Design a Car Door Unlocker with PIC IO Pin

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The discussion focuses on designing a circuit to unlock car doors using a PIC microcontroller. The proposed design involves using an NPN transistor that pulls the door unlock signal to ground when the PIC's output pin is high. Concerns were raised about the size of resistor R21, which was found to be too large, preventing the circuit from functioning properly until it was shorted. The necessity of D1 for circuit protection against reverse polarity was also clarified. The conversation concludes with inquiries about the safety of operating without a current limiting resistor for the transistor.
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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 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.
 
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to know if something is appropriate, you really need to know what the circuitry it connects to is.
 
david90 said:
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


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.

What's D1 for? Why is R21 so big?
 
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berkeman said:
What's D1 for? Why is R21 so big?

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?
 
if your transistor is not getting hot, it'll probably be fine.
 

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