The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
Relays from 20 mA to 200 mA can be driven by this sensitive circuit.6 or 12 V relays can be used...
This is a conventional double T oscillator, where the frequency is determined by the double T...
This circuit can be used to recognize a tone by its frequency. When this tone is present in a...