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Relays are mechanical switches. But instead of a lever or button, you switch them, and hold them in that position, by passing current through a coil.
Relays tend to come as Double Pole, Double Throw (DPDT) switches. That means that there are 2 separate switches, each of them having a common terminal (pole) that can be switched between 2 separate terminals.
This example circuit schematic diagram shows a Single Pole, Double Throw (SPDT) relay symbol that I came up with. By wiring up just one of the 2 switches in a DPDT relay and leaving the other side unconnected, you effectively have a SPDT switch.
One coil is used to switch both switches (at same time). So of course, you always need to power both sides of the coil.
There doesn’t appear to be a universal relay symbol, but most of the ones I have seen, resemble the symbol I drew here. Everything in the Gray box is part of the relay symbol.
Latching or Bistable Relay.
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