This circuit was found in a 90s Popular Electronics. It also serves for the analysis of microcontroller signals. The power comes from the circuit under test.
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,...