Repurposing steering wheel buttons

Has anyone repurposed their steering wheel stereo control buttons for other things? I have an aftermarket head unit that does not support steering wheel controls.
I wanted to wire up a horn to one of the volume buttons on the steering wheel.

 

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Subd

 

Its not quite that simple as wiring things up to the buttons. They aren't "separate" buttons. There are two banks of buttons, each of which implements a resistor network. There are two resistor networks. Three wires. One input, two outputs. You need to attach something to those wires that is capable of reading their resistance, and determining which button is pressed based on the measured resistance.

 

Do you know what voltage the input requires?
Do you think you could just make it so either output would trip a relay? I just want to be able to activate a solenoid with any of the steering wheel buttons.

 

Its just a resistor network. It doesn't have any input requirements. Give it one volt, give it a thousand volts. Its all good.
You can't just tie it up to a relay like that, because there is no "off" position at all. Even when all of the buttons are NOT pressed, the input ties through to the output via a small resistor.

If you want to make use of it, you MUST implement an analog to digital converter (ADC) to control transistors that feed relays.
You can make a fairly simple ADC with an LM339, a 7486, and a box of resistors.

LM339 is a quad-comparator http://www.onsemi.com/pub_link/Collateral/LM339-D.PDF in PDIP-14 package.

What a comparator does, is it... compares voltages. When the "varying" input exceeds the static input in voltage, the output turns "on".
You'll need the resistors to implement voltage dividers. First off, to convert the steering wheel button resistance into a voltage. You do this by tying the output line via resistor to chassis/grd. Then you also feed that output to the input of the comparator.

Next, you need to make voltage dividers to establish *reference* voltages for the static inputs to the comparator. I.e., from a 5V reference, one resistor to chassis/grd, and another to the static input. The voltage on the static input will be a proportion of the 5V based on the ratio of the two resistors.

At this point, the output lines will light up when you press steering wheel buttons. Only problem, is that when you press the lowest button, its corresponding output will light up. When you press the next output, its corresponding output will light up AS WELL AS all lower outputs. So now comes the 7486, which is an XOR logic gate. Well actually, a QUAD XOR logic gate.

http://www.ti.com/lit/ds/symlink/sn7486.pdf

Here is how you can wire it up;
http://sub.allaboutcircuits.com/images/04258.png

But note that the picture also implements a priority encoder (with the diodes) you can leave out the diodes on the xor gate outputs, and feed your transistors with them. You will need to keep a pulldown resistor on each of the outputs though.

Your transistor: I suggest an N-channel Field Effect Transistor (NFET). For instance, a 2N7000 https://www.fairchildsemi.com/datasheets/2N/2N7000.pdf

With the NFET, you use a positive input voltage to pull something from high to low. What that means, is that you apply a constant 12V to one side of a relay coil, the other side will hook up to the collector leg of the NFET, the emitter leg of the NFET will hook to chassis/grd, and the output from your xor gate will feed its gate leg.

NOW you have a relay that switches on and off when a particular button is pressed.

If you want to simplify it so that all the buttons do the same thing, then you can leave out the XOR gate. You won't need a quad-comparitor, a single will do just fine, so maybe an LM311 https://www.fairchildsemi.com/datasheets/LM/LM311.pdf

You still need to set up a voltage divider on the static input pin, and the resistor from steering wheel output to grd, but then you can use the comparator's output pin to control the NFET's gate directly.

 

Whoa, Thanks. Better break out my trusty bread board haha. I might have some comparators left over from college. I'll have to look. I think I will just wire it up so that any button will trigger the relay.

 

Holy crap. I thought for sure I was going to go over the head of anyone on a car forum. Nice to find someone who knows what a comparator is.

 

Recent mechanical engineering grad. So it's still fresh in my head. Kinda

 

Here is the schematic of the bank of resistors for the Tacoma radio controls.

If you took apart the controls you may be able to solder the factory wiring so that you're not going through the network.

You would certainly have to have the button pick up a relay as the contacts for the switches here aren't rated for much.

If I were doing what you want, I would look to see if I could, for instance, take the yellow wire and solder it directly to the hinge side of the on hook wire. You would also have to disconnect that side of the switch from the rest of the network. The other side of the off hook switch is already directly connected to red. I'd insulate the black and blue wires because voltage will be present there.

Then you would bring 12V to your red wire and connect yellow to the input of your relay. Remember to properly fuse your work! Burndowns are no fun!



I graduated with an EE degree about 10 years ago, but was concentrated in power and have worked with that my entire career. I deal with big wiring, high voltage and megawatts daily, I remember building comparators in college, but I'm doing good right now just to remember roughly what one does.

 

I haven't done it on the Tacoma since I don't have steering wheel controls on my base model, but on my GTO it has the same style resistor network for 6 buttons. I took an arduino and put 5v through the resistor network and could read which button that was being pressed. I then wrote the logic so that a <1 second press would pass the same resistance onto the radio via a digital potentiometer, but holding for >1 would perform a secondary feature by having a transistor ground a relay. I have them set up to trigger an auxiliary input, turn on/off my radar detector, and mute my radar detector.

 

just like cruise control works...two wires, and different resistance for 4 switch positions

 

Anyone seen a write up of how to take apart a 2nd gen steering wheel to get to the buttons?
I am thinking about getting new leds in there and maybe re purposing.

Another idea is to purchase the PAC SWI-RC Steering Wheel Control Adapter that lets you use the buttons for aftermarket radio's.
You can hook that up, but instead of connecting it to the radio just set it up so that when a electric signal comes through the PAC SWI-RC Steering Wheel Control Adapter harness it activates something else.

 

Post is a little old, but I was looking for information about what type of steering controls the Tacoma used. Thanks for the information. I'm planning to use the JoyCon USB steering control here: http://www.mo-co-so.com/Joycon-EXR-Steering-Wheel-Interface-p/mcs-joy-exr.htm