We found this code key or coded keyboard in a publication from the 90s. The circuit uses components that are still common today and is powered by a 12 V voltage according to the relay used.
This simple 555 component tester was found in the November 1992 issue of Radio Electronics...
The basic idea of this 1998 documentation project is to use the channel selector of an old analog...
This circuit, found in a publication of the year 1968, uses a MOSFET at the input to increase...