Ridiculously stupid question. DPST switch

Discussion in 'The Projects Forum' started by CRNUT, Feb 7, 2008.

  1. CRNUT

    Thread Starter New Member

    Feb 7, 2008
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    This would have been easy for me back in the day but I am lost right now. I want to control 2 12V light bulbs with a SPDT switch. The switch needs to light one bulb in one position and both bulbs in the other position. Every diagram I can think of will light both bulbs in both positions. Can't be that hard maybe just a diode somewhere. Can anyone draw me up a simple diagram?
     
  2. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    No, with just a switch you will need a DPDT. Otherwise, it will require a logic circuit.
     
  3. CRNUT

    Thread Starter New Member

    Feb 7, 2008
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    So no easy way around this even if I control the bulbs with relays?
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Well if one of the bulbs is always on, you could just switch the other on and off.
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Yes, you could use a relay or two to make a logic circuit. But why not just use a different switch?
     
  6. CRNUT

    Thread Starter New Member

    Feb 7, 2008
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    Because the switch can't be changed. Its built in...
     
  7. SgtWookie

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    Jul 17, 2007
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    This is 12V DC, right?

    Is the switch simply SPST, or does it have a center off position as well?

    See the attached - it uses a single diode in addition to your two bulbs and switch.

    The diode will have about an 0.7v drop across itself, leaving one bulb slightly less bright.
     
  8. CRNUT

    Thread Starter New Member

    Feb 7, 2008
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    Correct 12V DC, with a center off.
    I want
    middle---nothing lit,
    bottom--one lit,
    top------both lit

    Two bulbs and preferably two basic 4-5 pin relays (automotive type)
     
  9. CRNUT

    Thread Starter New Member

    Feb 7, 2008
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    I can control power or ground to the bulbs as well...
     
  10. SgtWookie

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    No relays needed - just a diode. See the schematic in my last post.
     
  11. CRNUT

    Thread Starter New Member

    Feb 7, 2008
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    I should be about to do the same thing as your diagram but replace the bulbs with the primary relay coils and I should be ok?

    I don't think the switch is rated to handle the bulbs thats why I wanted to use relays...
     
  12. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Ummm, the 1N400x series is only good for 1 amp which may not be sufficient. Make sure the switch and the diode are rated at least 2x above the currrent of the lights.

    I = W/V
    2.1A = 25W/12V
     
  13. CRNUT

    Thread Starter New Member

    Feb 7, 2008
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    Radioshack 276-1661???
     
  14. SgtWookie

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    Jul 17, 2007
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    Ahh, ok. I was thinking you were working on radio indicator lights or something.

    Sure, in the schematic I posted, just replace the lamps with the relay coils.
    You'll need two SPST relays in addition to the diode.

    Also, use diodes in reverse bias across the relay's coils to absorb the reverse EMF when the current is interrupted, otherwise your switch contacts will have a short lifespan due to arcing.
     
  15. CRNUT

    Thread Starter New Member

    Feb 7, 2008
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    I am actually installing HID headlights on my four wheeler and trying to use the factory mounted switch built into the handle bar controls haha
     
  16. SgtWookie

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    Jul 17, 2007
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    Oh, OK! Yeah, the current draw of HID headlamps would've cooked the switch pretty quick!
     
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