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  #1  
Old 02-05-2010, 08:47 AM
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Custom 12v Electronic sleep timer circuit needed for Headrest DVDs

I have some headrest DVD players that the kids love. I was able to wire them through my existing AV pack (I an E39 with the have the Mk4 Nav + TV option). The trouble is, they'll only work when the ignition is in position 1, because that's the way I've wired them. Obviously I don't want them left on when I exit the car (usually).

So that's the trouble, sometimes, when the kids (between 2 and 4 years old) are watching a movie I'd like to keep it running when I pop out of the car for a couple of minutes. For instance, when paying for petrol. Obviously I don't want to leave the keys in the ignition, and I normally lock the car for the 3-4 minutes it takes to go pay.

What I'd like is a simple time circuit that can keep power to the headrest DVDs for a short period, say 5 to 10 minutes. Think of it a little like the BMW 'sleep mode'.

I have key-1 power and also permanent power in the center console where they're wired. I'm guessing what I need is some kind of transistor, capacitor, 555 circuit kind of thing where the circuit takes power from the permanent feed as long as the Key-1 power feed is live. But as soon as the Key1 power is removed it will keep drawing power from the permanent feed for a few more minutes. Preferably tunable to between 5 and 10 minutes.

Ideally, a pushbutton or switch that disables the circuit or simply shorts the timer to zero would also be useful if I'm home and leaving the car completely.

Well that's the limit of my not quite passable O-Level electronics grade knowledge exhausted. I believe I could solder one together from components if I knew what to build but other than that I don't really know where to start.

Is there anyone in this forum with enough electronics knowledge to ***-packet sketch a workable circuit for me that would do this?

many thanks
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  #2  
Old 02-05-2010, 09:28 AM
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What i've found is if your watching a dvd, without the engine running, as soon as you crank the engine, the power drops and the dvd resets anyway. Maybe thats just my PS2 and the way it's wired in.

So if you were thinking this sleep timer would save you the hassle of putting the DVD back to where it left off then it may not help.

Other than that, this should do you perfectly - push the button before you turn the ignition off, then it will supply power for anything up to 15 minutes - Only costs £8 too so worth a try

Universal Timer Kit : Timer Kits : Maplin

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Old 02-05-2010, 09:36 AM
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Aha - a 555 chip, a transistor AND a capacitor - just as I thought ;P

That's not a bad start thanks Neil, but from what I can tell, it's push on/push off. It's not triggered by the car feed. It's a good fallback, and I might just try it, but I'm looking for something slightly more automatic really.

Re: your comment about the resetting - it's a good point, I hadn't thought of that, but as far as I know, our DVDs don't do that if going from Key to start is any indication. I'm pretty sure the permanent power feed is, well, permanent and reliable. The power doesn't seem to drop or spike in any significant way that affects this. I guess if it did, all kinds of things would get reset eg. Nav.

This could be a factor of the larger battery on Nav setups?

Still hoping for a closer solution...
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Old 02-05-2010, 09:44 AM
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Quote:
Originally Posted by danieljsinclair View Post
That's not a bad start thanks Neil, but from what I can tell, it's push on/push off. It's not triggered by the car feed. It's a good fallback, and I might just try it, but I'm looking for something slightly more automatic really.
I disagree - you give it power, then it flicks the relay for a preset time period.

Quote:
Originally Posted by Maplin View Post
Q- I'm looking for something to switch on a 12v bulb when current is applied and switch the bulb off after a few seconds with the 12v feed still being applied. Will this item do this - martin
Answer- Yes the product does support that.

Quote:
Originally Posted by Maplin View Post
Q- Will this work with a 12 v ( car ) circuit, to turn on and off after a delay a water pump? Trigger is a pressure switch to this item. - Neil
Answer- this is a +12Vdc supply item so it will work with a car.

Quote:
Originally Posted by danieljsinclair View Post
Re: your comment about the resetting - it's a good point, I hadn't thought of that, but as far as I know, our DVDs don't do that if going from Key to start is any indication. I'm pretty sure the permanent power feed is, well, permanent and reliable. The power doesn't seem to drop or spike in any significant way that affects this. I guess if it did, all kinds of things would get reset eg. Nav.
Agreed - permanent is permenant. But when you crank the engine, the voltage WILL drop all around the circuit. And if it drops below the minimum required by the DVD player, it will effectively turn off. It wont reset completeley as there is still voltage there, but it may be at 10 or 11 volts and DVD player might require a minimum of 11.8 to function properly.

Just a thought, and something worth checking before you proceed.
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Old 02-05-2010, 09:51 AM
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Hi Neil
re: voltage drop - yes, I picked that up from your first post. It's well worth noting, and something I'd not considered until now. However, I don't *think* it's a problem - but I can easily double check. Even if it does, it's not a huge issue, because the kids just whine for the few seconds it takes to reboot and restart the movie. That's better than them whining for the entire time that I'm in the shop and then having to manually restart again

re: the maplin circuit - it *looks* like it will do it. But that question says, "off after a few seconds with the 12v feed still being applied", whereas my circuit would require it to remain on for a few seconds after the power has been released.

It actually has to stay on irrespective of timer as long as the power is released, and specifically NOT shut off "with the 21v feed still being applied".

I'm not clear whether that answers my question - I'll post the question on the maplin site....
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Old 02-05-2010, 10:45 AM
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Quote:
Originally Posted by danieljsinclair View Post
re: the maplin circuit - it *looks* like it will do it. But that question says, "off after a few seconds with the 12v feed still being applied", whereas my circuit would require it to remain on for a few seconds after the power has been released.
Ah - i see now. You need something to stay on for x minutes after a switched supply is removed.

Got me thinking - what else stays on even after you turn it off? INTERIOR LIGHT

How about this? Delay is only 10 seconds but you could change the resistor/capacitor to suit

Vehicle Courtesy Light Extender (1115)
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Old 02-05-2010, 10:58 AM
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Yep, you've got it.

The courtesy light does stay on for a few seconds, not long enough. Other devices like the Nav stay on for about 10 minutes. As you probably know, full shutdown (sleep mode) is about 16 minutes, but as far as I know that's all the devices doing their own shutdown modes. I don't believe there's a master snooze wire or anything like that

Again - the courtesy light solution is a step in the right direction, but I don't think it could supply enough power for twin DVD players for 10 minutes. I'm guessing, since my electronics isn't good enough to properly do the maths, but shooting from the hip, I'd say that would require a monster (even life-threatening capacitor) to augment it enough to deliver that amount of power for that period.

I think my first instinct, regarding a triggered timer solution is what's needed here.

Hoping somebody with better electronics know-how than me can sketch one out.
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Old 02-05-2010, 11:02 AM
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Maybe the best solution is to take this opportunity to teach the children that patience is a virtue. What hoops will you jump through when they start whining about the res and sound?
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Old 02-05-2010, 12:17 PM
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My suggestion would be to power the DVD setup via a relay that pulls it's main power from a fused always hot and activate the relay from a wire to a component like the nav that stays on for the extra 10 minutes or so. Simple yet effective solution, just make sure to get the proper wiring schematics and use a multimeter to know ecactly what wires to hook to as you don't want to inadvertently cause a battery drain or other problem.
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Old 02-05-2010, 12:36 PM
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Actually, somebody suggested in another forum that the glove box light stays powered for about 15 minutes which would serve very well.

The only trouble is I still need an instant kill switch for those occasions when I'm parking up.

Also, I don't know if that 'timed power feed' is available in the center console (I've tapped into the mobile harness). If it is, then I still need the kill switch. The trouble is, the only way to signal back on again is Key1.

The glove box solution might work if I can make do without the kill switch and turn the screens off by hand - that sucks. It'll drive my wife mad, and she'll demand I revert the change to the "off by key" default.
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