We found this circuit in a spy book from the 90s. Transistors admit equivalents and the coil can be designed as in the CB19425E project. Power is provided by cells or 9 V battery.
Found in a documentation from the 90s, this circuit has a typical accuracy of 2% with 20 V signals at...
This circuit, from a 1988 documentation, consists of an automatic temperature control using an NTC...
This circuit of a Popular Electronics from the 80's selects tones around 60 Hz, acting as a filter...