This circuit has the purpose of activating a relay when there is light in the LDR. We can use it in an alarm system or even as a remote control using a flashlight as a transmitter.
The circuit works well with 6V or 12V relays and LDR may be common. Note that the relay only stays on for as long as light is on the LDR.
For applications where the relay remains active for a long time, it is convenient to use power supply. Batteries can be used for momentary contact applications.
For greater sensitivity, if used as a remote control, install the LDR in an opaque tubing with converging lens, positioning the lens for the highest possible performance.
In Figure 1, we have the complete circuit of the light relay.
In Figure 2, we show its assembly on a small printed circuit board using sensitive 6 or 12 V relays.
Material list
Q1- BC548 - NPN transistor
D1 - 1N4148 - silicon diode
K1- 6V or12 V - Sensitive Relay
Common LDR-LDR
P1 - 10k ohm - trimpot or potentiometer
Miscellaneous:
Printed circuit board, power supply or batteries, wires, welding, etc.
