PURPOSE: To prevent the damage of element and malfunction due to noise, by insulating terminals inbetween perfectly in DC and AC electrical circuits by using a photo-electric conversion circuit including an optical fiber and a photodetector.

CONSTITUTION: An output signal of a microprocessor is converted into a light as an input and it is converted into an electric signal at a photo-electric conversion circuit consisting of an optical fiber 14C, a fiber connector 14A, a visual light cut filter 14B and a photodetector 4B, a signal is formed at a zero voltage detector 3, a bidirectional thyristor 1 is triggered via a signal amplifier circuit 2, and a series circuit consisting of an AC power supply 30 and a load 20 is conducted. The DC low voltage side of the microcomputer and the AC side of the load 20 are completely isolated in the electrical circuits of DC and AC, allowing to avoid the damage of elements and malfunction due to noise.