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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

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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!

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