This small analog TV transmitter was found in an Electronics Now from the 90s. The range is around 20 meters, and the coils are according to the channel on which you want to receive the signal.
This sonic relay or vox was found in a Popular Electronics from the 90's. The power MOSFET can be of...
Originally indicated as an alarm alert in a 1990s US publication, this circuit can be used as a...
Found in a 90s Popular Electronics, this transistorized circuit generates pulses from 2 Hz to 50...