This simple circuit is a continuity test that, in addition to testing resistors, can also test other components. Power is provided by batteries and the LED can be of any color.
This circuit was obtained from an English magazine from 1977. The magazine no longer exists, but...
We found this circuit in a 1997 documentation. Its purpose is to fan an inverter for fluorescent lamp...
Found in a Hands On Electronics of the 80s, this circuit makes use of an integrated circuit of the time...