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555 question

Discussion in 'Electronic Basics' started by aman, Apr 15, 2005.

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  1. aman

    aman Guest

    Is it possible to use 555 to generate a pulse which triggers after
    every 24 hours. Am I looking at an RC time constant which is not at all
    feasible ?
  2. John Fields

    John Fields Guest


  3. With a supercap, you could think of getting a few hours
    with 20% accuracy. Your 24 hours is out of reach, as
    Mr. Fields states. You need to be thinking about some
    kind of digital accumulation of a shorter interval that can
    be done more accurately.
  4. I would try a 4060 or 74HC060(CMOS).
    It contains a oscillator and a counter.
  5. Chris

    Chris Guest

    No, and even if you could how long would it take to calibrate. A week, 2
    weeks ....

  6. No, that's why Exar introduced the 2240, which was a 555 (though without
    some of the pins externally available) in a package with a 7-bit binary
    counter. Via feedback you can set the divider to whatever you want,
    and then you run the 555 at a decent rate. As people have pointed out,
    you can then adjust the clock almost immediately, and you don't fuss
    with the impossible of getting suitable components for such low timing

    I have no idea if the 2240 is still available.

    Of course, you might look at what the end game is. Having a detector
    to note when the sun comes up (or goes down) may provide the same
    effect, given that you specify 24 hours.

  7. Kim  Sleep

    Kim Sleep Guest

    I was trying the same thing, and was able to accomplish this with a huge
    electrolytic, and a huge value capacitor that would cycle every 12 hours,
    quite accurately.
    I once saw a circuit that pulsed a 555 every couple of hours, advancing a
    4017. Once the 4017 advanced the required number of cycles, it reset, and
    began counting again. so if you could get a 555 pulsing every 2 1/2 hours,
    with a 4017 having 10 outputs , your pretty close.
  8. Michael Black wrote...
    Exar's XR-2240 was second-sourced, often with the same part number.
    Plus there was Fairchild and TI's uA2240, and NSC's LM2240. In
    the old days you could find one of these at suppliers like Jameco.
    I looked and didn't see any, so perhaps it's the end of that era.

    In the back of my mind I think there's a similar chip... Aha, yes
    now I remember, it's Intersil's icm7240 series, also made by Maxim.
    These are still available. For example, the popular ICM7242 is an
    8-pin chip with a 555-style oscillator driving an 8-bit divider,

    To finish the story, there are some other single-ICs using the
    '2240 '7240 oscillator-plus-counter idea, but without the classic
    relatively-precise 555-style oscillator. Examples are the CD4060
    or 74HC6060, the mc14536 or mc4541, plus Philips' 74HCT5555, etc.
    These use a different type of simple CMOS relaxation oscillator.

    Unless you succumb to the PIC crowd :>), you may want to explore
    a two-chip solution, which allows higher oscillator frequencies.
  9. Byron A Jeff

    Byron A Jeff Guest

    Not by itself. There's no way to generate a RC constant with any accuracy
    over that long a period of time.

    It's tasks like this where flash microcontrollers really excell. Something
    like a small PIC or AVR can knock out this application with a single chip
    and no external parts except for maybe a transistor switch to get to the
    same 200 mA output current of the 555.

  10. Lord Garth

    Lord Garth Guest

    It might be easier to think backwards, use a presetable DOWN counter and
    look for the borrow or terminal count pin to go active. This eliminates the
    extra gates.
  11. Jamie

    Jamie Guest

    i guess if you pass it through a few flip flips, and use an And/Nand
    gate combined with the input train and output train to reset the
    flops after an event, that way you could set the duration with in a
    spec that is suitable for the timer and simply scale it longer via
  12. aman

    aman Guest

    I have another idea. Why not use a sound sensor triggered by an alarm
    clock. So the output of the sound sensor can trigger 555 monostable
  13. Lord Garth

    Lord Garth Guest

    If you make the 555 pulse once for every 1.5 minutes, a single 10 bit
    counter would cover more than 24 hours.
  14. Rich Grise

    Rich Grise Guest

    And $150.00 worth of development system, and however long it takes to
    learn to program it.

    Here's the _real_ answer:

    Good Luck!
  15. Jack// ani

    Jack// ani Guest

    Perhaps yes! Otherwise this is best way I ever came across.
  16. Byron A Jeff

    Byron A Jeff Guest

    Only if you want to spend it. Homebrew programmers like my Trivial Programmers
    can be breadboarded in less than an hour's time and for less than $15 in
    Radio Shack parts.

    My Trivial programmers are located here:
    You can program in assembly, no doubt. All of the development and programming
    software is free.

    While I recommend that if you're going to be doing this long term that you
    be familiar with assembly, for a quick head start writing in a high level
    microcontroller language like JAL may be the ticket. You can find JAL at Also Wouter van Ooijen has a JAL based simple blinky LED
    application for many type of PIC microcontrollers here:
    It's an answer for this application.

    However any investment of time or money in a microcontroller can be amortized
    over a bunch of projects. And even if it's truly a one off (which I doubt
    once the utility is realized) $20 and an afternoon learning the basics of
    how to program the part isn't that big an investment. The next project
    won't have a 4060 or the 555 as a solution. However it's likely that a
    bootloaded 16F88 for example can do this project, and the next one, and the
    one after that.

  17. Jack// ani

    Jack// ani Guest

    Well there can be many more tricks, all depends upon how easily you
    want the things to be done!
  18. Byron A Jeff

    Byron A Jeff Guest

    There's a much simpler way if you want to go that route.

    Buy a cheap battery driven clock with hands. Something like the module
    outlined here:

    Now remove the minute and second hand leaving only the hour hand.

    Set up to pass the hour hand through a slotted sensor like this one:

    Now with a simple 1.5V battery and a clock module you get a signal exactly once
    every 24 hours. Use something like a 555 to guarantee a single trigger
    and that's about it.

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