12v Electronic sleep timer circuit needed for Headrest DVDs

Discussion in 'General Electronics Chat' started by danieljsinclair, Feb 5, 2010.

  1. danieljsinclair

    Thread Starter New Member

    Feb 5, 2010
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    I have some headrest DVD players in the car that the kids love. but 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.

    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 cig-packet sketch a workable circuit for me that would do this? Ideally, an off-the-shelf kit that could be tweaked would be ideal.

    many thanks
     
  2. MikeML

    AAC Fanatic!

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    Designed using LTSpice just this morning. Write back if you have questions.
     
    Last edited: Feb 5, 2010
  3. danieljsinclair

    Thread Starter New Member

    Feb 5, 2010
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    Crikey! thanks. I figured this forum would be the place to ask - but now I'm wondering if I've over-estimated my own sub-average electronics knowledge :)

    I think I can read well enough though to get a parts list off that diagram, and with a little bit of a google refresher, probably decipher enough to solder that together.

    Tell me - are there places online these-days that can take a schematic and post you back a working circuit - or am I imagining that?
     
  4. danieljsinclair

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    Feb 5, 2010
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    Actually - looking again, sorry for being such a noob, but I only need to build the bit in the middle right? the dotted areas on the left and right are part of your test harness you needed to (presumably) simulate the circuit in software?

    In which case, I just need 3 resistors, a 555, a 220mf capacitor, but I'm can't seem to interpret M1 - is that a diode, or the push reset switch?

    no idea what the graph is showing me, but it looks the part :)

    Second question - if I wanted to adjust the 10 minute delay, is that simply a case of replacing one of resistors with a variable one? R2?
     
  5. MikeML

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    Yes, you will only build the middle bit. The rest already exists in your car.

    M1 is a P-Channel Power FET. There are dozens of possible substitutes. The output is the drain. It needs to have a breakdown voltage of >30V, and an ON-resistance of <0.1Ω

    The delay is proportional to the product R2*C2. R2 is about as big as it should be, but it could be smaller. As simulated the delay is about 500sec.

    I forgot the reset button. Wire a 1K resistor in series with a momentary pushbutton. Ground one end of the pushbutton. Tie the 1K to the node labeled "RC". You will only be able to reset it if the Key is off.

    In the plot, the red trace V(12V) is the battery voltage. The dark blue trace V(Out) is the voltage applied to your babysitter, showing it going off ~500sec after the key is turned off. The light blue trace V(RC)is the voltage across the timing network, showing how it gets pulled high by the key switch, and then decays after the switch is turned off. The green trace V(key) simulates the ignition key ON/OFF. All the traces are plotted vs time (in 1000nds of sec) along the X axis.
     
  6. danieljsinclair

    Thread Starter New Member

    Feb 5, 2010
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    OK, so a FET is another transistor type thingy. OK, I think I have all that. I'll print it off and see what my local electronics store can come up with.

    Thanks - you've been awesome!
     
  7. danieljsinclair

    Thread Starter New Member

    Feb 5, 2010
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    Mike
    I've been trying to track on of these widgets down and they're not as easy to find as I'd have thought (or I'm looking in the wrong place).

    Lots of N Channel, Dual channel and different pin configurations though. I have the spec at;
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRF7205-ND

    and I'm trying to find a UK supplier (without crazy shipping costs) for these, or any other FET with similar characteristics. I've been able to source all the other parts from;
    http://www.bowood-electronics.co.uk/

    What other names do these items go by? "international rectifier"? "power transistor"? Is the 30v voltage important?

    Am I right in noting that these devices can provide over 4W at 12v? - I think that should just be enough to power two DVD players from what I can tell.
     
  8. MikeML

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    Here is a list of P-Chan FET suitable for your task that Digi-Key has in stock....
     
  9. danieljsinclair

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    Feb 5, 2010
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    ah ok - so as long as it's more than 30v and within those other limits you mentioned, it doesn't really matter. I can choose something good for 200v by the looks?
     
  10. danieljsinclair

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    Feb 5, 2010
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  11. MikeML

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    I would try to find one in a TO-220 package. The ones you found are all surface mount.
     
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