This fast-acting circuit uses any phototransistor as a sensor. The circuit drives a 30 V relay, but changes can be made to the circuit to operate at lower voltages. Equivalent operational amplifiers can be used.

This fast-acting circuit uses any phototransistor as a sensor. The circuit drives a 30 V relay, but changes can be made to the circuit to operate at lower voltages. Equivalent operational amplifiers can be used.