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How do I design this?

  1. Dec 12, 2008 #1
    I'm designing an automotive device and one of it's function is unlocking the car doors. My car has a door unlock switch and it unlocks the doors by pulling the door unlock signal to ground when pressed. My device uses a PIC as the brain and it unlocks the car door by asserting a high to an IO pin.

    I need to design a circuit that can interface my PIC's IO pin with the car's door unlock signal.

    I do have a design but I don't know if it is good or not. Please download the pdf to see the circuit.


    When PIC's output pin goes high, the NPN conducts and pulls the car's door unlock signal to gnd thus unlocking the car doors.
  2. jcsd
  3. Dec 12, 2008 #2
    to know if something is appropriate, you really need to know what the circuitry it connects to is.
  4. Dec 13, 2008 #3


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    Staff: Mentor

    What's D1 for? Why is R21 so big?
  5. Dec 13, 2008 #4
    D1 is for circuit protection just in case I connect the power wires (gnd and 12V) backward.

    Yes R21 is too big. I just installed the circuit in my car and it did not work until I short out R21. R21 is too big thus Q1 collector voltage never goes below Vil to successfully trigger the door unlock signal.

    Is shorting out R21 and leaving Q1 without a current limiting resistor a good design? What are other alternatives?
  6. Dec 13, 2008 #5
    if your transistor is not getting hot, it'll probably be fine.
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