How relays work?

A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts. Relays are found hidden in all sorts of devices. In fact, some of the first computers ever built used relays to implement Boolean gates.

Relay Construction

Relays are amazingly simple devices. There are four parts in every relay:

• Electromagnet
• Armature that can be attracted by the electromagnet
• Spring
• Set of electrical contacts

In this figure, you can see that a relay consists of two separate and completely independent circuits. The first is at the bottom and drives the electromagnet. In this circuit, a switch is controlling power to the electromagnet. When the switch is on, the electromagnet is on, and it attracts the armature (blue). The armature is acting as a switch in the second circuit. When the electromagnet is energized, the armature completes the second circuit and the light is on. When the electromagnet is not energized, the spring pulls the armature away and the circuit is not complete. In that case, the light is dark.
When you purchase relays, you generally have control over several variables:

• The voltage and current that is needed to activate the armature
• The maximum voltage and current that can run through the armature and the armature contacts
• The number of armatures (generally one or two)
• The number of contacts for the armature (generally one or two — the relay shown here has two, one of which is unused)
• Whether the contact (if only one contact is provided) is normally open (NO) or normally closed (NC)

(Source: HowStuffWorks)