This circuit was found in an English publication from 1967. The circuit is for listening on a phone. Transistors are types of germanium of the time.
This circuit was obtained from a 1986 Spanish magazine Resistor. The magazine no longer exists, but...
Many reed switches can be wired in series to perform na efficient alarm as shoen in the figure....
This circuit is from an old documentation making use of a 3,579,645 MHz analog color television...