Many homes and offices have hallways that have a light switch on both ends. You can turn the light on or off by flipping either switch. This kind of switching arrangement is called a three-way switch.

Have you ever wondered how these three-way switches work? If you think about it, the switches are puzzling. If the light is on, flipping either switch will turn it off. If the light is off, flipping either switch will turn it on

Say you flip one switch to its On position and the light goes on. Now go to the other switch, flip it to turn the light off, and come back to the first switch. It is still in its On position, but the light is off. To turn the light back on, you can flip the first switch again.

In other words, sometimes the light is on when the switch is up, sometimes it is on when the switch is down. How can this be?

The answer is that both switches are single pole, double throw switches, and they are wired in series such that either both switches must be up or both must be down to complete the circuit. If one switch is up and the other is down, the circuit is open.

Sometimes electricians install one of the switches upside down or wire the three-way switch backwards just to confuse you. Then, the switches work backwards: if both switches are up or if both switches are down, the circuit is open and the lamp lights only when one switch is up and the other is down. But that's not the normal way to wire a three-way switch.

In this project, you build a simple circuit that uses two single pole, double throw switches to show how a three-way light switch works.