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LED Timer

Discussion in 'LEDs and Optoelectronics' started by Hakkas, Jun 30, 2017.

  1. Hakkas

    Hakkas

    5
    1
    Jun 30, 2017
    Hi.
    My project is an IR circuit, that when blocked, it activates a few led's.
    The problem is that i want these led's to not stay lit more than, say 1 minute. So i thought a 555 could be a solution.
    Is there a simple way to do this? I am pretty much a beginner at electronics, and the project is just a Velleman kit :)

    /Hakkas
     
    superled.com.cn likes this.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,073
    1,824
    Nov 17, 2011
    Welcome to EP.
    You'll need a monostable multivibrator. A 555 is easy to use, see e.g. here.
    If this doesn't fit your needs. Search for "monostable multivibrator".
     
  3. Hakkas

    Hakkas

    5
    1
    Jun 30, 2017
    Thanks! That seems to be what i need. But won't the trigger input be constant "high", resulting in give constant output? Or will 555 only see the high input as a "trigger" signal?

    /Hakkas
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,073
    1,824
    Nov 17, 2011
    If your trigger is much shorter than a minute, this is no issue.
    If your triger can be longer, simply add a capacitor in series with the trigger input as shown here.
     
  5. Hakkas

    Hakkas

    5
    1
    Jun 30, 2017
    Not sure i understand.
    Should it not be the LED output from my original circuit as trigger? If so, trigger will be constant as i mentioned.
    If not, what should be the trigger?
     
  6. Bluejets

    Bluejets

    3,431
    695
    Oct 5, 2014
    You set up a standard 555 timer as a monostable. i.e. it gets a short trigger signal and then it starts to time out for the set period, in your case 1 minute. Then the timer output goes low again until the next trigger.

    As your signal comes from an LED in the IR sensor that possibly stays on, then as Harald says and link shows, feed this through a capacitor to give you the necessary pulse.

    Talking electronics also has a great deal of info.
    http://www.talkingelectronics.com/te_interactive_index.html

    Another example here...http://www.circuitdb.com/?p=226
     
  7. Hakkas

    Hakkas

    5
    1
    Jun 30, 2017
    Thanks. Think i understand. The output will go low after time, even if trigger signal is high.
    Then when input goes low, and then high again it will start the timer?
     
  8. Audioguru

    Audioguru

    2,516
    563
    Sep 24, 2016
    Why are you talking about an "input"? The 555 does not have an input, instead it has a trigger input. The output of the 555 goes high when the trigger input goes low and times out only when the trigger input is high during the timeout. If the trigger input is still low after the timeout then the output will be stuck high.
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    9,073
    1,824
    Nov 17, 2011
    The 555 is triggered by a negative going edge (input from high to low). If you want to triger the 555 from a positive edge (input from low to high), then you'll have to add a simple inverter:
    upload_2017-7-1_8-24-49.png
    Here I've incorporated the capacitor to generate the trigger pulse.

    You need to develop an understanding of the 555's operation in this circuit. The article in the link I gave you describes the operation of the 555 as a monostable multivibrator quite well.
     
  10. Hakkas

    Hakkas

    5
    1
    Jun 30, 2017
    Audioguru: Please don't judge me. I said i'm a beginner. I assumed the trigger input is a "input".
    I am trying to learn how this works.
     
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