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

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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 :
    Click image for larger version

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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 :

    Click image for larger version

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


      • #4
        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, 09:11 AM.


        • #5
          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
            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.


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


            • #7
              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, 11:44 AM.


              • #8
                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
                  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
                    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.


                    • #11
                      my onlu concern, is that the reciever of the horn , would get noise from the RF Remote that i will have inside the wheel, but since the remote is at 2.4ghz, i could get a reciever that works at Mhz...


                      • #12
                        If you could find a compatible car with steering wheel controls and use the internal parts that would be the best way to do this. Otherwise do you need the remote to be IN the steering wheel? You could always mount it in a panel bolted to the steering wheel column and not need to mess with the internals of your wheel.

                        Baring either of those you could also use a multi channel remote if you can fit everything inside your steering wheel. (Does your car have an air bag in that year? I hope you are not disabling the air bag? )

                        A multi channel remote would have a bunch of inputs that you could then send to the receiver to decode. You can use something such as a Aduino Nano as a transmitter and another as a receiver and do whatever you want with them. These should be very small and have the ability to have as many outputs/inputs as you want and you could have a wireless or IR transmitter to do your work. They are easy to program and very flexible. In this type of setup you really only need to close contacts so it should be easy.

                        Something like this depending on your room:


                        • #13
                          Originally posted by settra View Post
                          my onlu concern, is that the reciever of the horn , would get noise from the RF Remote that i will have inside the wheel, but since the remote is at 2.4ghz, i could get a reciever that works at Mhz...
                          Irrelevant. If it's 2.4G it is digital and won't interfere. (Neither should analog for that matter.)


                          • #14
                            redhead- my car is from the 1992, so i wouldn't worry about airbags (it has none). of course i agree, that finding a part thatr feets my car, and has cable pass option, in the slip, and then use the OEM buttons, is by far the most STABLE, option.
                            but not neccecery the best... apart of the fact that i did searched for parts like that, but here in greece, i can never find what i search for, it would limit me to the design of the buttons, (now i have used BMW x5 buttons), AND it would still requiore external software/hardware to translate the car buttons, to computer actions.... in the build i have atm, i have a 2.4Ghz RF remote, that connects directly to the pc, and has direct actions on it. the buttons, are parallel-wired with the buttons of the wheel..

                            after all it was RF that did the trick. i went and bought a 1channel communication kit, that controls a relay (although i could have build one ) with pin-codes and staff liek that for no RF Noise. So now, all the signals from the steering wheel go out, wirlessly, and the OEM cables, carry just power. i will post a complete thread when i am done!