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I'm looking to build a circuit to convert an old steering wheel remote to a newer head unit. Both work with resistances, but have different values each.

So, what I would need to build is a circuit that, given an input resistance value, outputs another resistance value.

I don't have much experience with advanced electronics, so what I'm looking to know is whether and how this could be made with more simple electronics (resistors, transistors and so on) or if I'd need to use things like micro-controllers (for which I don't have any experience at all).

Could you give me some orientation in this matter? Thanks!

Edit: The steering wheel provides input with 2 wires. Pressing a switch closes the circuit with a determined resistance value. The head unit / output circuit expects this kind of behaviour, albeit with different values. There are adapters, but not for my specific remote.

Guilherme
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  • How are the two resistances related? – Ignacio Vazquez-Abrams Jan 31 '15 at 16:05
  • Basically I know that, given resistance A, I would need to output resistance B, but the difference of these values is not constant, e.g. some function could have a difference of 300 Ohm, and another of -2k Ohm. – Guilherme Jan 31 '15 at 16:08
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    That sounds awful. I recommend digging a little deeper into both circuits. – Ignacio Vazquez-Abrams Jan 31 '15 at 16:13
  • Is there a function that relates the two? Or is it completely random? If there's a function, you may be able to use op-amps to perform basic math for you. – horta Jan 31 '15 at 16:18
  • It's random unfortunately... – Guilherme Jan 31 '15 at 16:20
  • Well then you're going to need a microcontroller at least. Without any experience, you've got a long road ahead of you if you want to go down this path. If you do, I'd suggest starting with an Arduino because it abstracts away a lot of stuff you're going to want to avoid as a beginner. – horta Jan 31 '15 at 16:24
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    You should post some information on these devices. You description sounds like there is either a misconception somewhere, or this is way more complicated than it needs to be. – Matt Young Jan 31 '15 at 16:31

3 Answers3

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Other solution: use a digitally controlled potentiometer. This is a lot easier than the FET solution in my other answer, but you have very limited current.

Again, measure the sensing resistor with a voltage divider and a microcontroller's ADC. Calculate the resistance from the voltage and set the digital potentiometer's resistance accordingly.

Jasen Слава Україні
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Joris Groosman
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You could try a Generalized Impedance Converter which is a circuit consisting of a couple of passive components and 2 op amps. It takes a Zin and scales it depending on the components" impedances. In your case you would just use resistors. Here is an answer that explains how a GIC works much better than I can: https://electronics.stackexchange.com/a/130746/26678

You could use some pots as the comments suggest to adjust the scaling factor as needed.

Community
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FullmetalEngineer
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Some orientation:

A FET is a variable resistor, whose resistance you set by applying a specific voltage to the gate. Note that it can only handle current going one way, it's not suitable for AC.
The sensing resistor can be placed in a voltage divider so, that the change in resistance results in a change in voltage.

Are we there yet? No, unfortunately the voltage from the divider doesn't change linearly with resistance, and the resistance of the FET isn't linear with the control voltage either.

I would suggest reading the voltage from the divider with a microcontroller. That's not so hard with an Arduino, which has ADCs on board. Then calculate the sensing resistance from that, and apply a voltage to the FET. Unfortunately, since there's a lot of variability within FETs you can't use a formula or look-up table for the voltage. What to do? Have a small sensing resistor in series with the FET, and measure the voltage across it. That gives you a measure for the current flowing through the FET (I = V/R). Then measure the voltage at the FET's drain, the difference between the drain voltage and source voltage divided by the current gives you the resistance. This also means that you can find the required drain voltage for a given current. Vary the gate voltage until you have this drain voltage.

Joris Groosman
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