Which switch do I need? (SPDT, DPDT, etc...)?

Question

I'm making a house lamp that has a primary bulb at the top, and a secondary string of lights around the base. I need a single switch to have three states:
 1.) Middle state controlling only the string of lights at the base. (Power on)
 2.) Top state controlling both the primary bulb and string of lights. (Power on)
 3.) Bottom state, both lights off

Violet W · Accepted Answer

There is a 4-position switch that is quite common and is used for this type of application.  It comes either as a rotary switch, a push button, or a pull chain. It is called a 3-way canopy switch or 2-circuit canopy switch (not to be confused with a 3-way wall switch which is 2-position switch that won't work for your application). It controls the lights in sequence of off-low-medium-high-off... where
low = one set of lights
medium = the other set of lights
high = both sets of lights
off = no lights

Here are some examples.  If your voltage system is higher than 120VAC nominal, then check with the manufacturers to verify allowable voltage range:
rotary:
http://www.amazon.com/Gardner-Bender-GSW-68-Rotating-Switch/dp/B000BVXVZO
pushbutton:
http://www.ebay.com/itm/120491086431
pull chain:
http://www.mylampparts.com/Products/Compact-3-Way-Pull-Chain-Switch__SL19336.aspx

You might consider dropping into a lamp store an browsing their replacement-part offerings.

Anonymous · Answer

Any form of switch that has a common contact that can be joined to any one, and only one, of three contacts under a manual or an automatic / remote control system. Here's an example. Contact A (common). Contacts B, C, D (selectable). A can make contact with B or C or D. A cannot make contact with B and C, nor B and D, nor C and D (unless it is a 'make-before-break' action - which is not common) A cannot make contact with B and C and D. B cannot contact C, nor D C cannot contact D. The actual shape, design, mechanical or electrical arrangement(s) etc. are not relevant within the definition. Now if, for example, you had said "a 3-way toggle switch" . . . .or a "3-way slide switch" . . . for example, the story might be a little different. Despite their apparent simplistic nature, switches can actually get quite complicated. Just 'look' inside any PC: logically, there are millions and millions of 'switches' in there! Incidentally, you're not thinking of a 3-position switch, are you? This is where a ceiling light - for example - can be switched on or off from 3 positons; control from only 2 positions is much more common. .

Hankm · Answer

if you get a (dpdt center off ) switch you can do that with some juggling of positions ( swap position 2 and 3 or 1 and 3 )
connect string to both terminals of one pole
connect main light to only 1 terminal of other pole

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Which switch do I need? (SPDT, DPDT, etc...)?

3 Answers