Like bipolar transistors, SCRs and TRIACs are also manufactured as light-sensitive devices, the action of impinging 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 Figure below.
![](https://i0.wp.com/www.electricalengineering.xyz/wp-content/uploads/2023/01/lascr-symbol.png?resize=183%2C130&ssl=1)
Light activated SCR
Optically-controlled TRIACs don’t receive the honor of having their own acronym, but instead are humbly known as opto-TRIACs. Their schematic symbol is shown in Figure below.
![](https://i0.wp.com/www.electricalengineering.xyz/wp-content/uploads/2023/01/opt-triac-symbol.png?resize=118%2C100&ssl=1)
Opto-TRIAC
Optothyristors (a general term for either the LASCR or the opto-TRIAC) are commonly found inside sealed “optoisolator” modules.
Article Extracted from Tony R. Kuphaldt Lessons In Electric Circuits — Volume III Chapter 7 under the terms and conditions of the CC BY License.