Circuit Operation

The light-operated electronic switch is a common electronic device in automation equipment, security systems, electronic counters, various remote controls, etc. It is very sensitive to light, responds quickly to its variations and has reliability in use. The assembly uses a photoresistor (LDR-Light Dependent Resistor) as the transducer, LDR which determines the value of the threshold (switching) voltage at the base of the transistor. The RV1 potentiometer, connected in series with the LDR, by the position of its cursor, determines the light actuation threshold of the relay, so the sensitivity of the electronic switch. The potential in the electrical node LDR-RV1, taken from the base of TR2 by R2, depending on the voltage value, commands 'On' or 'Off' the respective transistor. Q2 is ordered from the collector of Q1. Topologically (electrically) analyzing, the assembly Q2-Q1 forms a switch with hysteresis, which makes the relay actuators not intermittent at the same time with small variations in light intensity around the threshold (adjusted from RV1).

The desired sensitivity is adjusted to light / dark, at the desired threshold, maneuvering the RV1 slider. LDR can be a photoresistor worth 4k7 ... 100k.

Bill of materials available in the download section.

Enjoy it !