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Need to turn on led for a few seconds than turn off and stay off

Discussion in 'LEDs and Optoelectronics' started by Sinewave72, Feb 2, 2016.

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  1. Sinewave72

    Sinewave72

    2
    1
    Feb 2, 2016
    Any help appreciated. looking for a simple solution in order to use 12v supply to turn on an led light so it stays on for about ten seconds than turns off and stays off until it is turned back on. They are for a light on the undercarriage of a vehicle. Even better if when turning on the ignition the light illuminates for only 10 seconds and than shuts off. Thanks :)
     
  2. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    You need a simple timer!

    give this page a quick browse, it should get you enough info to dig around for similar solutions.
    http://www.dummies.com/how-to/content/electronics-components-555-timer-chip-in-monostabl.html

    Keywords for you will be :
    555 timer, one-shot, monostable

    If you can figure out how to connect that to your lights, great! Otherwise you will need to get your hands on a relay or transistor (FET) . Whatever floats your boat.
     
    davenn likes this.
  3. ChosunOne

    ChosunOne

    411
    119
    Jun 20, 2010
    An even simpler timer, for this type of application, is a large capacitor in parallel with a resistor. I have lots of capacitors lying about, so I just use trial-and-error, but if you don't have that kind of stock at hand, you may need to figure out the specs to see what size cap and resistor you need.

    For instance: If I pair a 2200uF with, say, a 47Kohm resistor in parallel, and put that in series with an instrument LED (which I assume has its own resistor in series) I'd expect it to light for several seconds before the capacitor charges and stops the current flow. The 47K resistor in this example was usually high enough to limit the voltage at the LED so it won't light.

    When the voltage source disconnects, the capacitor discharges through the 47K resistor and it's ready to go again.

    If you know the current draw of your LED, then the values you need for the capacitor & resistor can be calculated.
     
    Last edited: Feb 2, 2016
  4. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    I like the simple idea, but think that using a resistor and capacitor to drive a transistor would be more suitable. smaller values of capacitor / resistor can be used and easily tuned without having to deal with the potentially high currents used with LED strip lighting or similar.

    Of course, this slight alteration to your idea requires using a transistor which can handle the current momentarily while in the linear region... Neither one of these methods will turn off cleanly and will fade as power goes out.
     
  5. Sinewave72

    Sinewave72

    2
    1
    Feb 2, 2016
    I found a 12 volt delay relay control module with a timer for less than 10 bucks online. Wipuld this be a possible solution?
     
    Gryd3 likes this.
  6. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    It absolutely would be :p
    But would be slightly less 'DIY' as building your own timer.
    Go for it!
    Relays are quite capable and are less prone to electrical damage as FETs and transistors.
    You might want to buy a spare though depending on how long you have the vehicle and the current of the LEDs. Relays have moving parts, and if they switch high loads countless times they tend to wear out.
    $10 and a spare part in the garage isn't a bad idea. (I would assume it would last a few years though... but I can't vouch for the quality of the part)
     
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