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Motorcycle Turn Signal project

Discussion in 'General Electronics Discussion' started by Derek R, Feb 1, 2017.

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  1. Derek R

    Derek R

    Feb 1, 2017
    I am trying to increase visibility of my motorcycle by adding red running lights that also function as a brake light to the rear turn signals on my bike. I have already picked out which lights I will use, Putco 1157 red LED's.

    I have clear turn signal covers/lenses and I am going to modify the housings so that each rear housing fits one 1157 bulb and one 1156 bulb. (1157 has two positive terminals, 1156 only has one).

    I will have 4 leads going to the rear turn signal housings, ground, turn signal, running light, and brake signal. I want to wire this LED so that it does not turn on when the turn signal is on.

    When I turn the turn signal on, the running light lead turns off (I believe it floats when it is off, I am going to double check tonight by checking the voltage of the lead and then checking for connectivity between the lead and the ground). I am thinking I could use the running light lead to disable the brake light from turning on while the turn signal is engaged.

    Originally I thought would be able to make a simple circuit with just one transistor, with the source connected to the brake power lead, the drain connected to the brake lead on the LED, and the gate lead connected to the running light. This way, when the running light is off, the transistor won't allow current to go into the brake lead to the LED. However, I wasn't sure if this would work because the gate and source would both be exactly the same voltage, and I'm also not sure if the transistor could possibly cause the brake lead to be activated by the running light lead.

    Does anyone have any ideas on what type of circuit I should build to make this work the way I'm thinking?
  2. Derek R

    Derek R

    Feb 1, 2017

    I just checked the leads to see if the running light floats or if it "flashes" or is grounded when the turn signal is flashing. I am measuring with a Fluke 115 True RMS Multimeter. I set the range to manual, one decimal place, and I saw the turn signal flash from 0 to ~6 to ~11.5V (its sampling rate isn't that high). The running light on the other hand flashed from 0 to ~2.5 to ~5V.

    This could be an issue. I think however, that the apparent flashing of the running light is due to the way I have the mirror turn signals configured. The mirrors only have one light lead, but I have them powered on constantly by combining the running and flashing leads into one lead using diodes, so that the leads themselves are isolated from each other.

    I thought about the above theory and decided to check the voltage of the flashing lead when the turn signal was off. If the flashing lead is raising the voltage of the running lead through the diodes, then surely the running lead should be raising the voltage of the flashing lead through the diodes. When I checked the flashing lead however, it was +.5mV at one time and -80mV at another, so I think it does definitely float.

    I think the only way that I will know for sure is if I disconnect my wire harness that has the diodes in it and check the voltages of the running and flashing pins.

    I am replacing the LEDs in the mirrors, so getting rid of those diodes is an option, as I can re-wire the mirrors to have separate running and flashing LEDs.
  3. tedstruk


    Jan 7, 2012
    If the bike is already wired for LEDs, make sure the diodes are the right ones for the LEDS, but if its wired to standard bulbs, you will need resistors made for the LEDS.

    What you are considering is done with more relays. Factory bike electronics are very touchy, even a weak resistor can burn up the regulator. When you add bulbs, you are increasing the resistance, and if you put too many bulbs on, your charging circuit won't do its job because its only designed to power what they put on the bike originally.

    I built a taillight out of a set of cadillac lights, but the drain on the generator was so high, that I wasn't getting any charge on the battery, I ran it for about 15 minutes and the battery sulfated.
  4. Derek R

    Derek R

    Feb 1, 2017
    The bike is made for standard bulbs, but I replaced the turn signal relay with a relay made for LEDs or standard bulbs (up to 130W). The new relay however does not have a switch time that is load dependent, so the LEDs do not flash too fast.
  5. Derek R

    Derek R

    Feb 1, 2017
    I'm now thinking I should change the way I go about doing this. Instead of trying to make sure that the brake light stays off when the running light lead is off, I could make the brake light off when the turn signal light turns on. Also this would make it so that when I have a turn signal and the brake on, my turn signal would change between red and amber.

    Would anyone on here be able to help me make a circuit with 2 inputs and one output that does the following:
    Input 1
    Input 2
    Output 1

    Output 1 is +14.4V (1A max) when input 1 is +14.4V and input 2 is 0V
    Output 1 is 0V when input 1 is +14.4V and input 2 is +14.4V
    Output 1 is 0V when input 1 is 0V and input 2 is +14.4V

    Input 1 would be hooked up to the brake light lead, input 2 would be hooked up to the turn signal lead, and output 1 would power the bright lead on the brake LED. 0V is floating.

    Would I need a separate power lead that is always +14.4V? I could do that if so.

    I wish I could edit my first post and say "see 5th post!" haha
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