This low power circuit was obtained from an American 80's documentation. The circuit indicates the voltage of a battery being trimmed by the trigger point. The transistors can be the BC549.
This circuit uses an audio output transformer found in old transistor radios. Power output is...
This circuit was found in an old documentation and can be powered by a solar panel. The touch...
An application for the previous circuit (CB109) is shown here. The central frequency is determined by C and...