We found this circuit in Ferranti's 1974 transistor manual. The components are from the period. The table gives the value of C1 for different frequency ranges.
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...