This circuit was found in a 1967 US documentation. It shows how to build a gain control by digital signals. The circuit can be used as a shield. FET can be a modern equivalent.
Here we have an interesting circuit found in a 1981 publication that causes two LEDs to flash...
This circuit functions as a thermostat triggering a relay when the boil control temperature...
For the 600 kHz range coil is formed by 35 turns of 28 AWG wire on a ferrite core with a diameter...