This circuit was found in a 1964 documentation. It shows how to achieve automatic gain control for an oscillator. The transistors are of the time and the circuit controls signals from 2.6 to 3.1 kHz.
When the temperature rises above the value adjusted by P1 the relay triggers on. The rated voltage for the...
This circuit was found in a Popular Electronics of the 90s. We can build it based on modern...
We found this circuit in a 1965 publication by General Electric. The zener is 8.2 V x 1 W and the...