This circuit was found in a 1967 Raytheon manual. The transistors are from the time. Equivalent modern transistors can be tried out.
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...