3 light activated relay

light activated circuit

The figure above is a simple light activated relay that is controlled by a comparator. During day time, the light dependent resistor (LDR) has a small resistance, setting the comparator into high output. As a result, the relay is activated, thus switching on any equipment connected to 220V source. During night time LDR has a large resistance, deactivating the relay and switches off the switch. Some uses of this relay circuit is driving motors during day and turning off during night.
You can replace the relay into normally close type for this circuit to be used in light activated circuits such as in lamps and lighting equipments that automatically turn off when it senses sunlight and on during night time

Part List:
R1, R3 - 150 kilo ohms 1/4W
R2, R4, R5 - 100 kilo ohms 1/4W
LDR - any light dependent resistor
U1 - LM393 or any similar comparator
Q1, Q2 - 2N2222, CS9013 or similar NPN transistor
D - 1N4001 or any rectifier diode
R - low current 12V relay Normally Open (N.O)
NPN transistor pins
LM393 comparator
Note: Some parts of the circuit are connected to 220V, so be careful in building the circuit.If you are not used with higher voltage, you can build a simple light activated LED light powered by 1.5V.


  1. I looked for a label or parts list to find out what the chip is. Is it an opamp? Or is it a comparator? What is the part number? Thanks.

  2. Thanks for pointing it out. Ill re-draw it for you.

  3. Has anyone tested this, does it work?