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This example shows how to power a 120-Volt alternating current (AC) light bulb
through a circuit of 5 V DC (DC) ruled by the Arduino. It can be used with any
other circuit of 120 V, with a maximum of 10 A (with the relay of the example).
What is a relay?
The relay is an electromechanical device that functions as a switch controlled by
an electrical circuit in which, by means of an electromagnet, actuates a set of one
or more contacts that allow open or close other independent electrical circuits.
From here extract important information: we can separate two circuits in such a
way that they work with different voltages. One to 5 V (Arduino) and another 120
v (the bulb).
9. Use a relay to turn on 120 V devices
As shown in the diagram below,
there are two circuits. Black wiring
operates at 5 V DC and the red one
at 120V AC.