In the figure we have a simple thermometer with NTC, using an analog current indicator. The adjustment is made in the trimpot and the power is done by batteries.
We found this oscillator that operates on the third overtone in a 1975 amateur radio...
With the circuit shown, obtained in a 1980 publication, a reed-relay is activated with a resistive...
This circuit is from a 1967 US publication. The configuration can serve as a basis for designs...