We found this circuit in a 90's EDN. The circuit is based on a control integrated that is still common in these types of applications. The circuit uses a PROM.
This circuit using an operational amplifier was found in old American documentation. It can be...
If the circuit oscillates, emitting sound, it is because the transistor is good. This simple...
Like the previous circuit, this is used in the analog part of the telephone lines. The same...