Convert resistance ladder

[electron86]

Hi, I'm a newbie on this forum with a fairly good, but basic understanding of electronics, but as a project I'd like to come up with a steering wheel control adaptor for my car to an aftermarket headunit. Basically the 5 switches on the steering wheel are connected to the headunit via two wires using a resistance ladder. However, the specification for the aftermarket headunit requires a different resistance ladder for the same functions.

So when the resistance on the two wires coming from the switches is say 390ohms, I need the headunit to see a resistance of 16kOhms as laid out below:

Function    Steering Wheel        Headunit Spec

Vol +       390 Ohms              16 kOhms
Vol -       0 Ohms                24 kOhms
Mode        3.2 kOhms             1.2 kOhms
<<          1.7 kOhms             11.25k kOhms
>>          850 Ohms              8kOhms

What would be the best way of going about this? I realise you can buy these as aftermarket, or that I could just replace the resistors in the switches for the required value, but I'd really like to understand the concept of converting between the two if nothing else.

Many thanks in advanced!

 

[duke37]

You could perhaps convert the resistance to a voltage by feeding in an accurate current and then use a LM3914 bargraph display chip in dot mode to switch in what resistors you want in the output. Some stable voltages will be required.

 

[Harald Kapp]

The relationship between the two resistance series is not linear:

Not something that you can express by a simple equation much less achieve by a simple circuit. Therefore I think the easiest way is to replace the resistors.

Harald