This circuit makes use of a stroboscopic tube, a component that no longer exists today. The circuit is from the 70's and can be adapted to use mains voltage.
Of course, other frequencies can be generated by changing the components. The circuit generates a...
This circuit was found in a 1974 publication. It provides a tone indication of the temperature sensed by...
This spy transmitter can be plugged directly into a power outlet. The power comes from a regulated...