Urgent! How to short delay circuit? - Make a LED blink after a period of time

Discussion in 'General Electronics Chat' started by Ayush Modi, Jun 27, 2015.

  1. Ayush Modi

    Thread Starter New Member

    Jun 27, 2015
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    I am trying to create a circuit that will make a LED blink after a period of time.

    I used two mono-stable 555 circuits (one with large capacitor, one with small) so after the long delay, it would trigger the second circuit but am not sure how to.

    Please help.

    Thank you in advance

    P.S. Sorry for bad grammar in title. Not sure how to change it...
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    for example you coud hook up the output of the first 555 to the reset input of the second one.
     
  3. Ayush Modi

    Thread Starter New Member

    Jun 27, 2015
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    Yes, I have already done that.
    However, I still need to push the PTM switch to make it blink once. (see picture)
    I am not sure how to do it without using the PTM switch, which is my actual question. Sorry I am not very clear. circuit.PNG
     
  4. ericgibbs

    Senior Member

    Jan 29, 2010
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    hi,
    This is one option, ref image.
    E
     
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  5. Ayush Modi

    Thread Starter New Member

    Jun 27, 2015
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    That for a moment I thought it actually worked...
    However, it blinks at the start and not when the PTM switch on the Left-hand-side is pressed. Thanks for the try though.
     
    Last edited: Jun 27, 2015
  6. ericgibbs

    Senior Member

    Jan 29, 2010
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    Thats fine.

    I would choose a much higher value for R1 than a 1K, say 100K and make the C1 100uF , much smaller.
    [A 1meg and 10uF would also give the same delay]
     
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  7. korchoi

    Member

    Jun 5, 2015
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    6
    You can cut the wire between R4 and the LED. Then between the two would be a BJT that would be set up in such a manner that a pulse from IC2 would allow the Pulses from IC1 to reach the LED. That way, one 555 modulates the other.
     
  8. Ayush Modi

    Thread Starter New Member

    Jun 27, 2015
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    Ok, so first of all, I looked up what a BJT is and found that it is a bipolar transistor, now I am not sure which one two use (NPN or PNP and which one in those categories)
    Second, I am not sure how I would do that as I am not so advance in this field.
    Thirdly, if you can, would you please draw a sketch of what you are trying to say or someone else can.

    Thank you for taking your time for me, it means a lot.

    P.S. I am reiterating to say what I am making if it helps in new ideas.
    A device that measures time after an LED blinks after a period of time.
    So I am basically making a reaction tester. Any new ideas would be great.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    A project I had published in a magazine a couple of years ago used a blocking oscillator type Joule thief circuit to generate about 50V from a single cell. This charged a small electrolytic, and when it got to about 32V, a diac salvaged from a scrap CFL dumped the charge into a bank of parallel LEDs.

    The capacitor I used was 47uF and dumped into 24 parallel LEDs, for just one LED simply make the capacitor much smaller and the blocking oscillator less energetic.

    There's nothing to stop you building a blocking oscillator for more than 1.5V input, but you have to ground reference the feedback winding and feed it to the base through a DC blocking capacitor, then the base needs a bias resistor from Vcc - and of course you have to phase the windings correctly.
     
  10. korchoi

    Member

    Jun 5, 2015
    59
    6
    there you go. There are better ways to do this "analog-switch" thing. I'll leave that silly easy thing for you to google.
    WP_20150627_17_57_03_Pro.jpg
     
  11. Ayush Modi

    Thread Starter New Member

    Jun 27, 2015
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    Hmm, thank you that is nice idea...still trying to figure out how to wire it all up but I'll see what I can do.

    Sorry, I did not know it by the technical name
    However, once I tried what you showed me very kindly, another problem arose
    circuit2.PNG
    As you can see, the circuit is short-cutting and the LED is not blinking.
    Why so many problems in a seemingly simple circuit.
     
  12. korchoi

    Member

    Jun 5, 2015
    59
    6
    I think IC2's output is actually TTL and the low level is not low enough to cut off the LED. try putting a darlington instead of a BJT, with the same pin connections.
    These small things are what stops the entire flow most of the time. In electronics, building things is not the most difficult part.The toughest part is actually fixing what is broken in them.
    Even that is just a missing solder connection here, wrong component position there, a single wrong-value resistor, foolish things like that.
     
  13. Ayush Modi

    Thread Starter New Member

    Jun 27, 2015
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    circuit3.png
    How many Darlington pairs needed? Already using three...
     
  14. Ayush Modi

    Thread Starter New Member

    Jun 27, 2015
    14
    0
    A great thanks to the people who helped me.
    But, I don't think that this circuit will ever work on what I intended.
    However, from your help, it did spark a new circuit idea in my mind.
    circuit4.PNG
    I also have a problem for this circuit, which I plead you to guide me on a different thread:
    http://forum.allaboutcircuits.com/t...017-decade-counter-once-and-then-stop.112915/

    I now ask the moderators to close this thread as I believe it probably won't last anymore, unless other members raise the issue again soon.

    Thank you for your help everyone
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    A less complicated way is to use the slow 555 output to switch the fast 555 reset pin up and down.
     
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