This circuit from a Popular Electronics from the 90s uses a common optocoupler and a general-purpose transistor to control a sensitive relay that can be from 6 to 12 V.
This sample and hold circuit configuration was found in a manual from the 70s. The operational amplifiers...
When the temperature rises above the value adjusted by P1 the relay triggers on. The rated voltage for the...
This circuit was found in Radio Electronics from February 1993. The circuit can be implemented with other...