Light activated SCR (LASCR) or a Photo SCR is just an ordinary SCR except that it can also be light triggered. Most LASCRs also have a gate terminal for being triggered by an electrical pulse just as a conventional SCR. The basic construction of an LASCR is shown in figure. The schematic symbols most commonly used for the LASCR are shown in figure. Some LASCRs have clear windows in their cases so that light sources from other devices can be coupled to them. Many have the light source encapsulated in the same package so that a relay is formed. When the light falling on depletion layers is strong enough, valence electrons are dislodged from their orbits and become free electrons. When these free electrons flow out of the collector of one, transistor into the base of the other.
The positive feedback starts and the LASCR turns on just like a normal SCR, the LASCR will continue to conduct even if the light source is removed. For maximum sensitivity to light, the gate is left open, as shown in figure. Trigger adjust can be included if an adjustable trip point is desired as shown in figure. The gate resistor diverts some of the light produced electrons and alters the sensitivity of the circuit to the incoming light. The devices are for low power applications.