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 1956 direct amplification radio circuit was found in a documentation from Sylvania that...
This conventional oscillator with the 741 can be precisely adjusted to produce a 400 Hz signal. The...
This circuit uses two diodes in a doubling configuration to receive medium-wave range signals. The...