Quiz Question

One of a continuing series


When you want to turn on the stair lights while going up the stairs, and you want to turn them off once you get to the top, you need this bit of minor technology. The lights are controlled by two switches. One switch is at the bottom of the stairs, and the other switch is at the top. The switches are “double throw” type. That means the switch is connected to two circuits, and the switch has two positions. When you toggle the switch it turns off one circuit and turns on the other. If the light was on, then throwing the switch turns the light off. If the light was off, then throwing the switch turns the light on. Neat.

Now suppose you have a three-story house, and you want to turn the stair lights on when starting up, and you want to turn the lights off when you get to the second floor, and also off when you get to the third floor if you are going that far. How are you going to be able to achieve that? Do you need a new kind of switch?

What if you have a 15-story house?

Post your answer in the comments below.


Some comments have been received, and it’s apparent clarification is needed. This Quiz Question involves single-pole, double-throw switches. Here is how a SPDT switch works:


Update and Solution

The two-switch feature can be extended to more than two switches, but the configuration I had in mind does not work. I had to go to Wikipedia for a workable solution. First, here is how the two-switch configuration works.




Switch 1 and Switch 2 are SPDT switches. In the first configuration no power is supplied to the load, which is typically a lamp. In the second configuration power is supplied to the lamp, and it turns on.

It is trivial to extrapolate from this and determine that flipping either switch turns on an off-lamp and turns off an on-lamp.

To extend this idea to more than two switch requires a different kind of switch, but still a mechanical switch. The following are from Wikipedia:

Readers are invited to visit the Wikipedia article, following which they will be able to install multi-way lighting switches for their five-story condo.


5 thoughts on “Quiz Question

  1. I think I know how this works, since you introduced me to the technology. You have one switch that controls the circuit and all all the other switches just send a wireless signal to the controller switch to turn the circuit on or off.

  2. Stairway switches aren’t double-throw switches, they are two-way switches.

    Double-throw means the switches act in tandem. (Turning one switch to one pole will do the same simultaneously in the other switch. Usually they are mechanically connected by a rod.)

    For 3-stories or more, the ‘off’ position should reset all the switches to off position by another circuit which releases them all to a off pole. Then an ‘on’ will turn on the bulb. (Meaning, the two-way switch will not work.)

  3. Yes, two-way switches are SPDT switches. They won’t work for more than two such switches. All I can think of is the “release” to ‘off’ position — when turning off one switch will turn off all switches — is a mechanism that will work.

  4. Pingback: Quiz Question | Skeptical Analysis

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s