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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.

    http://www.tennis4real.com/capacitivekey.pdf [Broken]

    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.
    Last edited by a moderator: May 3, 2017
  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?
    Last edited by a moderator: May 3, 2017
  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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