PURPOSE: To reduce the pulse width and variations therein, by arranging a detection circuit to be made up of two light emitting diodes connected oppositely in parallel and two phototransistors optically coupled separately thereto to minimize deviation in the pulse exchange threshold and the phase.

CONSTITUTION: A zero volt detecting circuit is made up of two light emitting diodes 20 and 21 connected oppositely in parallel to an AC power source 1, two phototransistors 22 and 23 optically coupled separately thereto and resistances 24 and 25. The threshold 1D (about 0.7V) is set for starting the energization of the light emitting diodes 20 and 21. Then, when the AC power source 1 is within ±1D, the light emitting diodes 20 and 21 both fail to energized and the phototransistors 22 and 23 are turned OFF. As a result, the input I2 of a microcomputer 7 moves to Hi and also is given Hi near the AC power source zero volt. Otherwise, the zero volt pulse of Lo is obtained.