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Thread: Single Wire Cruise Control Sensing

  1. #1
    Variable Bitrate
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    Single Wire Cruise Control Sensing

    Hello Board Members,

    Here is my current situation: I have an Optoisolated input card for my TruckPC. Each input has 2 terminals on it. It accepts voltages from 3-24 volts, AC or DC.

    I have a 2001 Ford Explorer Sport Trac. It has the normal cruise controls built into the steering wheel. Instead of having a set of 5 wires and a ground coming out, they used one single signal wire from the steering wheel to the cruise computer.

    The way this cruise works is when I press the ON button, the switch applies +12vdc to this "Signal" Line. When I press the OFF button, that switch grounds out the "Signal" Line. When I press RES, ACCEL, or COAST, each applies a certain resistance to the "Signal" Line (each button applies a different value of resistance).

    So in the end, I have a ground and a "Signal" wire to try and sense for my computer controls. I know this can be done with certain IC's and other ADC's, but I am looking for a way to use the optoisolated input board I have.

    My Idea for sensing this: (PLEASE HELP HERE!)
    One side of each of the inputs on the input board will be connected to the "Signal" wire. The other side will be connected to some sort of a referenced voltage. The first two references will be at about 5vdc, and the ON/OFF buttons can either sink or source this and be sensed OK. For the last 3 resistance variables, I am not sure what to wire to sense it. IF ANYONE has ANY possible ideas, please let me know! I do not want to rewire my CC system, because I would like it to remain functional. Thanks a lot for reading, I was being long-winded so I don't leave anything out.

  2. #2
    Variable Bitrate
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    So if the resistance is changing, I would believe the current is changing, since the 12V is likely to stay constant, right? Thus its a matter of sensing the current, not a voltage. Easiest way to do that generally is to put a small resistor in your signal line. This should be a couple orders of magnitude smaller than your button resistors (if they're already in the 10's of ohms range, this method may not work). Take both sides of the resistor and sense the voltage with a comparator. The voltage difference is proportional to the current, which is inversely proportional to the resistance applied by the buttons. You can set thresholds on the comparators to trip for each button. I dont know what your board is, but it may even be able to set comparators between inputs.

  3. #3
    lez
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    old video recorder remotes worked that way.........badly.

  4. #4
    Variable Bitrate
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    Its a cheap way to make a single wire bus (all passives), but fairly error-prone. I've used a similar current sensing scheme to decode a bus in a clothes dryer, and it was fairly reliable, though the bus itself did have a lot of glitches. I think its your best bet.

  5. #5
    Constant Bitrate
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    sounds like this input is digial ie on/off , just has a range of accepatable trigger voltages.
    can you charge a capacitor with your one wire and measure the time it takes to charge to threshold?

  6. #6
    Variable Bitrate
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    Yeah, my input is on or off only, and i can write the app to sense each input.

    I don't kno much about caps, would it cause any delay?

  7. #7
    Constant Bitrate
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    http://www.electronics-tutorials.com...apacitance.htm

    http://zone.ni.com/devzone/conceptd....5?OpenDocument


    I seem to remember the principal was used somewhere in antiquity- apple II+ joystick?
    embedded apps too...

  8. #8
    Variable Bitrate
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    Using the capacitor method you'd need to find a place to tap off that will give you the different voltages. If you can find that, if the input card has an ADC, why not just plug it in? No caps needed.

  9. #9
    Variable Bitrate
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    There is no ADC, its stricty a triggered on or off input. 3-24v will trigger it ok.

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