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Thread: delay 12v on leds

  1. #1
    Constant Bitrate
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    delay 12v on leds

    Alrighty,

    Bit of an off subject but here goes...

    Just replaced all my leds in my centre console after 8/10 of them have failed!

    These leds are fed by the original wiring in my dashboard, 12v feed when the ignition is on. I'm wanting to delay this feed for a few seconds to after the engine has started. I'm thinking the cranking time may be giving too many volts to the leds.

    What do I need to do to give a delay of about a couple of seconds? a cap and resistor? If so what sort and how do I wire them in. Can anyone shed some light on this subject?

    cheers
    Volvo C70 - Installation in progress *gulp*

  2. #2
    Self proclaimed spoon feeder TruckinMP3's Avatar
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    LEDs are current driven, meaning a resistor will likely be the answer to reduce the current. More info on the circut will be needed to calculate the resistor values correctly.
    TruckinMP3
    D201GLY2, DC-DC power, 3.5 inch SATA

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  3. #3
    Constant Bitrate
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    Isnt a resistor needed to allow the cap to 'fill up' correctly?

    Its basically a 12v source which i'm wanting to delay for around 2 seconds. So a cap would be the answer wouldn't it? It'll take a couple of seconds for the cap to completely charge thus giving a delay to the leds?

    I have about 7 leds on the circuit each with there own resistor.. these are all fine as they are though.

    Cheers for ya help.
    Volvo C70 - Installation in progress *gulp*

  4. #4
    Self proclaimed spoon feeder TruckinMP3's Avatar
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    OK I am game, why did the original LEDs fail?
    Most have a useful life of over 100,000 hours if the circut protects them from to much current. They are not voltage driven....
    TruckinMP3
    D201GLY2, DC-DC power, 3.5 inch SATA

    Yes, you should search... and Yes, It has been covered before!

    Read the FAQ!

  5. #5
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    point 1)
    ok LEDS mite not be voltage driven but excessive voltage can blow them easily ( if you take a standard 5mm LED and connect it directly to a 9v pp3 battery it WILL blow sometimes with a big bang - i have a black mark on my celing from one of these) resistors are measured in ohms but they also have a power rating and capacitors have a voltage rating

    point 2) a capacitor will not provide a delay like you are after adequately
    what you need is a time delay circuit, i will look in my collection of circuits for a simple one that mite be of use here, i also recomend putting in a voltage regulator in there to stabalise voltage

    just to check what u r after is : turn on ignition wait 2 secs turn on LEDS

    point 3) if all you are after is NOT blowing the LEDS just get a simple voltage regulator in ther that can handle upwards of 20 volts and u will be laughing!

    try a 7805 ( gives 5 volts which means u dont really need resistors on leds tho they are recomended) not sure of the upper limit of this tho
    amd k6-2 450
    64 meg ram
    5 gig hdd
    all in an old bit of metal
    all for under 20

    and now a 500watt inverter for 20
    thats 40 in total
    not too bad for a noob

  6. #6
    Constant Bitrate
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    you might have the best answer there!

    Well I haven't had much luck with all of my blue leds... Basically most of them either completely stop working after several weeks of use or some even start flickering

    One led had not been working for months and last night it came on just like new so dont know whats happening there.

    I'm sure i'm using the right resistors but obviously something isn't right.


    Yep thats what i want.. ignition on then 2 seconds later leds on. although a voltage regulator is another plan. Afterall i'm only wanting this 2 sec delay because i'm thinking when I crank the engine the voltage is rising too much.

    I've got all the resistors already soldered in so what voltage regulator should I use to keep it at a smooth as possible 12v supply. Is that all I need? (i'm short of space as well so these components need to be as small as possible.

    cheers for ya help!!
    Volvo C70 - Installation in progress *gulp*

  7. #7
    Raw Wave Laidback's Avatar
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    The voltage would go down when you crank, the up when the engine is started.

  8. #8
    Constant Bitrate
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    Quote Originally Posted by Laidback
    The voltage would go down when you crank, the up when the engine is started.
    yeah... I just see my voltmeter rise to sometimes over 14.5 for a split second as soon as the alternator has kicked in.
    Volvo C70 - Installation in progress *gulp*

  9. #9
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    well personally i would go for 5 v and a small resistor ( u can work it out ) cos that is easily stable. if you can post the details of your LEDs here ie: how many/ what voltage/current/power ratings they have and i will source u a regulator
    amd k6-2 450
    64 meg ram
    5 gig hdd
    all in an old bit of metal
    all for under 20

    and now a 500watt inverter for 20
    thats 40 in total
    not too bad for a noob

  10. #10
    Maximum Bitrate
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    get resistors so they will run at max brightness when the voltage is 14.5 - 15v so you won't blow them www.lsdiodes.com have a calculator so you can figure out what you need, thats what they recommend

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