Found in an old English publication, this circuit applies to the 240 V network, but can be changed to lower voltages. Transistors are mostly general-purpose, admitting modern equivalents.
This transmitter was found in an old publication on espionage. It applies a signal of a few...
This Motorola circuit uses only one transistor. The circuit is from 1965 documentation. The...
This circuit is from a Poptronics, American magazine of August 2000. The circuit can be assembled with...