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I'm thinking of using a switch in my car. The switch has 5 leads: +, -, C, NO, and NC. 12 V to the + and - light the LED (brightly). What I would like it to do is light up the LED dimly when the dash lights are on AND light it more brightly when the switch is on.

How would I do this?

Or would it be simpler to have it light up dimly when the switch is off using a resistor between NC and +, and then brightly when the switch is on (direct wire from NO and +)?

I'm thinking this could be a problem because since the device I want to turn off and on will be wired to NO; it will get power from NC (through +) when the switch is off. This wouldn't be good! Is there a way to prevent this? Or perhaps a better way to go about the whole thing (apart from having a switch with 2 LEDs)?

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  • \$\begingroup\$ Apparently, the LED is bright with the full 12 V applied (dash lights active.) It probably has a built in resistor for that purpose. Assuming you cannot gut the switch, you need to either lower the initial voltage, returning it back to normal when the switch is engaged or else work out a two state current source controlled by the switch state, I think. \$\endgroup\$
    – jonk
    Commented Jan 24, 2019 at 13:24

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It would be easiest to start with the dimmed and switch in a parallel resistance with the NO contact for the brighter current. This assumes the 12V supply is also switched on when the key is turned to the on position.


schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks for quick reply but ... where would I attach the wire for the positive of the devise I am turning on and off? - is that called the load? Would it attach at the base of the switch in your diagram and if so, wouldn't it get power from the middle of your vertical lines even when the switch was in the off position? \$\endgroup\$
    – Robert600
    Commented Jan 23, 2019 at 1:36
  • \$\begingroup\$ maybe the 2 resistors the current would have to travel through prevent that? \$\endgroup\$
    – Robert600
    Commented Jan 23, 2019 at 1:38

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