This circuit activates a buzzer when the voltage applied to the input falls below a preset value in R1. The circuit is from an 80's American documentation.
This receiver was found in a 1968 publication, but can be mounted with a BC548 and BC558 in place...
PTT or Push to Talk is the control that switches the speak-hear function on a transceiver. This circuit...
This small amplifier with 3 transistors can be used both as a stethoscope and as a spy. The...