MOC3021 Optocoupler Triac Driver IC

Triggering a triac to switch mains AC power directly from a microcontroller pin is dangerous without proper isolation, since a fault could otherwise put lethal mains voltage straight onto the control circuit. The MOC3021 solves this the same way other optocouplers do, using an internal LED and phototriac to pass the trigger signal across as light rather than an electrical connection, keeping the low-voltage logic side and the high-voltage triac side completely separate. That makes it the standard building block for DIY AC dimmer circuits, solid-state relay designs, and any project that needs a microcontroller to switch a mains-powered load through a triac rather than a simple electromechanical relay.