The circuit consists of a photodiode, amplified circuit, a decision circuit (schmidt trigger circuit) and an output buffer circuit. When the light comes, the circuit will sense the signal and give a corresponding output signal. PS103C/104C will generate a digital signal off/on corresponding to the incoming light which is on/off. The output buffer will give a TTL compatible output signal from the digital signal. With build-in regulator, this circuit has a wide operation range.