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 power supply protection circuit using a relay was found in a 1967 documentation. The...
The integrated circuit used in this 80's application is not common today. However, modern...
This circuit from a 1979 publication uses FETs which can be BF245 and integrated circuits, unlike...