Like bipolar transistors, SCRs and TRIACs are also manufactured as light-sensitive devices, the action of impi ging light replacing the function of triggering voltage.
Optically-controlled SCRs are often known by the acronym LASCR, or Light Activated SCR. Its symbol, not surprisingly, looks like this:
Optically-controlled TRIACs don’t receive the honor of having their own acronym, but instead are humbly known as opto-TRIACs. Their schematic symbol looks like this:
Optothyristors (a general term for either the LASCR or the opto-TRIAC) are commonly found inside sealed “optoisolator” modules.