Design a Car Door Unlocker with PIC IO Pin

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Discussion Overview

The discussion revolves around designing a circuit to unlock car doors using a PIC microcontroller. Participants explore the interface between the PIC's IO pin and the car's door unlock signal, focusing on circuit design considerations and component specifications.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant describes their design, which involves using a PIC to pull the car's door unlock signal to ground via an NPN transistor when the output pin goes high.
  • Another participant emphasizes the importance of understanding the connected circuitry to determine the appropriateness of the design.
  • Questions are raised about the purpose of diode D1 and the size of resistor R21, with one participant suggesting that R21 is excessively large and has caused issues in their implementation.
  • Concerns are expressed regarding the safety of shorting out R21 and whether leaving the transistor without a current limiting resistor is advisable, prompting inquiries about alternative solutions.
  • One participant notes that if the transistor is not overheating, it may be acceptable as a design choice.

Areas of Agreement / Disagreement

Participants express differing views on the adequacy of the current design, particularly regarding the sizing of R21 and the implications of modifying the circuit. No consensus is reached on the best approach to take.

Contextual Notes

There are unresolved questions about the circuit's performance and safety, particularly concerning the interaction between the components and the potential risks of modifying resistor values.

Who May Find This Useful

Individuals interested in automotive electronics, circuit design, and microcontroller applications may find this discussion relevant.

david90
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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?
 
Last edited by a moderator:
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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