Control steering wheel switches from microcontroller

Thread Starter

superfly

Joined Apr 5, 2017
3
I am totally stuck now!

I try to control a car media system, that kind that is controlled by a steering wheel switch.
The switch i just a bunch of different resistances, ranging from aprox. 80 - 4000 Ohms.
I have a analog output from my control circuit, 0-10V (max 1mA)
I've tried with different FET's and transistors to simulate the different resistances (eg. current), but I've only failed so far.
And to tell the truth, I'm not a very good circuit designer...

Do any one have a good idea on how to solve this?

I've found out that the input stage on the media system is a resistor ladder with a pullup of 340 Ohm to 5V and a pulldown of 51 kOhm to GND.
So effectively I just have to control a current of 1mA to 12mA to simulate the buttons, but how can I do this?
 

LesJones

Joined Jan 8, 2017
4,174
Make table of the resistance values for each button pressed. Is 4 K with no buttons pressed ? Also is there only ever one button pressed at a time ?

Les.
 

Thread Starter

superfly

Joined Apr 5, 2017
3
4k - no button pressed
1.8k - Down
1.2k - Up
700 - Left
420 - Right
230 - Volume -
80 - Volume +

And no, there can not be more then one button pressed any time.

//Eric L
 

LesJones

Joined Jan 8, 2017
4,174
You could try to use a small mosfet (Such as a 2N7000) to switch in resistors of those values. For the 80 ohm resistor you may have to use a slightly lower value resistor to compensate for the on resistance of about 1.5 ohms. Another possible solution would be to feed in a voltage from a D to A converter. The output would need to buffered to provide a low source resistance as the media system would be trying to pull it up to near 5 volts. These are the calculated voltages that would corespond to the button pressed.

4k - no button pressed 4.58 V
1.8k - Down 4.18 V
1.2k - Up 3.88 V
700 - Left 3.35 V
420 - Right 2.75 V
230 - Volume - 2.01 V
80 - Volume + 0.95 V

To avoid having to use 6 I/O pins you could use a 3 line to 8 line decoder chip to drive the gates of the mosfets.
I suggest that before you build anything that you make sure connecting resistoors of these values to the input to make sure it work. (In case there is something more than just resistors in the original controls.

Les.
 

Thread Starter

superfly

Joined Apr 5, 2017
3
Another possible solution would be to feed in a voltage from a D to A converter. The output would need to buffered to provide a low source resistance as the media system would be trying to pull it up to near 5 volts. These are the calculated voltages that would corespond to the button pressed.
This sounds just like the way I would try to do, bu the buffer part is where my inadequate EE skills lets me down. I just can't make a buffer to work properly...
 

Dodgydave

Joined Jun 22, 2012
11,277
4k - no button pressed
1.8k - Down
1.2k - Up
700 - Left
420 - Right
230 - Volume -
80 - Volume +

And no, there can not be more then one button pressed any time.

//Eric L
Can you do the same test but measure Voltages,? then you can use an op amp voltage control buffer or mosfet transistor.
 
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