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A resistance converter - An interface for car steering wheel controls to aftermarket radio

JimB

Super Moderator
Most Helpful Member
or do you maybe no even better examples how "to much isn't always more" ?
We can express this in English as "Less is More"

An example from the past week:
A few months ago a bought a new "hobby car", a Mazda MX5 to be precise.
The car is eight years old, and one of the previous owners has replaced the OEM radio with some aftermarket wonder, probably because the new radio has a USB port for an iPod type thing.
When it comes to in car entertainment, I am a simple soul and just want to listen to a couple of stations, and if they do not have something which interests me, I will switch off and just drive the car.

Here is the problem, the new radio is incompatible with the radio controls which are built in to the car steering wheel.
Having eventually found the information which describes the output from the steering wheel and what the radio expects, I was able to make a little converter so that I can use my steering wheel controls. And it works well.

The controls on the aftermarket radio are an ergonomic nightmare.
The rotary control is quite thin and difficult for my large fingers to grip.
If the rotary control is pushed, it takes you off into some setup mode. You get out of the setup mode by pushing on another tiny button.
All the push buttons for preset stations and other odd functions are tiny and close together.
How the HELL are you supposed to simply change stations or set the volume UP/DOWN as you drive down the road?
One really useful (yeah right!) feature of this aftermarket radio is that you can select the colour of the illumination of the frontpanel controls!
WHAT!
I need a feature like that like I need a hole in the top of my head!

So as far as I am concerned KISS - Keep It Simple Stupid

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Here is the problem, the new radio is incompatible with the radio controls which are built in to the car steering wheel.
Having eventually found the information which describes the output from the steering wheel and what the radio expects, I was able to make a little converter so that I can use my steering wheel controls. And it works well.
While I'm quite impressed that you made your own converter, I presume you weren't aware that such devices are commonly available for replacement radios, with versions for various makes of cars.

As a matter of interest, what did you have to do for the converter?.
 

JimB

Super Moderator
Most Helpful Member
I presume you weren't aware that such devices are commonly available for replacement radios, with versions for various makes of cars.
I am aware that there are various wiring converters available that can convert from the proprietary connector in the cars wiring loom, to the DIN standard connector on the back of the average aftermarket car radio.
In the case of a Mazda car to a Pioneer radio, there is a wiring converter, but it just swaps wires around between the different types of plug.
There may be a box which converts the steering wheel controls, but there was not one fitted in my car, and I did not find one during my googling of the subject.

As a matter of interest, what did you have to do for the converter?
The output from the steering wheel controls is a resistance, each control having its own well defined resistance.
The wired remote facility on the Pioneer radio also tests for an external resistance and acts according to the resistance it sees.
The range of resistances from the steering wheel do not in anyway match the resistances required by the radio.

One possibility would be to pull the steering wheel apart and change the resistances.
Two problems with that, first, the radio has two lines which need to be selected for different functions, where the steering wheel will only switch one line. Second, I did not want to risk a face full of airbag. (Unlikely I know, but...)

So what I did, I converted the resistance to a voltage by making a potential divider, the steering wheel controls being the lower resistance of the divider.
I used an LM3914 in "Dot Mode" to sense the various voltages, and used the 3914 outputs to operate 4066 analogue switches to switch the required resistance to the radio.

Have a look at the pictures to see what it looks like.

JimB

Controls Converter 002.jpg Controls Converter 004.jpg Controls Converter 007.jpg
 

Abi Hurairoh

New Member
So what I did, I converted the resistance to a voltage by making a potential divider, the steering wheel controls being the lower resistance of the divider.
I used an LM3914 in "Dot Mode" to sense the various voltages, and used the 3914 outputs to operate 4066 analogue switches to switch the required resistance to the radio.

Have a look at the pictures to see what it looks like.

JimB

Hi JimB,
I am looking for such simple converter you mention above to convert from steering wheel control resistance of 13 kOhm becomes resistance < 1 kOhm. The images are not so clear. I hope you could share the circuit diagram for this converter.

Abi H
 

Abi Hurairoh

New Member
upload_2018-2-20_13-13-3.png

Hi JimB,
One component I could not figure out is the one below resistor (red circle). Is this the capacitor C6 0.1u?

Thanks
Abi
 

JimB

Super Moderator
Most Helpful Member
One component I could not figure out is the one below resistor (red circle). Is this the capacitor C6 0.1u?
The quick answer is no.

My initial design as shown in the picture, used two diodes (1N4148) to pull down the "Ring" connection to the DEH4300 radio when either the SEEK+ or SEEK- functions were operated on the steering wheel.
A good idea in theory, but in practice it did not work.
I think the 0.6v drop across the diode was preventing the radio from correctly detecting that SEEK functions had been operated.

So I removed the diodes and implemented the pull-down on the ring connection by using sections IC4C and IC4D of the 4066N.

JimB


MX5 Converter Diodes.png
 

nickpl

New Member
Hi. What if i have 3 wires coming from the steering wheel control and 6 buttons and only 3 different resistance (thats why there are 3 wires: 2 "switching" wires and common ground) How much more complicated the circuit will be and if i could use one showed higher. Thanks
 

JimB

Super Moderator
Most Helpful Member
What if i have 3 wires coming from the steering wheel control and 6 buttons and only 3 different resistance (thats why there are 3 wires: 2 "switching" wires and common ground) How much more complicated the circuit will be and if i could use one showed higher.
I don't know.
I have no idea how your wires/buttons/resistors are connected, or how the radio responds to the steering wheel controls.

It may be possible.

JimB
 

alex722607

New Member
Hi, I am new and I am working on a steering wheel project of my own.

Background:I have a car where the steering wheel was replaced with a better/newer one from a different model. Needless to say the resistance is different from one to the other and I would like to fix that. I opened up the switch but I don't want to disturb it as some resistors are wired in series and others parallel.

I was getting no where until I discovered this forum. JimB I really like your design and I would like to build something similar. Apart from the volume control, the same principle is involved where the steering controls output a certain resistance (depending on button pressed) and the radio is looking for a different resistance value(depending on button pressed). I have all the values in front of me for both the stock and new steering wheel. The original volume control works by way of a rotary encoder style switch where you scroll up and down and the new wheel has buttons that output a resistance. That being the case, am I dreaming in thinking this will work?
 

JimB

Super Moderator
Most Helpful Member
The original volume control works by way of a rotary encoder style switch where you scroll up and down and the new wheel has buttons that output a resistance.
OK, so presumably there are two wires for the volume control, one wire for Increase Volume and a second wire for Decrease Volume.
(There will also be a Common wire, to which all the other wires are referred.)
I suggest that you use a multimeter and see what happens to the voltage on the Up/Down wires as the encoder is turned.
As a wild guess the voltage on each of the wires will sit at some value while the encoder is at rest and pulse down to zero volts as the encoder is turned. There will probably be a timing between the two wire pulsing, how this could be implemented in a simple converter like mine, I do not know.

JimB
 

alex722607

New Member
Im testing it right now and it is exactly as you say. 3 wire type, actually the same style rotary encoder that is used on computer mice. In this case its an 11mm enccoder and works off of timing like you said. With 5 volts on the center post, I took one lead of the multimeter and connected it to the ground source and the other lead I probed the outer posts of the encoder and sure enough it read 5 volts and as it turned it dropped to 0. Same thing on the other outer leg.

That being said, Im not sure how to replicate the signal. One way I could think of and its a little crude is to have an electric motor drive the rotary encoder and I think a stepper motor would be ideal, so when the volume button is pressed up or down the varying resistance controls the motor spinning forward or reverse. If this is something that could be done I will start a thread on it. If its too crazy please tell me.
 

JimB

Super Moderator
Most Helpful Member
One way I could think of and its a little crude is to have an electric motor drive the rotary encoder and I think a stepper motor would be ideal, so when the volume button is pressed up or down the varying resistance controls the motor spinning forward or reverse.
Hmmm... "a little crude" is a bit of an understatement!

If its too crazy please tell me.
In my opinion, yes, too crazy.

It should be possible to make something electronic which will emulate the pulses from the encoder, from a hardware point of view a little microcontroller may be an easy way.
How are your programming skills?

JimB
 

alex722607

New Member
I can read it but writing I am still learning. I mostly search online for similar things to get acquainted with the different languages. I've done a lot of searching but am finding next to nothing about replicating the encoder signal. Going to research some more and start a thread in the morning about this. Appreciate your guidance Jim B and I hope you will chime in!
 

dr pepper

Well-Known Member
Most Helpful Member
Here in the Uk at least comms between electronic modules in vehicles is done with can bus, froma around '04 on.
There are usually 2 busses a high and a low speed.
I worked out the comms for my last car and built and wrote some code to control a dash/car with a can interface.
You can for certain manufacturers get the codes required to control the radio, fiat and others are available on the net.
You did well to get that to work.
 

rjenkinsgb

Well-Known Member
Most Helpful Member

alex722607

New Member
Thank you for the lead. I have started a new thread on the topic. Everyone please feel free to chime in.
 
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