How do SWC work?

[tdi90]

Just wondering how SWC works. Does the stereo send power to the SWC stalk then by pressing a button it lowers or raises the voltage or resistance and sends it back to the stereo?
Or is there a feed to the SWC from the cars electrics that is then altered by the buttons and sent to the stereo?
Or am I far too bored at work?

[murphyv310]

Hi.
The head unit sends a low current but stabilised voltage supply to the wheel control. there is a bank of resistors on the wheel and each time you press a control there is a potential difference set up by the differing resistors selected, the resultant voltage is compared with a stable reference voltage and the processor then can work out what button has been selected by it's resultant difference between the stable voltage and the selected buttons voltage.

Very much an analogue way of working, very reliable though.

[tdi90]

Many thanks for your reply.

[Duotone]

Originally Posted by murphyv310

Hi.
The head unit sends a low current but stabilised voltage supply to the wheel control. there is a bank of resistors on the wheel and each time you press a control there is a potential difference set up by the differing resistors selected, the resultant voltage is compared with a stable reference voltage and the processor then can work out what button has been selected by it's resultant difference between the stable voltage and the selected buttons voltage.

Very much an analogue way of working, very reliable though.

The steering wheel controls are just normally open switches.

All the changes in voltage are done in the head unit.



This is the diagram for the Harmony but the wheel controls are all the same.



Dave T

[raykay]

For 75s & ZTs, the switches have 'built in' different resistances. As in post 2, when the switch is operated, depending on which one and which way, the different resistances vary the voltage in the circuit which is recognised by the stereo and it makes the relevant changes.

[Polly]

Originally Posted by Duotone

The steering wheel controls are just normally open switches.



All the changes in voltage are done in the head unit.







This is the diagram for the Harmony but the wheel controls are all the same.







Dave T

That diagram is rather simplified, and wouldn't work as it appears. Each switch must set up a different resistance, so that the head unit can tell which one has been operated. They are quite reliable, however if not used regularly, the switches can get dirty, which upsets the calibrated resistance and results in the switches having random effects.


Sent from my iPhone using Tapatalk

[tdi90]

Hmmmm, might see what I can make for my pick up truck. Wont do wheel mounted controls cos of the problems of wiring it it, I'll try and column mount something

[Duotone]

Originally Posted by Polly

That diagram is rather simplified, and wouldn't work as it appears. Each switch must set up a different resistance, so that the head unit can tell which one has been operated. They are quite reliable, however if not used regularly, the switches can get dirty, which upsets the calibrated resistance and results in the switches having random effects.


Sent from my iPhone using Tapatalk

Very true, my mistake.

Here is a picture of the circuit board used by the steering wheel control switches.

You can see the resistors [arrowed] used to vary the swc voltaage.



Apologies for any confusion caused by me.

Dave T

[tdi90]

I've ordered a column unit out of a mk3 mondeo for the silly price of £4.65 inc P&P so when it and new car stereo arrives I'll give it a try in my Navara.