Found in a 90s Popular Electronics, this circuit checks if a remote-control transmitter is emitting signals. The sensor is a general-purpose type found in many electronic devices.
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...