This is the circuit for the HM-10ª, an instrument sold by Heathkit. The circuit is from a Radio Electronics from May 1963. The indication is by an analog instrument.
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...