the fullinput DC to the output. Transistor T2conducts and the relay energises tolight up the bulb as mains connectioncompletes through the relay contacts.As LDR1 is in parallel to VR1+R3combination, it effectively appliesonly half of the total resistance ofthe network formed by R3, VR1 andLDR1 to the junction at T1 in totaldarkness. In bright light, it greatlyreduces the total effective resistanceat the junction.The circuit issimple and can beassembled on a smallgene r a l -purpos ePCB. Use a heat-sinkfor IC1. Make surethat LDR1 and thelight bulb are wellseparated.The circuit can beused for streetlights,tubelights or anyother home electricallighting systemthat needs to be automated.
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