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What to use for timing in alarm circuits?

Discussion in 'Electronic Basics' started by sundar, Mar 31, 2007.

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  1. sundar

    sundar Guest

    Hi,
    I would like to make a small timed alarm to gift a friend. What I have
    in mind is an input consisting of 3 'up arrow-marked' buttons so that
    the user can cycle through all values of input (hours, minutes and
    seconds). Then, when user presses the alarm enable button, this value
    is fed to a timing circuit that makes the alarm ring after the set
    time. Now, here is where I need help. The only reliable timing circuit
    I can think of (with my admittedly limited electronics knowledge) is a
    microprocessor, but that seems an overkill for such a simple thing. I
    have also heard about a programmable timer 8253, but it has always
    been explained in terms of interfacing with a uP, so I'd like to know
    whether I can use it as an independent timer.
    And as a side note, I would also like the user to be able to choose
    among a few alarm sounds. Would it be better to design something like
    here: http://www.noiseboy.net/nb.asp?page=elec/alevel/melodygen.htm
    or would it be more sensible to use one of those multi-tone generator
    ICs? I'm ready to take a little PITA if that would mean a significant
    reduction of cost.. :)
     
  2. Chris

    Chris Guest

    Hi, Sundar. You might not have heard about microcontrollers. They're
    one chip microprocessors with program memory, RAM and (usually)
    peripherals built in. They're an ideal solution for this.

    The fact is, you're looking at a timing circuit to measure seconds, a
    counter which can count down seconds, minutes and hours, a display and
    user interface, a countdown comparator which will activate the alarm,
    and a device which will provide multiple user selectable alarm
    sounds. Doing this with discrete logic ICs will result in an "Acres O
    Digital" project which will not only cost you big time, but also will
    take forever to put together and debug. (PITA)^3.

    I'd normally suggest a BASIC Stamp for newbies for something like
    this. But they can get a little pricey, so you might want to try the
    Picaxe, the BASIC Stamp's kissin' cousin which is available as a
    standalone chip for significantly less than the Stamp usually goes
    for. The Picaxe, like the BASIC Stamp, is built around a
    microcontroller that runs a line-by-line compiled BASIC which can be
    downloaded from any PC with a serial port. The PC takes care of the
    line interpretation, converting commands to tokens which are
    downloaded to the microcontroller. You can then disconnect the PC,
    and have the chip run the program on its own -- it retains the program
    when you power down, so it doesn't have to be tethered to the PC.

    The thirteen I/O pins of the PICaxe or the sixteen of the Stamp will
    be more than enough to achieve what you want. The sysem clock of the
    microcontroller should be accurate enough (within 1%) to satisfy, and
    the complexity of the program looks like it's within the range of
    either device. Neither has a built-in clock peripheral, though, so
    you'll have to count out seconds with the BASIC commands, and carry to
    minutes and hours in software. Either device can drive hi-intensity
    LEDs directly. You might want to use an extra transistor or two to
    drive the speaker.

    You can set up with a Picaxe-18 starter pack for £15.10 (inc. VAT).
    The BASIC Stamp itse;f is $50 or so, and the development kit usually
    runs somewhere around $70 to $120 depending.

    Either one will be a good solution. Here are some links to get you
    started:

    http://www.rev-ed.co.uk/picaxe/
    http://www.parallax.com/

    Good luck with your project

    Cheers
    Chris
     
  3. ehsjr

    ehsjr Guest

    You won't be able to beat the cost of a commercially available
    timer. Go here:
    http://www.target.com/gp/homepage.html

    Put timer in the search box.

    Ed
     
  4. jasen

    jasen Guest

    the 8253 is not really suited it's basically a programmable clock divider

    hmm, you'd want something like a 14553 but one that could count both up
    and down.

    the easiest way is may be to salvage the timer from an old microwave oven
    If you want to build from scratch, IMO best solution is the microcontroller,
    something like a ATTiny2313 could do most of the work.

    Here's how to get a melody out of it's predecessor with minimal external
    components (I wired it the out pin straight to the amplifier input)
    <http://geocities.com/jasen_betts/polyphonic/index.html>

    The same code should work on the new chip too... if I remember I'll test it
    tomorrow.

    Clock applications using microcontrollers have no doubt been done to
    death elsewhere on the WWW. No introduction to microcontrollers is
    complete without atleast 1 lcd clock.... :)

    Bye.
    Jasen
     
  5. Look in stores for an electronic cooking timer.
     
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