This circuit, from a 303 Circuits from the 90s, uses a telephone pickup coil as a sensor to capture power grid noise, thus making it possible to locate electrical wiring.
This circuit was obtained in the publication 101 Electronics Projects of 1979. The circuit can be...
With this name, we found this electronic stethoscope with a coal microphone in a 1957 Raytheon...
We find this oscillator in a 1980s documentation. The frequency depends on the crystal and the...