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Thread: get power (current) inside the steering whell, through the horn button.

  1. #1
    Variable Bitrate
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    get power (current) inside the steering whell, through the horn button.

    hello forum. i want to pass power, inside my sterring wheel, to do some stuff (in our case, asuming to power some LED's is ok). the problem is, that since my car is prety old, there are only two cables going inside my sterring wheel. the cables that power the horn. so i have to make due, with them.
    here is what I THINK the schemantics of my horn look like :
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    meaning, that , when you press the horn, the +12v rail that goes to the relay, closes, the relay energizes, and allowes current to go to the actual horn.
    now , what i want to do is something like this :

    Name:  honr + led.png
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    meaning, i still keep the horn button, only with the addition, that is bypassed, with a rail that poweres some LED's. becouse the voltage drop of the LED's in my case is about 12v, the relay ALWAYS get cureent (not only when the button is pressed), but its about 1v, so it dosent energize.
    ALL fine and good and tested up to now, and it works prety good, with one exception.
    when i turn on the car, the LED's turn on, and no horn sound. BUT when i press the horn button, the horn starts hitting, BUT it dosent stop when i realise the button...
    anyone has any idea what i have done wrong??

    or if there is a better way to pass power inside the wheel, at all??
    thanks in advance!!

  2. #2
    Confusion Master
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    The horn button is a simple switch to the horn.

    The output of the switch is still 12v not technically GND.

    When you start the car 12v goes to the switch and stops, it also goes to the LED's where it powers them but because they have a small resistance they stop enough power going through to activate the relay.

    However when you press the horn button enough power goes through to activate the relay, but when you let go the small amount of power that gets through the LEDs is enough to keep the relay closed.

    Basically the relay needs just a little extra push to switch than to keep it made.


    I'm sure oldspark will be along to give you a more technical explanation at some point. (and probably debunk what I have just said)

  3. #3
    Variable Bitrate
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    yes... i experiamented on the bench a bit, and i found out, exactly that... but how can i go around it?? any suggestions?

  4. #4
    Raw Wave
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    Rats - I had to be curious didn't I? But I know never to debunk a mod, nor THE Enforcer.

    Actually horn switches are grounding. IGN +12V is connected to the horn relay input AND coil - ie, +12V to the "B" terminal which is 30 and 86 using a conventional relay.
    The output H = 87 is the switched +12V which sounds the grounded horn.
    The S = Switch = 85 is the ground end of the relay coil which goes via the steering-wheel's slip ring to the horn button. The button's other side is to GND via the steering column shaft.

    A voltmeter on the slip ring (ie, the non-GND side of the horn button) will read +12V with IGN on because it is connected to IGN +12V thru the relay's coil.
    Hence it is possible to power LEDs and even charge batteries etc, but if the current approaches that of the relay coil's energisation current, the relay energise and sound the horn.

    Most automotive relay coils are ~60mA but horn relays may be even higher. Though a 20mA LED may not be enough to energise a 60mA relay, it might be enough to keep the relay energised, hence a LED or LEDs may be ok, but once the horn button is pressed, the horn may latch on. It's a case of knowing the relays pull-in current and release current and allowing for (say) a 14.4V supply, else experimentation - and adding adequate margin.


    Solutions include a resistor - or bulb (which may be better due to lower cold resistance) - in parallel with the relay coil to provide more current to the load/LEDs without going thru the relay coil, or using a lower resistance (higher current) relay coil.
    There are also cap or battery charging systems but they are suited for occasional loads only - eg, a few Amps for a transmitter or alarm etc. The duty cycle of the load and hence its average power/current consumption must be less than what the "trickle current" (maybe 10mA etc) can provide to charge the battery or capacitor between horn and load usage with IGN on.

    I have some vague idea (or recollection???) that current sensing solutions can also be used. IE - above a certain current means the button has been pressed so the current sensing circuitry activates the horn relay whereas below that current means it's merely the load (LEDs etc).
    (Resistive voltage dividers have been used but that's to detect which switch on the steering wheel has been pressed. It's not suitable for powering loads. But then there are PICs that consume 14uA etc that could use AC signalling thru the horn wire to control devices at either end...)


    I reserve the right to reject embarrassment re the above. Even ignoring the gig I've just returned from (relatively sober I hope - after all, I drove home), I know this "single wire" manipulation gets real tricky. The number of times I've had some "brilliant" idea only to later realise its obvious flaw!
    Last edited by OldSpark; 10-19-2013 at 09:11 AM.

  5. #5
    Maximum Bitrate rray's Avatar
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    Wow, you invented the latching relay. It might require you to add a normally closed momentary switch inside the horn button that breaks the LED circuit while the horn is pressed, and the LED's go off temporarily. Adjust to make sure the led loop opens before the horn blasts.

  6. #6
    Confusion Master
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    Maybe the question, "why do you want LEDs in your steering wheel?" should be asked.

    Also telling us the Make, Model and year of car may help us figure out a solution for you.

    Take me as an example.

    I have a BMW 3 Series E36, these did not come with Steering Wheel buttons, but the BMW 3 series E46 did. I wanted to retrofit the E46 Steering wheel with buttons into my E36, but needed a way to get extra wires through the steering wheel slip ring. The E46 one wouldn't fit the E36. But the I found the 5 Series E39 had steering wheel buttons and the slip ring was the same as the E36 one, but with the extra wires.

    http://www.mp3car.com/input-devices/...-retrofit.html


    So maybe a later year model might have a slinp ring with extra wires in that may fit.

  7. #7
    Variable Bitrate
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    audi 80 1992. i also need to retrofit buttons, but i am doing it with a wirless remote control INSIDE the steering wheel, so the extra power is just for the leds that light the button's. (not so neccecery)

    thanks Old'spark for the detailed analize. i will do some more experiments, and give back the results!!

    edit: in my mind, i though that the relay, was AFTER the the horn button. not before.
    with that sed, the resistor in parallel as you suggested, seems pretty appealing
    Last edited by settra; 10-19-2013 at 11:44 AM.

  8. #8
    Raw Wave
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    rray - very funny. I hope others get the jokes - especially about the NC switch.


    settra - the button is "in series" with the relay coil. Before or after makes no sense; it's merely in the grounding side (aka ground switching like some lighting circuits, most ignitions, most digital circuits, etc).

    Paraphrased albeit poorly, the load or component sits BETWEEN the supply (...obviously). EG - the relay coil or horn is between +12V & GND with its switch anywhere in series. The +12V relay coil is grounded thru the horn switch, and the relay contacts switch +12V to the horn (and the horn body is grounded thru the body etc). [ Hence a 3-terminal horn relay is a normal SPST relay with combined 30 & 86. ]

  9. #9
    Variable Bitrate
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    well. i have tried the resistor think, with a resistor of 12ohm value, but with no luck. although i should try a bit more before rouling it out, i though of something else :

    what if i completly disable the cables that go from the horn realy , to the steering wheel, and replace them with normal 12v power cables. that way i can use the power to do whatever i want inside the steeering wheel.
    now, for the actuall horn, i could have the button work like this : when i press it, it will power an RF or whatever transmiter inside the steering wheel. outside of the wheel, i have the reciever, that when recieves, closes the loop for the horn relay.
    that way, i am not limited at what power i want to use inside the wheel, and still have the horn work "flawless". although it is some work. what do you guys think ?

  10. #10
    Raw Wave
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    12 Ohm is low enough to power close to 50 LEDs, but you haven't said how you intend to control them.

    But yeah, the dedicated power with transmitters will work. That's what I meant with the PICs etc though I didn't clarify the dedicated 12V supply.
    The limitation is the slip ring - both its max current, and noise or bad contact and interruptions, but that's where the battery or cap circuit comes in.

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