This circuit takes advantage of the noise generated at the reverse biased junction of a germanium diode. Other types of diodes can be used. The signal is weak.
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...