Brief Description of a Relay
A relay is a device that accepts a voltage to close or open (normally closed or normally open, or both) an internal switch in the relay. That is to say, a relay is a switch that is triggered by a voltage. The relay can control much higher voltages than what it takes to control it. The voltage that the relay will accept to control the switch is in this case 5v. Relays also exists that accept all kinds of voltage values. The triggering voltage is used to fire a coil that actuates the switch, so a bit more current may be necessary than what the microcontroller output pin can handle, so a transistor may be needed.
Relays are also available as latching and non-latching. That is to say, non-latching is when the voltage is applied the switch triggered until the voltage taken away. Latching is then the relay receives the voltage and when the voltage is released, the switch stays triggered. The switch will un-trigger when another pulse of voltage is sent.
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