I'm beginning to wonder how I've ever found time to do the nearly 400 entries that this blog has accumulated over the last two and a half years. It's Friday already and I've only done one entry this week, on top of not much last week either.
One of the highlights of electricity is the two-way switching circuit. This is commonly used on staircases. The idea is that two switches (one at the bottom of the stairs and one at the top) can control the same light (e.g. at the top of the stairs). So you can turn it on by flicking the switch at the top of the stairs, then go down the stairs, and turn it off again with the switch at the bottom. This is often coupled into pairs, so there would be two switches at the top, one for the upstairs light, one for the downstairs light, and two at the bottom.
A simple 'truth table' for the switch would be: Top switch 'up', bottom switch 'up', light on; top switch 'up', bottom switch 'down', light off; top switch 'down', bottom switch 'up', light off; top switch 'down', bottom switch 'up', light on. So changing anything toggles the light from on to off or from off to on.
We have such a circuit in a bedroom. Two switches, one by the bed, and one by the door, control the same light (for people who don't like getting out of bed to turn a light on and off). Except that this light has never worked. I didn't really bother me, because there was another light in the room (on a single switch) which worked just fine, though I have wondered about having an electrician to look at it.
Anyway, I discovered a couple of weeks ago that actually it does work, but in a strange way. The light comes on when both switches are 'up', but only when both switches are 'up'. Any other combination and it's off. Maybe a wiring fault, I'm not sure. I guess what I did when we moved into the house was that I flicked the switch by the door up and down (and got nothing), flicked the one by the bed up and down (and got nothing), changed the bulb, repeated (and still got nothing) and gave up. I must have missed the one combination out of the four that worked.
So, now I know how to turn the light on, but it is a bit of a mystery as to what's happening. If I feel inclined, I might have a look in the switches themselves, to see if that gives me some clues.