# How light switches work?

Discussion in 'General Electronics Chat' started by inkyvoyd, Dec 7, 2011.

1. ### inkyvoyd Thread Starter New Member

Dec 6, 2011
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Hello, I'm sort of new to most electronics, but a recent interest in circuits led me to ask myself how circuits for certain light switches work. Besides the most simple, obvious case with one light and one switch, I've seen other cases which I'm curious about. The first one would probably be two switches and one light, each of the switches cancels each other out. (Say if you are inside a house and you want the porch light to be able to be turned on from both inside and outside, with this light switch installed both inside and outside the house, you can turn on/off the light from the inside, but you can also do the same thing from the outside). The second, and probably harder, would be 4 lights and one switch, where repeatedly flipping the switch turns on one more light every time. Say you flip it on the first time, and one light turns on; you flip it off and then on 2 lights turn on; you flip it on and off again, 3 lights turn on; you flip it on and off once more, 4; and if you do it one last time only one light turns on. If you leave the switch off for a while at say, right after 2 lights, and turn it back on, it'll be 1 light and not 3. I know I'm not very good at explaining things (and probably going to kill someone with those semicolons), but an explanation would be awesome.

2. ### indiansiva555@gmail.com New Member

Jul 24, 2011
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(Say if you are inside a house and you want the porch light to be able to be turned on from both inside and outside, with this light switch installed both inside and outside the house, you can turn on/off the light from the inside, but you can also do the same thing from the outside)
This can be achieved using a 2 way switch.(Google it....)
Hope its right...
Please correct me if i am wrong....

Dec 26, 2010
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The second type of "switch" is actually a digital control circuit using a logic IC, switching a number of lamps, typically using TRIAC solid-state switching devices for AC lamps.

This is fairly complicated, but the other switch systems used for stairs, corridors etc. are sometimes fairly simple arrangements using passive switches. This Wikipedia link shows some examples. http://en.wikipedia.org/wiki/Multiway_switching

4. ### debjit625 Well-Known Member

Apr 17, 2010
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It will be better if you understand circuit diagrams ,anyway give it a try ,if you don't understand ask us...

That can be done by digital logic like using electronic circuits,relay logic,etc.

Good Luck

Dec 26, 2010
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Note that apparently electricians in different countries have different terminologies for the Single Pole Double Throw (SPDT) switches used in some of these arrangements.

Here in the UK we call an SPDT switch "two-way", but in the USA the same item is referred to as "three-way" apparently. As I believe a famous man is said to have said (G B Shaw? Oscar Wilde? or both?) "We are two nations divided by a common language".

6. ### SgtWookie Expert

Jul 17, 2007
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Adjuster, you bring up an interesting point that I'd never thought about. I have heard those light switches called "3-way switches" since I was a youngster, and never considered how incorrect that term actually is.

It originated because the switch had three terminals rather than the usual two terminals, instead of going by the number of throws. Perhaps they're afraid to correct the terminology now as it would throw thousands of electricians into a tailspin.

7. ### GetDeviceInfo Senior Member

Jun 7, 2009
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pretty sure they're termed 3 way to describe the number of operations. Off/on in one position/on in another position. Two way is on/off.

8. ### sheldons Well-Known Member

Oct 26, 2011
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These switches are single pole double throw..spdt used for two way switching as in the example shown above ,and it is referred to as a two way switch...one pole,2 positions,normally open or normally closed but if you look on the web here
http://www.handymanwire.com/articles/3wayswitch.html
they are referred to as 3way switches.

Last edited: Dec 7, 2011
9. ### inkyvoyd Thread Starter New Member

Dec 6, 2011
25
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Thanks for all the helpful responses.
Is the digital logic behind my second question simple, or complicated? I'll probably be able to understand 10 or 20 logic gates put together if it's possible to explain it that way.
PS: debjit, that was helpful, thanks.

10. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
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That would make sense.

Switch one: flip 1 turns light on
Switch two: flip and light turns off
Switch one: flip and light turns on

So there's "Three ways" to toggle the light's state, but it still doesn't make as much sense as "double throw", which probably doesn't make any sense to those not familiar with the term.

Dec 26, 2010
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I too was quite unaware of the difference in terminology (UK vs USA) for these switches, until looking into this thread. Perhaps on an international forum it would be best to stick to SPST, SPDT etc, which are unambiguous.

In notes aimed at readers without much background in the subject, these terms can be written out in full, and defined as necessary. Finally, as we are always telling newcomers, it helps to show a diagram. This should avoid any possible confusion e.g. with a Warsaw shot-putting competition.

Last edited: Dec 8, 2011
12. ### GetDeviceInfo Senior Member

Jun 7, 2009
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Now with that being said, hydraulics use the 'way' term to describe directional valving. I've been a licensed Millrwight for 25 years and have worked extensively with hydraulics, but never subscribed to the 'way' descriptor. Guess I just saw it differently.

13. ### sheldons Well-Known Member

Oct 26, 2011
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ok single pole single throw(spst)-one pole and one switched contact(could be nc or no-but then you are wandering into the "make before break" catagory..)single pole double throw(spdt)-one pole and 2 contacts-n/o and n/c then you have dpdt etc the list goes on....

Dec 26, 2010
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For SPST / 1 way switches, the normally open/closed distinction is important for momentary types (e.g. push-buttons, limit switches, micro-switches), and of course relays. This applies no more and no less whether you call them SPST or one-way switches.

Similarly, multi-throw mains switches are usually not Make-Before-Break, because in some applications this could be dangerous (it might cause momentary shorts). There may be rarer applications where it is a momentary disconnection which must be avoided.
The Break-Before-Make vs Make-Before-Break distinction is therefore also relevant, but again, nothing to do with whether the switch is called SPDT, or UK 2-way, or US 3-way

15. ### sheldons Well-Known Member

Oct 26, 2011
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i was referring to some low current microswitches ive come across recently,but it seems we have opened a bit of a debate here.....