Centronics relay card:

A relay is still one of the simplest devices for switching large electrical loads with the aid of a relatively small amount of energy. Since a computer is perfectly suitable when it comes to generating switching commands according to a certain pattern, the combination of a PC and a number of relays results in a multi-purpose, flexible, switching system. The relay card presented in this article is the safe link between the PC's parallel port and other equipment. The fact that the port may be controlled with very simple commands allows you to experiment with the circuit to your heart's content.