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.