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Cruise controle converter

Discussion in 'Automotive Electronics' started by kerrmotorsports, Dec 21, 2013.

  1. kerrmotorsports

    kerrmotorsports New Member

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    I need help designing a module that will convert different voltage readings in to a separate 12v output. What I'm trying to do is adapt a Jeep cruise control button set up to work with a GM computer. What I have so far is the Jeep has 5 outputs:
    ON is 909 ohms
    SET is 6650 ohms
    RESUME is 15400 ohms
    CANCEL is 0 ohms
    COAST is 2940 ohms

    Each mode needs to output 12v to the GM computer with the ON signal being constant and the others momentary.
    Is this possible?
     
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  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Not without a hell of a lot of work or a microcontroller (but even that is still a hell of a lot of work).

    A brute force way might be to start with an LM3914
     
    Last edited: Dec 22, 2013
  3. crutschow

    crutschow Well-Known Member Most Helpful Member

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    What is the Jeep output when no button is pushed? Is it an open circuit?
    Is that resistance measured to a common terminal on the Jeep button?
    Does the GM computer have 5 inputs? If so you may just need to attach a transistor to each of the Jeep outputs to generate the 12V.
     
  4. dave

    Dave New Member

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  5. Mickster

    Mickster Well-Known Member

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    If I understand the OP correctly, the Jeep cruise control switch is using only a couple of wires and a resistor ladder inside the switch to provide the different states, not five separate outputs. The reason for this, is to cut down on the number of tracks required in the rotary coupling behind the steering wheel.
     
  6. kerrmotorsports

    kerrmotorsports New Member

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    I"m not to concerned with the amount of work. The GM only has 4 inputs but set and coast share the same input so I was just going to combine the two. Mickster is correct is a common way for cars to reduce wiring. And the circuit is open when no switches are pressed.

    So the best I can figure is to start by building a voltage regulator that outputs 5 volts to power the micro controller. (which I don't know how to do but can easily figure out). Use a micro controller and program it convert the inputs to separate outputs. (Which I really have no clue how to do). Use transistors on the micros outputs to make them 12 volts that I can do lol
     
  7. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Expanding on Mike's idea, you could use a LM3914 dot/bar generator which has ten evenly spaced input voltage levels for the ten outputs. You add a series resistor from 12V to the the Jeep switch and from the common to ground. For example 6.04k ohms in series with the 12V input and 7500 ohms series with the ground will give output voltages of (see post below).

    You connect the LM3914 Rh to the 12V which will give an input sensitivity of 10 equally spaced trigger points from 1.2V to 12V. The outputs are connected to ground when active so you would need a PNP transistor or P-MOSFET at each of the 5 used outputs (2, 3, 4, 6, 8) to generate the +12V signal. You put the LM3914 in the DOT mode so only one output is active at a time.

    Edit: There was an error in the voltage calculations. See below for revised chart.
     
    Last edited: Dec 24, 2013
  8. kerrmotorsports

    kerrmotorsports New Member

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    Thank you this really helps. Is there any program I can use to design the circuit?
     
  9. crutschow

    crutschow Well-Known Member Most Helpful Member

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    There are Spice programs to simulate a circuit but none I know of to design a circuit.

    I suggest you take a shot at designing the circuit and then let us review it.
     
    Last edited: Dec 24, 2013
  10. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Here's the revised chart for post #6
    upload_2013-12-24_14-47-32.png
     
  11. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Below is my first iteration at simulation of an LM3914 circuit that, I think, does what you want.

    The glitches in the outputs may be just a simulation artifact and may not show in the real circuit. If they do, they can be suppressed by judicious capacitor decoupling.

    The circuitry at the left inside the dashed square is to simulate the sequencing all of the Jeep switches. It's not needed in the actual circuit.

    Note that to minimize failures from accidentally shorting any of the outputs, you should add about a 1k ohm resistor in series with each of the outputs (not shown). (I couldn't show them on the schematic since it gives floating node errors).

    Edit: The 1k output resistors, both to ground and to the output, should have a power rating of at least 1/2W. Also updated the schematic.

    Cruise Control Ckt.gif
     
    Last edited: Dec 25, 2013
  12. kerrmotorsports

    kerrmotorsports New Member

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    Wow this is a huge help, thank you. Is that ltspice your using?
     
  13. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Yes. I'm using LTspice, a free download from Linear Technology. If you want to play with the simulation I can attach the files for you. But you have to add the CD4000 logic and LM3914 files since they are not part of the normal LTspice part's library.
     
    Last edited: Dec 25, 2013
  14. kerrmotorsports

    kerrmotorsports New Member

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    That would be cool. I played with LTspice a little but didn't like how it didn't have buttons or switches
     
  15. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Here are the files for LTspice.

    The .asy files go in the lib/sym directory (Digital for CD4000 and SpecialFunctions for LM3914). They generate the package icons on the schematic.
    The .sub files go into the lib/sub directory. Those are the simulation model files.
    The .asc file is the one you simulate.
    Edit: To use the CD4000 parts you have to add a spice directive to the schematic: .lib cd4000.lib. (which is included in the .asc file below).
     

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    Last edited: Dec 25, 2013
  16. kerrmotorsports

    kerrmotorsports New Member

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    So I took your schematic and built it again so I can better understand whats going on. I tried to recreate the Jeep switch tho I know it wont work in simulation but it helped me understand it. I think I forgot the two resistors you mentioned but if you find any other errors I'm all ears. sorry for the crudeness its my first time using LTspice.
    EDIT: I didn't understand the V+
     

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  17. crutschow

    crutschow Well-Known Member Most Helpful Member

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    V+ is simply a net label for the +12V line. You generate it by hitting key F4 or right clicking over the wire you want to label. Anything with the same label will be connected together, even though there is no visible wire. Thus the V+ net name on the LM3914 V+ and Rhi terminals would be connected to the +12V.

    You need to add the additional 6.04k ohm resistors in series with the V+ and the 750 ohm to ground at the output of the Jeep switch module.

    All your transistor outputs should be connected to the respective transistor collectors, not ground as you show. Alternately it may work to use those resistors to the outputs as the protection resistors and just remove the ground connections shown. It depends upon whether an open circuit to the GM module looks like a 0V signal or not (it probably will).
     
  18. kerrmotorsports

    kerrmotorsports New Member

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    With some more playing around I found some mistakes and fixed them. Now it actually works. I just wish they had a click able switch that i can watch real time. Heres the updated model. Now I just need to figure out how to make the on output stay on.
     

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  19. crutschow

    crutschow Well-Known Member Most Helpful Member

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    If you look at my simulation you will see that the ON output stays on for any of the 5 switches being closed (that's the purpose of the 3 diodes). Thus the ON output will stay high as long as the ON switch is closed, independent of the position of the other switches.
     
  20. kerrmotorsports

    kerrmotorsports New Member

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    I noticed that's why the diodes were there. I need the on command to stay on until the on button is pressed again or until the module loses power. Is there a latching transistor I could use? Or a relay?
     
  21. crutschow

    crutschow Well-Known Member Most Helpful Member

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    There are no latching transistors and you could possibly use a latching relay but the easiest is to add a flip-flop to give alternate action to the ON button as shown below. The flip-flop triggers when the ON button is released but that shouldn't be a problem. C1 and R2 reset the flip-flop at power on otherwise it could randomly power up in the on state.

    There are two flip-flops in one package but you will only use one of them. Note that you must connect all unused inputs (not outputs) of both flip-flops to ground when you build the circuit, otherwise the circuit will likely malfunction. Also the model doesn't show the chip power and ground which must be connected. I also added a filter to the power line to suppress any transients from the vehicle power.
    Edit: The negative voltage for V(3) is an artifact of the LM3914 model and will not occur with the real circuit.

    Cruise Control Ckt.gif
     
    Last edited: Dec 26, 2013

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