This circuit is from a 1972 Motorola publication. The used transistor can still be found, but it can be used equivalent. The circuit is powered by the amplifier with which it must operate.
This circuit uses a crystal phone and can be built with more modern transistors like the BC548. The...
The resistors in this circuit marked with * must be within 1% tolerance. The circuit is from the...
The existence of field effect transistors at the input of the integrated circuit (operational...