This circuit was found in a publication by Edelman from 1924. The power comes from the battery and the listening is through a high-impedance headphone.
This signal injector and tracer (follower) was found in an old American documentation. The...
The basis of this circuit is the HA2400 released at the time. The circuit is from a Radio...
Found in a 1980s English magazine, this trigger circuit can have different types of inputs...