Time Delay

Discussion in 'The Projects Forum' started by hi452, Apr 4, 2013.

  1. hi452

    Thread Starter New Member

    Apr 4, 2013
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    I am working on a project where I am planning on using a 555 timer to turn on a light for a designated amount of time and then turn off after the press of a button. I do believe this is possible through the mono stable capabilities of the 555 timer.

    So getting more into specifics, I have a time of about 100 seconds in mind to use as how long the light is on. Therefore using the formula of

    T=1.1*R*C

    I figured on using a 100K ohm resistor and a 1000 uf (.001 f) capacitor. This turns out to be about 110 seconds, which is close enough in my case. (I was shooting for the 100-150 seconds range in all reality anyhow.)

    So bought these parts
    • LED
    • 555 Timer
    • 1000 uf Capacitor
    • 100k Ohm Resistor
    • Momentary switch

    Hooking them up in this way

    [​IMG]

    (Sorry about the bad schematic, apperently Microsoft word isn't the best for making schematics.:rolleyes: Pin 1 is top left)

    I have made sure all components that are polarized are installed in the correct way. I am supplying at least 5v, and I know the 555 timer is okay. But yet when testing it I have no response to the switch most of the time. Occasionally it will turn on the led for about 3 seconds (at most) and then the led goes off.

    What am I doing wrong here? The calculations say about 110 seconds (and probably longer due to the leakage in capacitors) and I am getting about 3 when it does turn on, which is totally inconsistent. And the more interesting part i when I completely remove everything connected to pins 6 & 7 I get the same results, which tells me they aint doing anything.

    So anyone have some suggestions that would cause this and a solution to this?
     
  2. antonv

    Member

    Nov 27, 2012
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    I think you should tie pin 2 high with a resistor (10k or so) and trigger it by grounding pin 2 briefly.
     
  3. absf

    Senior Member

    Dec 29, 2010
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    Something like the attached should work. BTW your LED is connected backward on your schematic.

    Allen
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    A 100nF cap across pins 1 and 8 would be a good addition, as would a switch conditioner for the trigger input. Such a switch conditioner could consist of the 10k resistor already in your schematic plus a 100nF cap in series with the trigger lead. Then, add another 10k resistor as a pullup between the 100nF cap and pin 2.
     
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  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The purpose of the differentiator is to create a short negative-going pulse on pin 2 when the switch is depressed. Pin 2 needs to go back high before the oneshot times out.
    For a 100 second oneshot, he should not need a differentiator. All he has to do is release the switch in less than 100 seconds.
     
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  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Ron H is correct. I didn't pay attention to the long time the OP wanted.
     
  7. hi452

    Thread Starter New Member

    Apr 4, 2013
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    Sorry I put that in there wrong. New guy alert. :)

    So do I need the 10K resistor from pin 2 to high or not?

    I built it according to plan, but yet it doesn't work. It stays on for about 15 seconds and then goes off with the 100K ohm resistor in place. When I replace the 100k ohm resistor with a 10k ohm resistor, it works for approx 8 seconds, which is what it should. So am I doing something wrong here or is the capacitor not charging correctly with the 100K resistor. I thought that this would be reliable up to like 10 minutes, but just getting a 110 second delay is turning out to be harder than expected.

    Thanks for your responses.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The problem is that large electrolytic caps have large leakage current, you can imagine it asi if you had a resistor in parallel with the cap, so the cap discharges faster. For such long time it is better to use an oscillator and a counter in some arrangement to time the 100 seconds.
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    It should work fine built according to the schematic posted by absf. According to the calculation, 1000 μF and 100 k should produce 110 seconds. If actual time was off even 30% I might think it was due to component tolerances, but I think you have a wiring error or a bad 1000 μF cap.
     
  10. hi452

    Thread Starter New Member

    Apr 4, 2013
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    I have tried physically building this, checking everything at least 5 times and it didn't work. So figuring that I am just an terrible builder, I tried copying your schematic into Multisim for evaluation of what is happening. Here is what I did:

    [​IMG]

    When I go to simulate it, It doesn't work.:eek::eek:There is no voltage on the output. It is not my LED. Any clue what is going wrong here? Or does anyone know any quirks about Multisim that I may have screwed up or settings I should change?

    I am sorry to keep bugging you guys or gals about this, but it is really starting to get frustrating that this will not work. I do NOT doubt your intelligence, but something is wrong and I would love to know what.

    EDIT: IF you cant read the pin assingment names on the 555, here they are from top to bottom for the left side. (The top pin is vcc and bottom pin is ground with the right pin being the output)

    RST
    DIS
    THR
    TRI
    CON
     
    Last edited: Apr 5, 2013
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The 100k resistor is in the wrong place. It should be between the cap and Vcc.
     
  12. tracecom

    AAC Fanatic!

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    How are you building the circuit? Solderless breadboard or what? Post some pictures of your build.
     
  13. hi452

    Thread Starter New Member

    Apr 4, 2013
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    Yes I am using a solderless breadboard. I used a continuity checker when the circuit wasn't energized to make sure that I was getting good connections to all the wires/pins, and used a multimeter to test voltage, so i doubt it is either of those. I will get the pics of my build later as I too the thing apart until I found out why this isn't working. I figured I was wasting time doing it that way as it turned into a game of trial and error.

    But I am more worried about why I cannot get this to work. I changed it like you said but it still doesn't work.

    [​IMG]
     
  14. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I know nothing about multi-sim.

    Attached is a photo of a breadboard that I built that worked; also attached is the schematic.

    If you build it like mine and it doesn't work, you have bad component(s) or a bad breadboard.
     
  15. hi452

    Thread Starter New Member

    Apr 4, 2013
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    Okay well thank you for what you have given me so far. I will try to physically build your schematic tomorrow as it is getting a bit late where I am. Once again thank you and goodnight. :)
     
  16. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You can leave off R4 and C4.
     
  17. antonv

    Member

    Nov 27, 2012
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    It sounds to me like your circuit is working, but as someone mentioned, you may have excess leakage current through the electrolytic.

    Do you have another one you can try, even if it has a different value?
     
  18. Ron H

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    Apr 14, 2005
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    Hi452, pay attention to this! Kubeek speaks the truth.
     
  19. tracecom

    AAC Fanatic!

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    Using a 1000 μF cap is not terribly accurate, but can work. I just breadboarded the circuit again with the first 1000 μF cap and 100 k resistor I found in my junk tray, and it timed out at 112 seconds. Then, I substituted a 100 μF and a 1M; it ran 137 seconds.

    Timing is not the problem the OP is having. He can't get pin 3 of the 555 to go high in the first place. I still think it's a wiring error, one or more defective components, or a defective breadboard. It might also be a power problem; he says he is using a 5V supply, but it might be sagging below the 4.5V minimum the 555 needs.
     
  20. hi452

    Thread Starter New Member

    Apr 4, 2013
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    Thanks for trying that. It gave me a bit more inspiration to continue building. :)

    I do understand and if I ever do this again for a lengthy period of time, I will definitely go for something other than these huge capacitors. The leakage is terrible.

    But on a happier note, I did finally get it working. I decided to switch to a standard 9 volt battery as my power supply as tracecom said that power might be an issue. I do not know if that was the issue, but building it today workes like a charm. I also took out and replaced the switch with another, so who knows what it really was. I am about 10 seconds over the optimal point, but these capacitors are leaking, and I can accept that. For my applications it will work brilliantly.

    Here is some pics of my build, just to brag a bit about it.
    [​IMG]
    [​IMG]
    (In pictures I was activating it by shorting pin 1 to pin 2 with a wire temporarily, thus why the switch isn't there. I have since put in a momentary NO push button switch)

    Thank you all for your advice and time.:D:D
     
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