Suitable for clandestine listening and in SMD assembly, this circuit was found in the Radio Eletronics magazine of August 1988. The listening is in headphones and the amplifier is electret.
We have many versions of biofeedback with the 555 IC on this site. This is another one. We found it...
Using an operational amplifier of the time, this circuit was obtained from a 90s Radio...
This circuit was suggested by Popular Electronics from the 90s. The circuit is quite simple using...