Light Controlled Switch with Solar Cell

This project is another in the series of light controlled circuits see Switches section.

This circuit is special because in the presence of darkness the Battery current drain is about zero. Then in the presence of sufficient light the Transistor is turned on and the Relay is energized. A 3V Lamp is used to demonstrate the switching of the Relay contacts.

The circuit and wiring procedure given is for Lamp operation in the presence of light, but you only need to change the wire to turn the lamp on when there is no light on the Solar Cell.

Study project Light Controlled Switch with CdS Cell in Switches section to determine the operation of the Transistor when controlled by a CDS Cell. The difference in this circuit compared with project Controlled Switch with CdS Cell, is that here the Solar Cell produces the required base current directly from light striking the Solar Cell surface.

The Relay coil cannot be fully energized by a single Transistor, as the current generated by the Solar Cell is weak. We therefore use a 2SA and a 2SC Transistor in this test circuit. The next logical step would be a Transistor with a light control built in. We've got them!! They are called "photo transistors".

You know, this is why electronics is such a challenge, just about the time you think you have seen it all, somebody develops a new batch of goodies!