This circuit was originally recommended to give the call signal on intercoms. However, it can be used with other applications. The circuit is from 1978 documentation. The transistor may be the BC548.
We found this interesting circuit in a 1969 American documentation. It shows a solution with...
Here is a high-quality intercom for demanding readers. The circuit is from a 1982 documentation. Of course,...
This circuit can be found on this website in similar versions. The power supply is given in...