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 accelerates the tripping of a relay when energized. The circuit uses the CA3018...
The circuit shown is from the National Semiconductor 1994 Linear Applications Handbook. The base...
This ultra-simple circuit has the frequency determined by the LC network. Any general purpose JFET...