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Resistor ladder

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I am trying to use oem steering wheel radio control buttons to change pages or set/reset timers km's etc. on my Ecumasters ADU . It's off a Subaru Forester 2009-2012 . 2 wires from the board are doing all the functions .

From a little google research I have found that +5v at the beginning then the resistors . Here is were I kinda got confused as I found 2 , 3 different layouts but none similar to my needs .

Could I have some advice how to set it up ? were would the 2 oem wires come into the connection ? Also would this be possible to be done through 1 AN input or depending on functions ? And were would this/these wires be connected in the ladder ?

The though and the plan was to use 1 page up 1 page down , start/stop lap , hold for reset lap (,for the 4 buttons) and the remaining 2 to have from the ecu boost map selection .


Hi Dimitri,

I would approach this as I drew on the schematic attached. This way, depending on the voltage you measure at ana_in you can determine the state of you switches. E.g. if the voltage is +5V, the the pin is pulled high and no button is being pressed. If the voltage is 0V, the page_up button is pressed. For all in-between values you can determine the state based on the resistor values you select. R1 in my drawing is your pull-up and R are the resistors that "pair-up" to the respective buttons. Obviously, choose your values wisely so as to limit the current to appropriate levels. Your ECU will most probably have pull-ups/pull-downs, I just included it in my schematic for demonstration purposes.

This design though allows you to press one button at a time, if you press more than one, the ECU will detect the button that provides the path of least resistance and ignore the others.

Hope this helps :)

Attached Files

Thank you .

That is what I have found online but my issue is that as you can see through the attachment my oem steering wheel control i have 4 wires . 1 black, 1 red , 1 white and 1 blue . From a little bit testing I think that red black are lights . And when I have +5v on either white or blue and an on the other I can do the page up and down . Nothing more . So this is were I get lost . If I am correct were will the 2 wires from the steering control come into the diagram ?

Attached Files

Have you tried to press the buttons and measure resistance across your wires, e.g. across white and blue? If you can see discrete values for different buttons actuations then you know that you can read that on your ECU. Then you could potentially connect one of the wires to an analog input with an internal pull-up and the other to a corresponding sensor ground. That would in theory allow you to press the buttons and see discrete voltage readings on your analog in pin.

Also, what voltage are you supplying to the red wire (I assume you ground the black)?

It is a bit hard to give a more detailed answer without having a more clear picture about the hardware in question (or even better having a play with it :D). Let me know how it goes.

So I have had a few hours of play with it. I have done the following . +12V to red and power ground to black and it illuminates . Resistance test shows only through the white and blue . The following numbers come back no buttons pressed 4.7kΩ, +button 88.5Ω , -button .192kΩ (192Ω) , Mute 22.1Ω , Mode .334kΩ (334Ω) , UP Arrow .601kΩ (601Ω), Down Arrow 1,142kΩ .

So I tried having the +5V and the AN Input with a resistor of 100ohms in between them and then to the blue wire , sensor ground to he black . The + button only worked with a a little adjustments , which is what I wanted which was page back. But then when I tried to add a 200ohm resistor and a different AN although only + and - were operating they both did the same function . No matter what fine adjustments they both did the same function specifically page back .

When I only connected a 200ohm resistor again the only + button was working with any AN input that I used . It was doing what the input was supposed to bur only seperaterly and only through button + .

I know for sure that all buttons work as when with no resistor all the buttons work but do the same function .

Maybe there is a need for extra sensor ground or something . Any thoughts ?

It is good that you can see different resistance for different presses.

You write in your last post that you measure this resistance between blue and white, and when you connect to your ECU, you try to measure voltage between the blue and the black wire (through the various resistors you tried with), is that correct? If that is the case, I would suggest to connect your white wire to your sensor ground as it is between white and blue that you would measure resistance and hence voltage, not between black and blue.

Also, how do you see the correct response that each button should have? I anticipated that you can see discrete voltage measurements to your analog in pin on the ECU and given these values you can tell your ECU what to do, e.g. if A

Sorry my bad yes I have the sensor ground to the white not black as I wrote . I have tried either pull up/pull down resistors , reversing blue and white wires .

I haven't quite understood your last question .

I am pretty sure that it has something to do with the wiring or the position of the wires or something in those lines (as this is something completely new to me) . The problem is that I have to add different an inputs for each function as well as I have 2 different devices (ecu for map changes and digital dash for page view and lap timing) .When I have 1 an input and with a resistor only 1 button works (which is what I want) when I add the other an input 2 buttons work but they do the same function .

Can you share a quick hand-drawn sketch of what you are doing with the analog, as it will clarify some things?

Sure thing. For sure there is a mistake and hoping you could find it .

Attached Files

The issue I can see straight away is that you are trying to connect your steering wheel controls to 2 electrically separate devices, which I would advise against. Essentially you provide sensor power to the controls from both devices (dash + ECU), connect ground to the ECU (sensor ground), and try to get an analog reading on both the dash and your ECU.

This is problematic as you are creating a circuit with elements that shouldn't really be connected together, e.g. you are tying up the two +5V sensor supplies together via some resistors etc. The sensor power and sensor ground of a device, is meant to be used as a pair, and often you find that ECUs have multiple sens_5V and sens_gnd pins, which are meant to be paired up, e.g. you could find sens_5V_1 & sens_gnd_1, sens_5V_2 & sens_gnd_2 etc.

The easiest set-up that I would suggest involves some CAN configuration, but if done correctly is robust and simplifies wiring. I would read the presses from one device, e.g. ECU, and transmit the necessary data to the other device e.g. your dash display. This way you connect your resistance ladder to e.g. the ECU, process the readings there and then communicate via CAN to the dash (or you can do it the other way around if the more complex data is needed in the dash etc). Based on the info you provided I draw a quick sketch, see attached proposed diagram (assuming that your red and black wires are taken care of).

What are your thoughts?

Attached Files

I haven't yet done anything as I got stuck at work . In the above examples I have given I had only the screen connected . In reality even if I have just the dash settings controlled via the steering wheel control would be ok . But I believe that my issue might also be my clock spring and maybe I might have to upgrade to the newer one .Not 100% sure on that , just a thought

Ok from what I saw in the drawing you attached you have the analogue reading connected to distinct 2 inputs in the ADU, is that right? Also, which specific ADU model are you using?

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