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 is like the previous one except that it can be triggered electronically. The circuit is...
Found in 90s Radio Electronics, this audio indicator makes use of an integrated circuit that is...
Found in the October 1992 Radio Electronics magazine, this oscillator is controlled by switch S1....