This variable oscillator with power step for alarms was found in an American documentation from the 90's. The power transistor must be equipped with a heat sink.
The coil of this receiver determines the range. It can have 2 to 10 turns of wire 20 to 24 in the...
This circuit was found in a Radio Electronics from January 1965. The relay is 9V, but we can...
This crystal or galena radio circuit uses a high impedance magnetic earphone. The coil can have 80 +...