Found in Motorola documentation from the 90's, this circuit has a high input impedance. The power supply must be symmetrical. The circuit can be implemented with modern operational amplifiers.
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...