CW Monitor (CB21439E)

This circuit for monitoring telegraphy transmissions was found in a 1975 CQ magazine. Transistors may be modern equivalents.

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ELF Monitor (CB21055E)
This circuit monitors extra-low frequency (ELF) fields. We found it in an American documentation from the...
Controlled Oscillator (CB12682E)
We can use any type of resistive transducer with central resistance in the range of 10 k ohms to 1...
Triac Double Dimmer (CB16915E)
This circuit controls two lamps so that when one increases in brightness the other decreases. The...