Circuit to switch LEDs on 12V but not 7V

Discussion in 'The Projects Forum' started by losinj, Oct 25, 2010.

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  1. losinj

    Thread Starter New Member

    Oct 25, 2010

    I'm after a little circuit to switch on some LEDs in a DIY high-level brake light on a motorcycle. The circuit will need to trigger from +12V when the brakes are on. When the brakes are off there is +7V unexpectedly still present, which is enough to make the LEDs glow at reduced brightness, but I need them to be completely off when the brakes are off.

    Could this be solved with a simple transistor circuit (and no relay)? The LEDs each have a 510 Ohm series resistor, and draw about 80mA in total.

    If it is possible to power the circuit from the existing connection to the brake light, that would be even more excellent as then I wouldn't need to wire up a separate 12V feed. (I've seen some tutorials on Schmitt trigger circuits, but I haven't a clue how to apply one in practice, or if it would be suitable.)

    Any help gratefully received.
    Last edited: Oct 25, 2010
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    The 7V is probably the taillight power.

    Changing out lights for LEDs by DIY isn't encouraged in the US, so isn't often discussed on the forum. See the readme tacked thread in General Electronics Chat. There are way too many rules imposed by the .gov on color, brightness, and viewing angle of every light on a vehicle.

    If it is an aftermarket replacement for your vehicle, you should attempt to contact the supplier for support.
  3. bertus


    Apr 5, 2008

    Never change the lights on a vehicle yourself.
    Go to an authorised dealer.

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