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.
This circuit was found in an old documentation and can be powered by a solar panel. The touch...
This circuit was obtained from 1967 documentation. It uses a sensor to determine the trigger...
This sophisticated logic probe for 5 to 12 V circuits (CMOS and TTL) was found in a 1980s...