Sensitive Light Controlled Switch 2

This project is a switch that responds to even a slight change in the intensity of light hitting the CdS. By nature, the CdS is not a very "fast" component. It does not react (change resistance) quickly to changes in light intensity. However, by using all three Transistors to amplify the changes, we can make a very sensitive light controlled switch.

After you have finished the wiring, adjust the Control to the point where the Lamp comes ON (but no higher). This adjustment is very important to the sensitivity of the switch. Now when you move your hand over the CdS cell the light should go OFF.

You can see from the schematic that the three Transistor amplifier energizes the Relay coil, which in turn closes the Lamp/3 volt Battery circuit.

The Diode in the circuit protects the 2SA Transistors from damage by a "spike" of electricity caused by the collapse of the magnetic field in the Relay coil, when it is de-energized.