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Automotive - using turn signals as combo turn signals and running lights

Discussion in 'Electronic Basics' started by Greg Bender, Dec 15, 2004.

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  1. Greg Bender

    Greg Bender Guest

    I have a vehicle equiped with turn signals, but no running lights. I would
    like to be able to use the turn signals as combination turn signals/running
    lights. For a variety of reason, I cannot replace the pigtails or the
    sockets to accommodate dual-filament bulbs. Instead, I've come up with the
    following solution:

    Use a relay (one per side) and connect the turn signals to the normally
    closed circuit (power to the turn signals coming from the tail light).
    Activate the relay by use of the turn signal lever on the steering column.
    The automotive flasher unit then turns the turn signals off by switching the
    relay to the normally open circuit (to which I have nothing connected).

    Standard automotive flashers (the thermal type) require some load in order
    for them to operate (flash properly). My design above does not place a load
    on the flasher, and thus my turn signals don't flash. One option would be to
    place an artificial load on flasher by using a low resistance, high wattage
    resistor. This works, but generates a tremendous amount of heat (which I
    really want to avoid). I've looked into creating my own flasher using a 555
    chip. But since I'm very new to electronics, I'm not sure if this would
    solve my problem, either.

    Any help is appreciated!


    Greg Bender
  2. Rich Grise

    Rich Grise Guest

    This looks like something I've always wanted to try. Use one 555 (or half
    of a 556) to run a chopper at about 100 Hz or so, with a duty cycle enough
    less than 99% to dim the lights visibly - to make this work, you might
    have to use a brighter bulb type.

    Anyway, use that 100Hz pulse to chop the 12V to both of the lights, like
    with a MJ2955 PNP power transistor.

    Then, use another 555 (or other half of the 556) to make your flash rate,
    and simply drive another MJ2955 to turn the one light to full-on for the
    duration of the flash. You'd use a couple of diodes to make an "OR"

    I'm way too lazy to draw a schematic, but it's such a simple circuit in
    concept, that there will probalby be at least two or three up in a matter
    of hours. ;-)

    Good Luck!
  3. Greg Bender

    Greg Bender Guest

    Thanks Rich! Hopefully someone will post a schematic.
  4. Guest

    Hey Greg
    Rich's idea was to dim the lights but if you want to do it easier you
    can skip the chopper/dimming half of the circuit. Just turn them on
    and off. The manufacturers' datasheets for the 555 on the internet
    have example circuits. The 555 by itself won't deliver enough current
    for the turn signals, so you will have to drive a relay or power
    resistor from the 555.
    Or if you want to make it really simple you can go back to the resistor
    idea and I would suggest instead of using a resistor use a 12 volt bulb
    that draws enough current to make the flasher work. Bulbs make great
    power resistors. Stash it under the dash or hood where it won't bother
    There is one disadvantage to the whole thing. With your car's original
    wiring you have the safety factor of knowing when one of your turn
    signal bulbs burns out, because the flasher rate will slow down or stop
    flashing altogether. If you change your circuit so that you use a 555
    or a flasher with an independent load, you will have no feedback from
    the turn signal lights, so you won't know if one of them burns out.
    Makes it more likely that you will get stopped by the police, and they
    will find the recreational substances.
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