I need to power a relay for 15 seconds when ignition turns off

[adown]

Hi Everyone, I am out of ideas here. I am looking for a way to dump the air in my motorcycle air shocks when I turn off the ignition. This will be accomplished by energizing an air solenoid. That is why I need to power it for 15 seconds, then turn it off. I thought I could do it with a simple delay circuit with discrete components, but I can't figure out how. I know it would be simple to do with an Arduino, but I need to make it as small as possible, and I lack the programing skills necessary. Thanks for any ideas, adown

 

[Reloadron]

Using discreet components you could likely get where you want to go using a 555 timer configured as a "One Shot". When the Ign is turned off it would serve as a trigger to the 555 and the 555 would output a pulse with a 15 second duration. I would make the duration variable so you have some room to work with. The 555 output would drive a simple transistor to turn the solenoid On / Off. The transistor determined by the solenoid current. This could all be put in a very small package.

I haven't played with any micro controllers in a few years but a small 8 pin uC would be all that is needed and a few lines of code. The uC would drive the transistor that turns the solenoid On / Off.

No matter how you do this the circuit will always draw a small quiescent operating current. A Google of 555 Timer as Monostable Multivibrator should bring up a dozen circuits for you to think about as far as discreet component approach.

Ron

 

[crutschow]

Using a CMOS version of the 555 (LMC555) will reduce the quiescent current when the ignition is off.

 

[adown]

Hi Ron, thank you for replying. That was my intention, but then I started confusing myself with the reset. When the trigger goes from hi to low with my ignition being shut off, that would send the one-shot, but if my ignition stays off, wont it keep triggering the one-shot, until it goes hi again? Thanks again, adown

 

[crutschow]

............................
When the trigger goes from hi to low with my ignition being shut off, that would send the one-shot, but if my ignition stays off, wont it keep triggering the one-shot, until it goes hi again?

You can avoid that by capacitively coupling the trigger. See below for a simulation of the circuit. The values shown give a one-shot time of about 16s.

Edit: Depending upon the relay you will likely need a transistor driver at the 555 output to power the relay coil. And don't forget the suppression diode across the relay coil (cathode to plus voltage).

 

[dr pepper]

Older fiats use a relay that does this, I think more like a minute, they use it so you can shut the electric windows after you shut off the ignition.
It shouldnt take too much sussing to get one to work.

 

[Reloadron]

Hi Ron, thank you for replying. That was my intention, but then I started confusing myself with the reset. When the trigger goes from hi to low with my ignition being shut off, that would send the one-shot, but if my ignition stays off, wont it keep triggering the one-shot, until it goes hi again? Thanks again, adown

You use the type design posted by Crutschow, here is another similar example. That is how you go about it using a 555 timer design.

Ron

 

[Reloadron]

Older fiats use a relay that does this, I think more like a minute, they use it so you can shut the electric windows after you shut off the ignition.
It shouldnt take too much sussing to get one to work.

My GMC truck has that feature. However, I believe that id done through the on board computer system. Never gave it much thought.

Ron

 

[adown]

Thank you for the solution. That is exactly what I had planned at the beginning, but I couldn't figure out how to keep it from re-triggering. Do you think I need a diode on the trigger input to flatten out the turn on pulse? I am going to use a power FET to drive the solenoid instead of a relay.

 

[Reloadron]

Thank you for the solution. That is exactly what I had planned at the beginning, but I couldn't figure out how to keep it from re-triggering. Do you think I need a diode on the trigger input to flatten out the turn on pulse? I am going to use a power FET to drive the solenoid instead of a relay.

No, I would use the posted circuit as is. There are a few ways to capacitive couple the trigger. The circuit crutschow posted should work just fine as drawn. Just remember you will want a diode across the solenoid coil.

Ron

 

[crutschow]

............................
Do you think I need a diode on the trigger input to flatten out the turn on pulse?
......................

The trigger pulse is about 5ms as is, so that's plenty of time to trigger the 555.

 

[adown]

Thank you all very much for all your help and guidance. You are a great asset to this forum!!

 

[adown]

EDIT-- Disregard the findings below. I hooked it up to the motorcycle as it would be in the circuit, and it worked as originally drawn. I guess my ignition pulls lower than just an open switch on my trainer.

I wired this up on a trainer, and I couldn't get it to work as drawn. I noticed if I unhooked the trigger that would be going to my motorcycle ignition and then grounded that wire, it would trigger. The problem is my ignition just goes to open, it won't take the trigger to ground. So I wired it up as shown on my trainer, and did get it to work with out any problems. I just don't know if this will produce a significant ground on my electrical system. Can you please advise me. Thanks, adown

 

[crutschow]

EDIT-- Disregard the findings below. I hooked it up to the motorcycle as it would be in the circuit, and it worked as originally drawn. I guess my ignition pulls lower than just an open switch on my trainer.

I wired this up on a trainer, and I couldn't get it to work as drawn. I noticed if I unhooked the trigger that would be going to my motorcycle ignition and then grounded that wire, it would trigger. The problem is my ignition just goes to open, it won't take the trigger to ground. So I wired it up as shown on my trainer, and did get it to work with out any problems. I just don't know if this will produce a significant ground on my electrical system. Can you please advise me. Thanks, adown

The ignition switch does just go to open but it has a load connected to it consisting of the ignition circuit and any other devices powered from the ignition. It's that load that pulls the voltage low when the switch opens, similar to the resistor you added in the trainer (whatever a trainer is).

 

[eTech]

Hi Everyone, I am out of ideas here. I am looking for a way to dump the air in my motorcycle air shocks when I turn off the ignition. This will be accomplished by energizing an air solenoid. That is why I need to power it for 15 seconds, then turn it off. I thought I could do it with a simple delay circuit with discrete components, but I can't figure out how. I know it would be simple to do with an Arduino, but I need to make it as small as possible, and I lack the programing skills necessary. Thanks for any ideas, adown

Is there a potential safety issue if the air dumps while moving? Caused by some failure of the of the circuit?