An optically triggered silicon controlled rectifier (SCR) circuit (20) has a number of semiconductor layers diffused into an N- substrate (21). The layers form an SCR (50) having a P+ anode region (25), a P+ gate region (24), and an N+ cathode region (27). An adjustable base-shunt resistance, in the form of a P- channel depletion mode MOSFET (Q3), connects between the SCR gate region and the cathode region. The MOSFET includes a MOSFET gate region (35), a P+ drain region (24), a P-- channel (26), and a P+ source region (23). The substrate also accommodates a PN photodiode (22, D1) which connects to the MOSFET gate region for switching the MOSFET on and off in response to incident optical radiation (L) thereon. The SCR gate region also comprises photosensitive material. When sufficient optical radiation illuminates the photodiode and the SCR gate region, the MOSFET is turned off and the SCR is triggered, permitting anode-to-cathode current to flow. When insufficient optical radiation is present, the MOSFET is turned on and provides a low-resistance shunt path which prevents the SCR gate region from triggering the SCR.