This sensitive circuit for triggering relays was found in an old English publication. It works with alternating signals applied to its input. Transistors are general purpose types.
This circuit uses an MW radio tuned to around 540 kHz to detect atmospheric signals, for example, from...
This project was found in a 1982 publication. It is a small transmitter that can send signals from...
This circuit is from a 1960 book by Hugo Gernsback. The transistors are germanium from the time....