In the figure we have one of the many options found in this section for an intercom using ordinary phones not in use. This was obtained in a Popular Electronics from the 90s.
This is an old test circuit for integrated TTL counters, feeding a Nixie tube, common at the time....
This traditional crystal radio circuit, but which has a modern version with diode, was found in an...
This headphone amplifier using a pickup coil can be used as an extension for devices such as TVs,...