We found this circuit in a Texas Instruments documentation from the 80s. The circuit uses simple power source and the frequency depends on C1 and R1 basically.
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...