This interesting circuit found in American documentation generates a busy signal, without you having to take the phone off the hook if you do not want to answer. The components used are common.
We found this circuit in an American publication from the 1990s. The circuit has a Darlington...
The sensor for this alarm is an LDR and it consists of an oscillator that goes into action when...
This circuit was found in a computer publication from the 1970s. The transistors can be the BD135...