[pyfgcrl123]

How about electromechanical relay switch. As far as digital electronics goes, relay can, with some mental gymnastics, be considered grandfather of transistor (which, in turn, is one of the most important inventions of all time, but too complicated).

[lebuffon]

They were invented (I believe) to "relay" telegraph messages over long lines so it was indeed a digital amplifier. (Former Western Union employee)