Emulate momentary switch on timer

[Izerous]

My goal is to emulate a physical switch being pressed in a vehicle when it is first started but not again until turned off and on again. My thoughts are to have a timer controlling a relay wired in parallel with the physical momentary switch.

-When the 12v source is turned on it should wait x seconds
-after x seconds trigger a relay to emulate someone pressing the button.
-keep relay in on position for 3 seconds
-then turn off and not fire again until the circuitry looses power and re-energized again.

 

[MikeMl]

I followed your aborted thread on AAC.

One problem I see is that the way that was discussed there you would have to leave 12V applied to the 555 even with the key off. I dont like that approach.

Here is how I would do it. Note zero standby power, and it can drive a low resistance load.

 

[alec_t]

I can see that providing a 3-sec 'on' time for the load, but how is the preceding x-sec delay obtained .

 

[MikeMl]

I would just feed it with the "accessory" output from the ignition key switch. That circuit usually is not powered while the key is in the start position, so anything powered by the delay circuit will not have power during cranking, but will get power for 3sec after the key is moved from the start to the run position.

In modern cars, lots of things (radio, AC, blower, some lights) are automatically turned off during cranking so that the starter gets the full available output of the battery...

Note that my circuit resets the timing network when input power goes away...

 

[Izerous]

Thank you for your schematics/directions. I'm hoping to build this in the near future will post results at the point in time.

 

[MikeMl]

Thank you for your schematics/directions. I'm hoping to build this in the near future will post results at the point in time.

Almost any modern PFET will work in the circuit (Vds > 20V, Id >10A, Vgs>16V) . If you dispense with the relay, how much current are y0u switching?

 

[Izerous]

I need to measure what is going through the physical momentary switch as I honestly do not know.