Found in an English publication from the 1980s, this circuit can be built with equivalent transistors such as the BC549. The signal must be applied to an external amplifier.
Carbon microphones are no longer used in most projects. However, finding one in the scrap metal, it...
This circuit produces a strong tone of audio or ultrasound, depending on the values of components C1 and...
This alarm from an old 1978 publication uses PNP transistors and driver and output transformers that are...