The dark detector circuit shown here can be used to produce an audible alarm when the light inside a room goes OFF. The circuit is build around timer IC NE555. A general purpose LDR is used for sensing the light. When proper light is falling on the LDR its resistance is very low. When there is no light the LDR resistance increases. At this time the IC is triggered and drives the buzzer to produce an alarm sound. If a transistor and relay is connected at the output (pin3) of IC1 instead of the buzzer, electrical appliances can be switched according to the light.
Circuit diagram with Parts list.
- The LDR ,R4 can be any general purpose LDR.
- The circuit must be assembled on a good quality PCB or common board.
- The circuit can be powered from a 9V PP3 battery.
- The POT,R3 can be sued as a volume controller.
- Mount the IC1 on a holder.It will make replacements easy.