This two-tube circuit, one being a double, was found in Radio Electronics magazine from July 1958. The distance to the microphone depends on noise and losses. The cable must be shielded.
This circuit uses unijunction transistors and drives a small speaker through an output...
The resonant circuit depends on the crystal frequency that can go up to a few megahertz. The...
This relay without lock uses a general-purpose transistor. The relay is according to the supply....