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 solar cell powered circuit generates a standard 100 kHz signal. The circuit is from a 1960...
This very simple circuit from a 1995 documentation lets you know which input button was triggered when...
This headlight reminder for your car was found in the May 1970 issue of Radio Electronics...