LDR Based Automatic Lamp

● This circuit is based on Light Dependent Resistor (LDR) which will turn on LED's when ever darkness is sensed in room. You can put this small lamp in your child's room as a simple emergency lamp to avoid panic situation in case of sudden power failure. As it provide ample of light in the room so that your child can easily come out of room.

● Important feature of this LDR based automatic lamp circuit is it automatically become on by sensing the darkness so no need to search for switch.

● And it is powered by a rechargeable battery which charges automatically by the AC power supply so no need to worry about the battery backup.

● You can use adapter readily available in market or make your own power supply with the help of transformer and bridge rectifier.

● And when light comes it automatically off the lamp so need to off the lamp manually.

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Fig. 1: Prototype of LDR based Automatic Lamp Circuit on Breadboard

This circuit explains the principle of operation of LDR (light dependence resistor). LDR is a device whose sensitivity depends upon the intensity of light falling on it. The resistance of LDR decreases when intensity of light falling on it increases and vice versa (resistance increases when intensity of light falling on it decreases). In dark or in absence of light, LDR exhibits a resistance in the range of mega ohms, which decreases to a few hundred ohms in presence of bright light.

This phenomenon can be measure with the help of multimeter, for measuring keep your multimeter in ohm or resistor measurement region. When you cover the LDR its resistance will be very high and when you place it in light it decreases. We are utilizing this property of LDR to act as a sensor, since a varying voltage drop can be obtained with varying light.

Working of circuit

As we know LDR offers low resistance in daytime and high resistance in darkness. Now in day time LDR offers low resistance therefore transistor T1 is in saturation state and base of transistor T2 is at low voltage and it will not conduct means transistor T2 is in cut-off mode. So LED’s connected at the output does not glow.

And in night LDR offers high resistance so transistor T1 will be in cut-off mode which will make the base of transistor T2 high and it will start conducting and the LED’s connected at the output start glowing. We have use a diode D5 and current limiting resistor R1 to the charge the battery.

Power supply

In this circuit 230V mains power supply is step down with the help of transformer T1 which will give us 9V AC supply. Now transformer output is rectified with the help of bridge rectifier made with help of diode D1 to D4. C1 is used to remove the AC components and IC 17806 is used to provide us regulated 6V power supply.