This receiver was found in a 1924 book by Gernsback. The circuit uses three tubes and is powered by 3 batteries of different voltages, common at the time.
This circuit, from 1999 documentation, prevents the powered device from receiving power when it...
This circuit is intended for monitoring an environment. It fires a relay with a sound, but only if...
Found in a Popular Electronics from the 90's, this circuit uses integrated from that time, today...