We found this circuit in an American documentation from the 90's. The circuit makes use of general purpose NPN transistors. The 0.005 uF (5nF) capacitors determine the frequency.
This is the battery-powered version of the previous circuit. The circuit is from the same Radio...
This is the modern version of the Galena radio in which the detector is a germanium diode (which can...
This circuit triggers a remote doorbell when the phone rings. The circuit is for the 110 V mains...