A switch is a signal-carrying circuit that can send output signals to two or more different locations, depending on the state of the mechanism. These may be toggle circuits, where the state of the switch changes after each use, or permanent switches that retain the same state through many uses until a change is made with a separate signal.
More generally, any circuit may be referred to as a switch, if it can alter its output based on stored information. For example, the following simple mechanism based on an eater2 was discovered by Emerson J. Perkins in 2007. It either reflects or absorbs an incoming signal, depending on the presence or absence of a nearby block. The block is removed if a reflection occurs.
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The switching signal here is a glider coming from the northwest, converted to a Herschel by a high-clearance syringe variant found by Matthias Merzenich. The syringe is not technically part of the switch mechanism; any standard Herschel source can deliver the signal to the block factory (the two eater 1s on the right side of the pattern). A ghost Herschel marks the Herschel input location. Alternate converter mechanisms could also be used to place the block.
An earlier example of the same type of one-time switch mechanism, also mediated by a block, can be found in the NW34T204 H-to-G. See also bistable switch for a very robust and versatile toggle switch with two input lanes and four possible outputs.