This sensitive light relay can be operated at a good distance by a flashlight, working as a remote control. Another application is in alarms, triggering a siren when the light of a place is lit.
The circuit works with both 6 V and 12 V relays and the sensor is a common LDR. The sensitivity adjustment is done through a trimpot or a potentiometer.
The circuit can be powered by batteries or power supply. No batteries will be recommended if the relay is energized for a long time, because in this condition the consumption is greater.
In Figure 1, we have the complete circuit of the device.
The relay used may be any sensitive type for 6 V or 12 V.
In Figure 2, we have a suggestion for the printed circuit board.
Material list
Q1, Q2 - BC548 or equivalent - general purpose NPN transistors
D1- 1N4148 - silicon diode
LDR - Common LDR
K1- 6 V or 12 V - Sensitive Relay
P1- 100k ohm - trimpot or potentiometer
R1 - 100 k ohm x 1/8 W - resistor - brown, black, brown
Miscellaneous:
Printed circuit board, power supply or batteries, wires, welding, assembly box, etc.
