One Shot 555 Timer

Discussion in 'The Projects Forum' started by ryfitzger227, Feb 12, 2012.

  1. ryfitzger227

    Thread Starter New Member

    Dec 4, 2011
    6
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    Okay this is simple.. I thought...

    I'm using a 555 timer as a "Safety Switch".

    A microcontroller will be programmed to send power to a 555 timer right after a button is pressed. The 555 timer's output will be connected to a micro relay coil. The coil will need to be energized for about 10 seconds.

    Think of it this way:
    Once power is applied to a 555 timer, I want an LED to be cut on for 10 seconds, and cut off (NO FLASHING). Once power is taken off, and applied again, this "system" will restart.

    I would like a simple circuit to do this..

    Thanks!
    -Ryan
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    The basic 555 monostable looks like this. We can narrow down the detail as we go, but this will give you the first introduction.

    How familiar with circuits are you?

    The 555 Projects

    555 Monostable
     
  3. ryfitzger227

    Thread Starter New Member

    Dec 4, 2011
    6
    0
    Okay.. I read over that, but i'm still confused.

    I'll be honest, i'm 14 years old, but I've been working with the 555 circuits for about a yr and half now.

    Your circuit confused me. I'm using one led. Not 2.. I'm actually building a timing circuit for a local racetrack that my mom and dad own. Our old timing system broke down. I need it by Thursday. I haven't even built the test circuit yet! I still have to build the PCB!
    Therefore, I'm on a very short deadline. Please give me a SIMPLE schematic I can look at. I know this sounds bad, but I really don't have time to read all of the post you have and learn how to do it.

    IF you can, please create a simple schematic for me.

    Again:
    Connect power to a 555 > LED illuminates for 10 seconds > LED shuts off > LED does not cut back on until power is disconnected and connected again.

    Thanks!
    - Ryan
     
  4. BSomer

    Member

    Dec 28, 2011
    433
    106
    Did you read the write up Bill did on the monostable 555 circuit? It explains everything you need to know about how it works. If you only want one LED, remove the red LED and 1K resistor tied between it and V+. Then what will happen is the green LED will come on when the 555 is triggered. The 555 cannot be triggered again until the timing circuit has completed.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I can give you a better circuit. It is based off of what I linked, and the link has a good theory of operation if you want to try wrapping your head around it.

    I'm currently stepping out the door, and won't be back for around 16 or so hours, but I will get back with you. Promise.

    For what it is worth I love helping the younger techs. I used to be there 40 mumble years ago.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Gee, time flies when your having fun...

    [​IMG]

    R1 - 10KΩ
    C1,2 - 0.1µF
    R2 - 470Ω
    CR1 - 1N4001
    Q1 - 2N2222A
    K1 - Small Relay
    For 10 Sec if Ct = 100µF then Rt = 91KΩ
     
  7. douglasgb

    New Member

    Feb 16, 2012
    8
    0
    Bill,

    First, thanks for taking the time to share your knowledge! Great help for those of us embarking on the journey.

    I am building an analog to digital converter based on the ADC0804 in self-clocking/free-running mode, but to initialize the chip I need to momentarily ground two of its pins after powerup.

    I've read your 555 Monostable chapter but admit I need more time to digest it and experiment. I also need to experiment with the ADC0804 to see if the pins can start off grounded and then float, or if they need to float, be grounded, and then float again.

    But in the meantime I was wondering if the circuit shown above could be used for this purpose. Removing R2, CR1, Q1 and K1, wouldn't the output of pin 3 be ground upon powerup and then go high?

    But part of me thinks there should be a way to do what I need with a simple resistor/capacitor pairing... depends on what the ADC0804 wants I guess.
     
    Last edited: Feb 16, 2012
  8. Wendy

    Moderator

    Mar 24, 2008
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    No problem, and yes, a simple RC will do the trick. I use it all the time.

    The old Commodore 64 used a 555 for a power on reset too. It is a very old technique.

    Thing about this site, do not be afraid to ask questions. We do not allow trolls, and don't mind going through basics now and again.

    If there is something you want my personal attention on just PM me. I may not be able to help, but I will try.

    The 555 can't power the world, so I used a relay. It will start on for a set amount of time, then go low. Many reset circuits need the opposite, but that is not hard either. A simple transistor will do it well.
     
    Last edited: Feb 16, 2012
  9. douglasgb

    New Member

    Feb 16, 2012
    8
    0
    I will be putting together the prototype in the next few days and will see what behavior the chip requires. I can comprehend RC ramping up when power is applied, so it would start low and end up high and maybe that will do it. My worry is if the chip needs float-ground-float, I'm not sure how to 'wait' and then ground and then float again.

    I'd love to do it with as few components as possible to keep the board small.

    You mentioned doing this with a transistor... along with a resistor and cap, or by itself? It is just to square off the signal?
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    No, to invert the signal.

    A 555 makes a great Schmitt Trigger. You can find many other chips that do this too (Schmitt Triggers). Their job is to square off analog to digital.

    555 Schmitt Trigger
     
  11. douglasgb

    New Member

    Feb 16, 2012
    8
    0
    Well good news for me... I built my prototype and played around with the chip, and it's okay if the two initializing pins start (powerup) at ground, and after a moment are allowed to float. I imagine this simplifies things greatly, however I'm not sure how to accomplish it!
    I will do some research and thinking about it this weekend, but the guidance in your post #8 has me thinking R+C provides delay and then switches Q, which stops grounding the pins. I can probably figure R and C values to determine the time, but any hints on Q would be appreciated! This is a 5v chip.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    You can not allow pin 2 to float. That is an illegal and unstable condition. The Q output is the only one allowed out of the package.

    If you want to start a new thread please do so, and I will be glad to assist. We do not allow hijacks of other peoples threads, the thread belongs to the person who started it.

    In other words, start a new thread and we can discuss it. :D I like helping where I can, and there are a lot of other people with the same point of view.
     
  13. douglasgb

    New Member

    Feb 16, 2012
    8
    0
    By Q I meant transistor, sorry. But I don't think I need one. For now I am going to experiment with an RC circuit. If I can't figure it out I will start a new thread as you say. Thanks!
     
  14. douglasgb

    New Member

    Feb 16, 2012
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