Simple automotive switch circuit help

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  #1  
Old 08-22-09, 11:30 AM
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Location: Princeton, NJ
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Simple automotive switch circuit help

Apparently my brain is getting old. I used to be able to design simple circuits without fuss but I'm in need of some assistance with this one.
The design is to turn a simple SPDT Center off switch (aka on/off/on) into an on/on/off . The switch has just 3 poles. The center pole is isolated when "off" and the up/down connect the center to the outside poles respectively.
Can't this be done with a relay? What I have is a GPS that I would like to have battery/ignition/off . Kind of an on/auto/off if you would. Trouble is my circuit that works is charging the coil of the relay when OFF. This is obviously a problem so if somebody could help with a sketch or good description I'd really appreciate it.
 
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  #2  
Old 08-22-09, 12:45 PM
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It's easy with a center-off SPDT switch if you will settle for batt/off/ign. You want to feed the "selected" terminals from the power sources (battery or accessory) and the common goes to the GPS.

v From battery (always hot)
|
|
o (Top or Bottom)

o (Common) ------> To GPS

o (Bottom or Top)
|
|
^ From accessory power

If you insist on batt/ign/off, use a SP3T switch and don't connect the third "selected" terminal to anything.

Mouser.com and other electronic part vendors have SP3T (on-on-on) toggle switches in various sizes, styles and terminal types.

Mouser toggle switch selector. Select SP3T from the Contact Form filter, and your termination style and other filters as desired.
 
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