Relays and solid state switches allow a person to use a small voltage to control a larger voltage. The following is my attempt at explaining how relays work and how they compare to solid state AC switches. Please remember that working with mains voltage (110VAC) can be dangerous. Let's be careful out there.

Standard disclaimer: I'm not responsible for anyone hooking these circuits up and the results that follow their use.

Relays

Let's start with explaining how relays work. If you are at this page you probably know how a speaker works, right? There's a coil in the back of each speaker and that coil, when voltage is applied to it, moves the speaker cone. The same is true in a relay. There is a small coil in it that, when voltage is applied to it, moves a SWITCH instead of a speaker cone. So if you get a 5 volt relay, if you apply 5 volts to the coil, the switch side of the relay gets moved and changes it's connections (i.e. on or off). If you look on the back of a relay package (one that has a picture on it of how the relay is wired), you'll notice a funny loop-to-loop line. That's the symbol for the coil. It usually has two connections (or pins) to it. The other lines show how the switch is wired. You might also see abbreviations like N.C. and N.O.. These stand for "Normally Closed" and "Normally Open." If a switch is open, that means it is NOT making a connection. If it is closed, that means it IS making a connection.

Ok, let's tie these two terms (N.C. and N.O.) to a relay:

If a switch is N.O., that means when voltage isn't applied to the coil (the coil is at rest), there is no connection (the switch is open). It is called Normally Open because the coil is at "rest" and this is the normal position for it.

So if you apply a voltage to a N.O. relay, the switch inside closes and makes a connection. Remove the voltage and the switch opens.

If a switch is N.C., that means when voltage isn't applied to the coil (the coil is at rest), there is a connection (the switch is closed). It is Normally Closed because the coil is at "rest" and this is the normal position for it.

So if you apply a voltage to a N.C. relay, the switch inside opens and breaks the connection. Remove the voltage and the switch closes.

Relays are mechanical because they have moving parts inside of them. That's why you can hear them click on and off. Because they are mechanical, they are "slow" when compared to electronic switches (i.e. solid-state switches) which have no moving parts.

While a relay can be used to switch more than just 110VAC (because it's a physical connection), one has to take care when choosing which relay to use. The switch's contacts are usually rated at a certain voltage and current carrying capacity and not staying within those specifications can result in something as simple as the device not working to fire and danger!

Solid State Switches

Solid state relays do the same thing (switch 110VAC), only electronically. They still have the "control pins" (where the coil would be if it were a relay) and the input and output pins of the switch. However, there isn't a coil inside. Remember we are talking about switching 110VAC here. Because of this, one has to select the solid state switch designed to accomplish this task. One can purchase these devices from many electronic parts stores. Remember to simply check the ratings and make sure you provide yourself with a safety margin. For example, if the part will handle up to 3 amps of current, don't run 3 amps. Instead run something like half that. This way no one will get hurt because of a device failure and the device will work longer than running it at it's maximum rating.