# How to pull line to ground for 3 seconds on car start?

#### rj2k21

Joined Nov 26, 2021
4
I'm trying to simulate the pressing of a button after starting a car. The button is a momentary button that just shorts the input wire to ground. The input wire seems to be floating, I've measured 3-6v on this but being automotive maybe this could get as high as 14? Being more of a software person I initially thought of using a microcontroller and writing a program to detect the power on, wait and pull an output low and somehow use this to pull the actual input wire low. I think this might be over complicating the problem.
How simple of a circuit could be used to use the 12v ignition feed to power a circuit to pull this signal wire to ground for say 3 seconds (this isn't critical) and then leave it in it's original 'floating' condition which would allow the actual physical switch to be used if needed?

Thanks.

#### MrChips

Joined Oct 2, 2009
25,949
Welcome to AAC!

Sounds like a job for a 555-timer IC.

#### Tonyr1084

Joined Sep 24, 2015
6,864
Made a circuit years ago that delayed the action of a relay by four seconds. Using just a capacitor, resistor and transistor, the cap would charge to a level high enough in four seconds to activate a 24 volt relay. Why 24 volt? First, it was what was on hand. Second, its higher voltage would require a higher input voltage to trigger. Commonly, a relay will take about 70% of its rated voltage to pull in, whereas they commonly hold to about 35% of their rated voltage before it will drop out. Using a 12 volt relay would have shortened the time frame to trigger this action. Partly dependent on the circuit is the amount of current the relay would pull. Sorry, I didn't measure that factor, but the drawing below illustrates how I used mine to delay turning on a gauge that had color memory issues at startup. D1 would insure that the relay would hold in the on position once triggered. Q1 and K1 (the relay) would drain C1 over a short period of time and be ready to delay again. You could use the relay to momentarily short your circuit to ground. Depending on whether you want to hold it grounded or let it float open - that all depends on which set of wires you use. C and NO (Normally Open) will close after four seconds in my circuit. C and NC (Normally Closed) will remain closed for four seconds, then go open. In your case you might want a DPDT relay (Double Pole Double Throw).

R1, marked 102 is code for a 1, a zero and two more zero's for a value of 1000Ω, or 1KΩ. Using a higher resistance will increase the delay time. So will increasing C1 to a higher capacitance.

And yes, welcome to AAC.

#### crutschow

Joined Mar 14, 2008
29,537
If the timing is not critical than a single N-MOSFET should work (LTspice simulation below):
RTest is just for simulation purposes and is not needed for the actual circuit.
The delay time will vary some with the actual Vgs(th) value of the MOSFET used, so R1 can be changed to tweak the delay time if needed (larger resistance gives longer time).

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#### rj2k21

Joined Nov 26, 2021
4
Thanks for the fast replies. I like the idea of using just a few discrete components to get this working.

I really feel like an idiot asking what probably seems like simple questions, I'm fine with digital logic but practical analog electronics always feels like a black art!

How can I get those examples working using a +12v, GND and the signal wire that needs pulling to ground?
crutschow does this need a 'pulse' to be switched to pull the signal wire to ground? Would this work by just supplying a constant 12v from an ignition power source?

#### crutschow

Joined Mar 14, 2008
29,537
Would this work by just supplying a constant 12v from an ignition power source?
Yes, if you don't mind the timing starting from the moment the ignition is turned on, and not when the starter is first engaged.
The circuit would need to be slightly modified to work that way.

#### MisterBill2

Joined Jan 23, 2018
11,643
I'm trying to simulate the pressing of a button after starting a car. The button is a momentary button that just shorts the input wire to ground. The input wire seems to be floating, I've measured 3-6v on this but being automotive maybe this could get as high as 14? Being more of a software person I initially thought of using a microcontroller and writing a program to detect the power on, wait and pull an output low and somehow use this to pull the actual input wire low. I think this might be over complicating the problem.
How simple of a circuit could be used to use the 12v ignition feed to power a circuit to pull this signal wire to ground for say 3 seconds (this isn't critical) and then leave it in it's original 'floating' condition which would allow the actual physical switch to be used if needed?

Thanks.
"Input wire" to WHAT? the requirement does not make much sense, and given no clue as to how much current could be expected to flow, no answer will be reliable.
So let us know what the wire connects to and what the monetary grounding is supposed to achieve. And also, is this to happen every time, or just occasionally?

#### rj2k21

Joined Nov 26, 2021
4
"Input wire" to WHAT? the requirement does not make much sense, and given no clue as to how much current could be expected to flow, no answer will be reliable.
So let us know what the wire connects to and what the monetary grounding is supposed to achieve. And also, is this to happen every time, or just occasionally?
I meant the input wire to the switch. The other side of the momentary switch is grounded, pressing the switch just connects the 'input' wire to ground. I am just trying to effectively 'press' the button automatically when starting the car, if that makes sense?

#### Tonyr1084

Joined Sep 24, 2015
6,864
I'm trying to simulate the pressing of a button after starting a car. The button is a momentary button that just shorts the input wire to ground.
So at present you start the car then press a button. But you want to start the car and moments after starting you want the button to be automatically pressed for just a moment. Right? If so - crutschow has given you a pretty straight forward solution, but you might need a little more circuitry. Otherwise moments after starting the car Crutschow's circuit will push and hold the push button down (or simulate being pushed down) until you shut the car off. I think you're asking for it to "Push" the button but then release it.

Would help if we understood the nature of your project. More details would be helpful, like what you're trying to achieve? What kind of circuitry are you trying to delay? There might be a better alternative than the push button altogether. Instead of grounding your circuit momentarily (till after the start) you might want to delay powering your circuit until after the car has started. We're doing some guesswork here, and that might not lead to a suitable solution. The guys here are pretty darn smart AND clever. Someone will likely know the best solution to your problem. My take was to delay the start of something; which is what my circuit did. Delayed power on for four seconds; give or take some "EH".

#### LowQCab

Joined Nov 6, 2012
1,931
What is the device being controlled ?
And preferably with a Schematic-Diagram of the internal Circuitry,
or at least a rudimentary Wiring-Picture.

Why does this Button need to be pressed when the Starter is Operated ?
Or, are You wanting to turn "something" On when
the Alternator-Voltage-Increase indicates that the Engine is running ?

Does it need to turn Off when the Engine is not running ?,
or maybe operate a second time to turn it Off ?

Don't leave us hangin'.
.
.
.

#### MisterBill2

Joined Jan 23, 2018
11,643
OK, here is how to detect that condition and have the conditions to operate that closure: 1.ignition power on, 2. engine cranking power was on and now is off., while ignition power is still on. 3. The oil pressure switch has not yet operated and condition #2 is true.
This describes the condition of an engine that has just been started and has run just a very few seconds. It is based on the guess that three seconds is not a critical time, and that four or even five seconds would be just as acceptable a time.
An alternative would be to initiate the time period the first time the engine speed exceeds 500RPM, or some similar value. That would need to set a flag so that it would not happen again until after the engine had been switched off.
Of course, if there were ever any engine starting problems it would cause problems.
AND, if this is to switch on the air conditioning or a sound system I will provide no more answers at all.

#### rj2k21

Joined Nov 26, 2021
4
To give this some context, the button is just to disable/enable the auto stop/start system of the car. It really isn't needed where I live and rather than have to press the button every time the car starts to disable it, I just want a way to have this feature off by default with the option of enabling it. The car does not save its state and defaults to on.
The wire going to the switch only has to be grounded to switch the state of the stop/start feature. Attaching the wire going to the switch directly to ground does disable the stop/start but obviously it could not be enabled with the button if needed as connecting it to ground is the same as holding the button in permanently.
Coming from an IT/programming background the obvious choice was to use a microcontroller which could be powered on with the ignition, use an output pin to pull the state of this wire to ground for x seconds, I haven't worked out the minimum time which would work, but 1/2/3 seconds would all work fine. I thought something like this might work.

But using a microcontroller just to effectively 'press' a button seemed like overkill for such a simple task, so I thought I would ask here if there was a simpler solution using a few discrete components to achieve the same result.

#### MisterBill2

Joined Jan 23, 2018
11,643
OK, now that the application is known, probably a simple relay with a delayed release, operated by the starter motor command through a diode, with a large enough capacitor value to hold the relay operated for a few seconds after the start command is ended. It can be a very small relay with a single normally open contact and a high coil resistance. The capacitor can be possibly a 470 mFD 16 or 20 volt device, and the isolation diode need only handle possibly as much as an amp, (1N4001) The closure would be on during cranking time, but that should not matter at all.

#### iimagine

Joined Dec 20, 2010
501
I believe the TS wants a one-shot circuit to trigger and releases the push button by ~3 secs once the car started and remains running.
A Relay might not be needed if you can measure how much current through the button

#### MisterBill2

Joined Jan 23, 2018
11,643
I believe the TS wants a one-shot circuit to trigger and releases the push button by ~3 secs once the car started and remains running.
A Relay might not be needed if you can measure how much current through the button
With the additional understanding via the additional information, it seems that what I proposed in post #13 will be satisfactory.

#### iimagine

Joined Dec 20, 2010
501
hold the relay operated for a few seconds after the start command is ended
If the car remains running, there wont be a start command-end

#### MisterBill2

Joined Jan 23, 2018
11,643
If the car remains running, there wont be a start command-end
Certainlythere will be an end to the "Crank Engine, (start) command. That happens when the engine catches and runs up, and either the driver or the computer decides that it is running.