PURPOSE: To control optionally an operating time by an external signal by providing a means controlling the charge/discharge current of a time constant circuit based on an input signal.

CONSTITUTION: When a negative trigger pulse is inputted from an input terminal 3, a transistor (TR) 1 is turned on and a TR 2 is turned off, the electric charge of a capacitor C1 starts discharging to a TR 6 through a resistor R1. The moment the potential of the capacitor C1 reaches a VBE+VF from a negative potential, the TR 2 is turned on and the TR 1 is turned off (VBE is a base- emitter voltage of the TR 2 and VF is the forward voltage of a diode D2). The electric charge is discharged because the resistor R1 is connected to the emitter of an emitter follower circuit comprising the TR 6, the base of the TR 6 is used as an input terminal 7 for a control signal and the base voltage is controlled to control optionally the operating time externally.