Abstract: A solid state trigger circuit provides variable period output signals within a timed sequence of signals for controlling conduction angle firing of thyratron type switching devices in an A.C. phase line. A bi-stable multivibrator receives timing signals derived from the alternating current in the phase line which are out of phase with each other. The timing signals oscillate the multivibrator at a desired timing rate to produce an output of the multivibrator which has voltage transition spikes at the beginning of each positive circle of the timing signals. A saw tooth wave form with a linear voltage input control signal indicative of a desired time period pulse width provides a square wave trigger output signal with signal pulse periods equal to the desired pulse width.