Abstract: A trigger circuit for use in providing a trigger pulse to a sweep drive circuit in a streak camera. The trigger circuit comprises a low voltage DC power supply, a resistor, a charge line and a photodetector switch all connected in series. The photodetector switch includes a slab of a semi-insulating semiconductor material which becomes photoconductive when actuated by optical radiation. In use, the trigger circuit is coupled to the sweep drive circuit of the streak camera. In the absence of optical radiation, the switch is nonconducting and a voltage from the DC power supply builds up in the charge line. When the switch is actuated by optical radiation it becomes closed causing the voltage to be discharged to the sweep drive circuit. The switch returns to a nonconducting state (i.e. an open state) after about 1.5 nanoseconds.