Adjustable temperature sensing for freeze protection — a fail-safe electronic switch with no-neutral 2-wire design.
A 2-wire device wired in series with a 120VAC relay coil. It monitors temperature and controls whether the relay energizes. When warm (above setpoint), the TRIAC fires and the relay pulls in. When cold (below setpoint), the TRIAC stays off and the relay drops out, energizing the heater.
The entire electronics rail is powered via a capacitive dropper (C2, X2 rated) — no neutral wire needed. The TRIAC's own latching delay window (~0.75ms per half-cycle) is used as a deliberate charging window for the storage capacitor C3, keeping the 5V rail maintained even when the TRIAC is conducting.
BSS131 MOSFETs block the AC-coupled gate drive in both polarities when the comparator output is high.