A switch is a tap that controls the flow of current, preventing it, allowing it, or diverting it. A person may control it, or a moving object such as a door. Other switches operate in response to light and movement as in security systems. In the case of electronic switches, we use small amounts of power from external sources to trigger them.
Relays and electronic switches amount to the same thing in electronics. A separate low-power circuit triggers them often using the power of electromagnetism.
In other instances, they are solid-state relays without moving parts. In that case, a small external voltage across their terminals activates them.
Early Telegraph Switching
The earliest relays formed part of long-distance telegraph circuits. Their role was to repeat a signal from one circuit to another one in order to send it on. There is no general agreement whether Joseph Henry or Edward Davy built the first relay.
However, Samuel Morse was the first to patent one. This acted “as a digital amplifier thus repeating the telegraph signal and allowing it to propagate as far as desired.” In this way, he overcame the hitherto limited range of early telegraphic systems
Electronic Switches in the Digital Era
The 1950’s arrival of digital logic transformed the role of electronic switches. This form of reasoning introduced Boolean functioning, whereby a switch is either on or off, and a gate is either open or closed.
Today we have a variety of digitally active devices including transistors and logic gates.
Their function is to toggle their outputs between a pair of logic levels. These may be different signal lines, computers, and even networks. We still find them in public switched telephone networks though. In technical jargon, electronic switches are either on or off, closed or open, or connected or not connected.