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 conventional humidity or water alarm circuit was found in an English documentation from 1974....
The frequency of this traditional crystal oscillator (XTAL) depends only on the used crystal,...
This circuit of industrial applications has an interesting characteristic: the relay is only...