The circuit presented uses two common transistors and is powered by the circuit being tested. The circuit is for TTL logic, since its supply is 5 V.
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,...