This circuit was found in a Popular Electronics from the 90s. It is based on components still common today and an optical coupler for the triac. The sensor is a thermistor.
This circuit is from a 1980s Motorola documentation. The circuit can be made with modern...
This well-known circuit triggers a relay when light striking the phototransistor is cut off. The...
This circuit generates a pulsed tone whose frequency depends on C2 and whose flashing depends on...