This circuit was found in a Hands On Electronics from the 90s. The microphone is a small 75-ohm speaker. A lower impedance can be used with a transformer.
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...