I want to be able to close the electric tailgate on my car using my keyfob (this functionality is not possible from the factory).

Coming from a software developer background, of course I could use some micro processor to solve this, but I want to solve this problem without using one.

Below is a part of the schematic of the electric tailgate with 3 wires added by me, but what I'm struggling with is what should be in between.

How it works at the moment:

  1. Tailgate button is pressed on keyfob;
  2. 12-14V (car battery/alternator voltage) comes through the 1 wire, unlatching the lock mechanism using V254 (holds unlatched for about 0.5s) and the tailgate module opens the tailgate using the motors attached to the hinges;
  3. By pressing the momentary button E406, the wire 3 essentially gets connected to GND via wire 2 and the tailgate module now knows it needs to close the tailgate

What I want to do is simulate a button press of E406 if I actuate the lock latching action twice withing a given time frame (1-2s)

I'm assuming I will need some transistor as a switch to connect wires 2 and 3 that is controlled by some logic (capacitor charging up to a needed voltage from 1 wire?). I don't have much (or at all) experience with making-up logic circuicts.

enter image description here


1 Answer 1


I'll give you some pointers, it's not difficult to do as discrete components.

  • A monostable give you the time limit. You are correct about charging a capacitor, there are many way to do it (from a 555 to a capacitor reaching the Vth of a MOSFET to discrete ICs doing the monostable work to comparators actually looking at a capacitor charging);

  • For the logic you would like to a state machine. Well, it's trivial but it's there. In fact probably a counter would do it, some 74 part, for example;

  • The monostable would reset the state machine at timeout; some kind of power up reset would be useful;

  • Probably some kind of debounce for the signal (and protection from horrible electrical noise)

  • The signal increments the counter and arms the monostable, on the count of two trigger some output: maybe one other monostable to shape the output.

It's an interesting design exercise. However a PIC10F or a tinyavr would do it in a simpler way!


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.