Copyright 2006, all rights reserved

Turning on a light from two switches — how it works

Q: How can one light be controlled from more than one switch in different locations? (Lana, Perth, Australia)

Figure 1. A clear light bulb showing the tungsten filament. Courtesy of James D. Hooker, Lighting Equipment News.

A: It does seem puzzling. I’m at the bottom of the stairs, and flip the switch. The light comes on. I climb the stairs, and flip the switch. The light goes off. Odd. I return later to the top of the stairs, and flip the switch to go down. The light comes on. Nice, but how does it work?

Actually, it’s fairly straightforward, given a simple diagram or two to see the logic. But first, let’s consider just a light bulb, powered by a battery as shown in Figure 2.

Figure 2. A battery powering a light bulb. Charges flow from the battery to the bulb; charges light the bulb by colliding with the tungsten atoms. The many collisions heat the tungsten wire, causing it to glow. Courtesy of Advanced Control Technologies, drawing modified by author.

Here’s how the light comes on: Chemicals in the battery separate the negative charges from the positive. This creates a difference in voltage between the two terminals, which pushes charges from the negative terminal through the wire to the light bulb. There the charges collide with the atoms in the small tungsten filament, agitating the atoms, which heats the tungsten wire to extreme temperatures.

For example, the tungsten in a 60 watt bulb gets to about 4500 degrees Fahrenheit (2500 C). Anything that hot radiates light as well as heat. In the case of tungsten, about 10% of the radiation is visible light. The electric charges continue through the light bulb back to the positive battery terminal.

Figure 3. Same as Figure 2, but with the addition of a single switch. The switch is CLOSED, and the circuit behaves the same as in Figure 2.

Now, what happens if there’s a switch in the circuit as shown in Figure 3? No real difference. The battery pushes charges from the negative terminal through the wire to the light bulb as before.

Suppose I flip the switch as shown in Figure 4. The battery attempts to push the charges as before, but the open switch stops the charges since there is no more wire for the charges to flow along. No charge can reach the tungsten filament; the light goes out.

Figure 4. Same as Figure 3, but the switch is OPEN, blocking charge flow to the bulb. The light goes out.

Now, to answer the question: Take a look at Figure 5. We’ve got two switches (A and B) controlling one light. The light is on. It’s not difficult to see why. Charges come roaring in from the negative battery terminal. There’s a closed path for the charges to follow through Switch A — along the wire between switches, through Switch B and along the wire to the light. The charges colliding with the tungsten filament cause the light to shine.

Figure 5. In this figure, two switches (A and B) control the one light bulb. The light is ON because a path exists for the charges from the battery to flow to the bulb. Both switches are CLOSED.

Suppose I flip Switch A. Figure 6 shows the result. The path is broken. Charges still attempt to flow from the battery through Switch A and along the wire (the red one this time) between the two switches. But then stop. They can’t get to the light, and the light is off. The same thing happens if I flip Switch B instead.

OK, now I’m in the situation shown in Figure 6. My light is off, no matter which switch I flipped earlier to turn it off. How do I get it back on? If I flip Switch A, I’m back to the situation of Figure 5, and the light is on. If I flip Switch B instead, then the circuit looks like Figure 7, and the light is on. Either switch turns the light back on again. Clever, eh?

Figure 6. Switch A is OPEN, blocking charge flow to the bulb. The light is out

By the way, three or four switches can also control one light.

Please note: working with electricity is dangerous. This article just describes the logic of a 3-way switch, and is not intended, in any way, as wiring instructions.
A reminder: always disconnect or turn off the power at the main service (fuse or circuit breaker) panel before you doing any electrical work.  Check to make sure it's off.

A disclaimer: Neither I or USA Today is responsible for any damages or injuries resulting from information obtained from this site.

Figure 7. Instead, I closed Switch B, and now a path exists for the charges from the battery to flow to the bulb and light it. 

Further Reading:

The three-way switch by W.J. Rayment, HomeHumor.com

Incandescent lighting by Rod Nave, HyperPhysics

How does a light bulb work by Lois A. Bloomfield, How things work

The three-way switch how-to by Jonathan Johnson, WebMoth.com

How to wire 2, 3, or 4 switches to control 1 light, HandymanWire.com

(Answered March 14, 2006)