Three-way and four-way light switches
Going to the upper floor is quite straightforward. At the bottom of a dark staircase you turn the light on, then when you reach the top, you simply turn it off behind you. Cool and useful.
As a child I was always wondering how do they manage to connect two switches to a single light bulb making it possible to turn the bulb on and off from both places. But no mater how many simple circuit diagrams I sketched, I never found the solution. Must be some sort of magic.
Then, possibly years later, I learned about three-way switch. This proved useful, because when I next time recalled my old staircase-light-bulb problem the solution become evident in only few minutes. Here it is.
Using two three-way (SPDT) switches you can control your staircase light bulb from two distant places.
On the picture, switches are in on position - alter any of them and the bulb will light off.
Simple and clean. Only you need to pull some more wires around your house.
After solving my problem, I somehow lost further interest on this mater. Without real reason I made myself believe that the maximum number of switches that can control a single light bulb is two. I was thinking that is not practical, if even possible, to put three or more switches in the circuit.
Well, I was wrong. Recently I re-thought this problem and found that it is possible to add as many switches as wanted and control the light bulb from many places without using a single relay. Surprised I made a quick search on the internet and found hundreds of pages showing the same thing the four-way switch in action.
The four-way switch simply exchanges two wires like in the picture below.
Usually the four way switch is made by some internal wiring of a DPDT switch.
And, finally, how can it be used to turn light on/off. Here is how.
Charming and simple. Only the four-way switch is somewhat harder to obtain... But anyway, never assume something is impossible.
Danijel Gorupec, 2006