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555 Timer — Parallax Forums

555 Timer

J^3J^3 Posts: 121
edited 2008-12-31 01:29 in General Discussion
Good Evening Everyone,

··· I am trying to build a circiut using the 555 timer to produce a square wave that has an on time of .5 seconds, and an off time of .5 seconds.· After reading the datasheet I found some formulas on page 11 to calculate the size resistors to use given a certian size capacitor.· Either I am doing some bad math, or what I want isn't possible in that application (astable operation).· If any of you have some experience with this component I could sure use some help.· Thanks in advance.

P.S.· Why do they use subscript for the parentheses in some occasions and others they don't.·


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There are 10 types of people that understand binary, those who do, and those who don't.
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Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-12-29 15:18
    You can't normally generate a symmetric square wave with a single 555 timer. You need a diode as shown in this monograph:
    www.kpsec.freeuk.com/555timer.htm
  • ercoerco Posts: 20,260
    edited 2008-12-29 15:41
    Mike: Great reference, that's my favorite 555 site.

    JJ: You can't quite get 50%, but close. Per Mike and that great site, you must add a diode in parallel with R2 to get below 50%.

    Without a diode, it looks like R1=10K, R2=100K, C1=10 uf (that's .00001 F in the calculations) will give you 0.645 hz and a duty cycle of 52.4%. You want 1 hz, so you can either use a third-smaller cap or resistors, depending on·what parts you have handy.

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    ·"If you build it, they will come."
  • J^3J^3 Posts: 121
    edited 2008-12-29 16:48
    Thanks Mike, and Erco.· I will check out the site.· What about the parentheses being in subscript, is that indicating anything special, or is that just the way they typed it.

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    There are 10 types of people that understand binary, those who do, and those who don't.
  • Mike GreenMike Green Posts: 23,101
    edited 2008-12-29 17:02
    Re: parentheses subscripted

    I think they just didn't have a word processor that handled the parentheses properly.
  • J^3J^3 Posts: 121
    edited 2008-12-29 17:18
    ··· By the way, I am trying to build this circiut as a means of generating a steady pulse for a clock project.· I have most of my code written for keeping track of time and the date, but the pulse generator on the PDB looses a couple of minutes after a few hours.· I still need to add the switches for setting the time and date, and I also want to have a temp display.· Here is what I have so far if you are interested.· Mike,·thanks for the site, I will be reading more articles from the site and passing it on to freinds at school.

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    bs2
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    2x16_Lcd.bs2
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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2008-12-29 17:45
    This thread is not BASIC Stamp related and is being moved to the Sandbox Forum.

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    Chris Savage
    Parallax Engineering
  • kwinnkwinn Posts: 8,697
    edited 2008-12-29 19:15
    JJ, the 555 is not a good choice for a clock to keep time and date. Even with the best quality capacitors and precision resistors it will not be stable enough. There are chips that let you use a 32Khz watch crystal to produce timing pulses for that purpose.
  • J^3J^3 Posts: 121
    edited 2008-12-29 19:45
    kwinn,
    ··· Can you reference some of those chips, so that I may review them.· Thank you for your reply.

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    There are 10 types of people that understand binary, those who do, and those who don't.
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2008-12-30 01:08
    JJ

    Here is the Link to it

    ·Here for·the·DS1302Time Chip


    Make sure order one of theses as well
    Here·for 32.768 kHz crystal.

    Look at a DS1302 Time Chip that PARALLAX sells that should work for and there
    Demo code for this chip·as well

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    ··Thanks for any·idea.gif·that you may have and all of your time finding them

    ·
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    Sam

    Post Edited (sam_sam_sam) : 12/30/2008 1:25:38 AM GMT
  • kwinnkwinn Posts: 8,697
    edited 2008-12-30 02:21
    JJ - If you are using a microcontroller to build a clock the DS1302 and 32.768KHz crystal Sam mentions above are good choices. If however you are using counter chips and a display then you need a 1pps signal such as that produced by a SER3618TR-ND 32.768KHz oscillator and a 74HC294 counter that will divide the 32.768KHz to produce 1 pulse per second. I found these parts at "http://dkc1.digikey.com/ca/digihome.html", but I am sure there are other choices out there.
    If you need more help choosing parts please post detailed information on what you want to accomplish.
  • charlieknoxcharlieknox Posts: 27
    edited 2008-12-31 01:29
    From your previous posts it appears you already have the tools to accomplish your goal.· You mention the "PDB" and pulse generator.· That tells me you are using the Parallax Professional Development Board, which ALREADY has a real time clock on it.· See page 188 of

    http://www.parallax.com/Portals/0/Downloads/docs/books/sw/Web-SW-v2.1.pdf

    Hopefully that will get you on your way.· On the other hand, maybe you are avoiding using it for some reason ???

    Good luck with your project !!

    Charlie
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