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I think I want a relay

Discussion in 'Electronic Design' started by [email protected], Aug 23, 2005.

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  1. Guest

    I ride a <a href="">motorcycle</a>
    for a living, as a funeral escort. We're not allowed to use sirens, but
    we can use air horns. We use a Whelen electronic air horn.

    I'm getting my own bike--a retired CHP BMW R1100RT-P. All the CHP gear
    has been stripped, which means I need to re-outfit it for escort use. I
    want to tweak the air horn a bit, and get a relay that will "stutter"
    the air horn, like the phaser siren sound, something like <a
    href="">this,</a> only not
    quite as fast. Maybe 500 cycles per minute, or about 8 per second.

    The bike I'm buying should have separate buttons for the siren and the
    air horn. I'd use the siren switch & button for this stutter tone, so
    that the air horn would have two different power inputs (I'd crimp
    splice the inputs into one). I could probably put the thing together
    myself if I knew what I needed, and it could be built on a Radio Shack
    project board. Inputs and outputs would be 12 volts DC.

    Any ideas or suggestions?
  2. mc

    mc Guest

    The real question here is the electrical requirements of the air horn. You
    might need a rather big relay, which implies a rather slow relay -- or else
    some kind of transistor switching. How much current does the horn draw?
    And how does it work? That is, are we really powering an air pump?
  3. Guest

  4. ehsjr

    ehsjr Guest

    Yes - use a power mosfet for this. Gate it on with
    a 555 for the stutter, or straight from a switch for
    regular horn.

  5. At one time my cars used to bristle with my DIY gadgets, but just
    about all essentials have been built in for a decade or so now. The
    only device in my present car my 'Granny Horn'. A foot operated
    microswitch enables a low duty 555 astable which activates the
    existing horn relay, delivering a series of very short (hence
    'gentle') beeps. Useful not only for grannies about to step off the
    curb but also in sharp bends on country lanes hiding horse riders and

    By far the hardest part of that project was getting wires through the
  6. ehsjr

    ehsjr Guest

    Oops - meant to add:
    Use 1.5K for R1 and Make R2 with a 33K in series with
    a 4.7K. Use 2.2 uF for the cap. See

    Use the diagram for the Astable, about 1/2 way down the
    page. The Mosfet gate goes to pin three of the 555
    through a 330 ohm resistor. The source and drain go
    in series with the horn and the supply, source to +,
    horn between drain and minus. Add a 220K resistor from
    source to gate. An SPF9Z34 will work for the Mosfet.

  7. mc

    mc Guest

    OK, you could use a 555 timer chip driving a power MOSFET, but make sure the
    power MOSFET is rated for 10 amps or more and is on a heat sink.
  8. ehsjr

    ehsjr Guest

    heh - I was wrestling with that for about a half hour, trying
    to cram my entire body under the dashboard to find the &^$*)
    existing wireing harness hole, when my neighbor came over to
    ask whatinthehell was I doing, based on my disheveled appearance,
    bleeding fingers, dirty shirt etc. I told him and he said "piece
    of cake". The guy has a belly about the size of Nebraska - yet
    he was done in under 5 minutes. He repairs cars for a living, and
    was able to kneel on the ground and locate the existing bulkhead
    hole by feel and stuff the wires through.

  9. I read in that ehsjr <>
    Yes, to repair cars efficiently you need to be ambidextrous and able to
    make a mental image of what you can JUST get two fingertips of your left
    hand to. It helps to have had a LOT of practice.
  10. Jim Thompson

    Jim Thompson Guest

    It always amazes me what different skills each hand is most adept at.

    I'm right-handed, yet there are many things my left hand is better
    at... like removing jar lids ;-)

    ...Jim Thompson
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