Found in a 1980s American documentation, this circuit operates with 5 V signals requiring 55 cycles of 500 Hz for tone recognition. The circuit can be adapted to operate with other frequencies.
This circuit is from Popular Electronics from October 0, 1981. The diodes can also be 1N4004 and...
This switch or band selector for receivers was found in a Popular Electronics from the 1990s. The...
This circuit, published in 1978, can use any general purpose NPN transistor such as the BC548 and...