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24Hr/7 Day Timer

Discussion in 'Electronic Basics' started by Bob, Nov 16, 2005.

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  1. Bob

    Bob Guest

    I have a project that requires turning a plastic disc (about the same size
    as a CD) through 360 degrees in 7 days or 24 hours using a battery powered
    system of some sort. The smaller and cheaper the better! Can anyone suggest
    the best way I should approach this problem?

    Thanks

    Bob
     
  2. John Fields

    John Fields Guest

     
  3. Chris

    Chris Guest

    Hi, Bob. The ideal solution for this would be a small AC gearmotor.
    They're made for this kind of service, and they're inexpensive.

    But if you need a battery-operated solution and the moment of inertia
    of your disc isn't too great, an "easy button" relatively cheap
    solution for one day rotation might be a 24-hour ham wall clock:

    http://www.eham.net/reviews/detail/2794

    It might be worth picking one up on ebay and just trying it. You only
    need the clock movement -- the clock face itself isn't necessary. The
    bigger clocks would probably have enough torque on the movement hour
    hand to pull a CD-sized piece of plastic around with no problem, at
    least while the battery was fresh. To get more torque, you may want to
    wire up a "D"-sized battery to replace the AA. It would last longer,
    too.

    For a 7-day movement, some kind of gearing might be in order.

    Another inexpensive solution which might give you more "oompfh" would
    be to use a small 5V stepper motor, 3 or 4 "D" batteries or a 6V
    lantern battery, and a PIC with some logic level FETs or darlington
    transistors. It would be trivial to program in the long time delay
    between steps. You could also use a switch to differentiate between
    1-day and 7-day movement. The trick would be to pulse the stepper
    coils for only a fraction of a second each time there's movement, like
    the battery-operated clocks, to reduce long term power dissipation.

    If you'd like to use a cheapie 4000-series CMOS and 555 solution to
    replace the PIC, it would actually be quite a bit bigger and more
    expensive, due to board space, number of ICs and construction hassles.
    Also, a PIC with a ceramic resonator (+/- 0.5%) would be more accurate
    and stable than the R-C oscillator of a 555 or CD4060 with a tweaker
    pot. If you needed even more accuracy, you could use a crystal for the
    PIC.

    If you're looking for that type of solution, or if this isn't enough,
    you should post again with more detail on your project requirements.

    Good luck
    Chris
     
  4. Ben Jackson

    Ben Jackson Guest

    Do you mean switchable 24h or 7d? Also, you need to specify how many
    steps you need. You could sleep for 23:59:59 and turn the disc one
    turn at 60rpm in the final second! An AC timing motor could probably
    do it in zillions of tiny steps. A solution with a PIC and a hobby
    servo might only give you 1000 steps (1/3rd of a degree accuracy...)
     
  5. Bob

    Bob Guest

    Thanks very much for the suggestions, only I am not clear whether a PIC
    solution would be best since the two posts seem to contradict each other. Do
    you know of a circuit/PIC program on the internet that you could recommend
    this?

    I like the clock idea but I guess I would need to have a gearbox as well as
    the disc must rotate very slowly to give an almost continuous but slow
    rotation during the period.

    A selector switch to operate the unit in either period would be ideal but
    the alternative of making two units is ok.

    Do you think it would be possible to put an invertor on a12V battery to
    power an AC gearmotor? I'm open to all possibilities!

    Bob
     
  6. Invertor? Highly depends on the available power. AC motors are relative
    powerhungry and invertors need also some to function.

    Guess the wall clock is the best option if... it only can turn your disc.
    Short hands weight next to nothing and I don't know about your disc. But as
    you will remove the oher hands, guess you have a good chanche. As for the
    turning, you have a 12 hours rotation already. Adding mechanical gearing is
    pretty expensive and requires some skills in that field. So what about
    electronic delay? The quarz wall clocks I'm aware off all use a simple drive
    mechanism. It's a coil in which a small piece of iron is driven to and fro.
    That iron drives the wheels but it is driven by short current pulses through
    the coil. The X-tal and a chip provides that pulses through two pins. One
    per two seconds each pin gives a short (positive) pulse while the other is
    kept low. So you get one tick/second. So the "only" thing you have to do is
    putting some divide by fourteen circuit between the drive pins and the coil.
    Once you got the idea, you will find out you only need one divide by seven
    circuit and a capacitor. (And an amplifier/comparator to bring the 1.5V
    pulses provided buy the clock chip to the level of your logic circuit.)

    petrus bitbyter
     
  7. Rich Grise

    Rich Grise Guest

    Buy a 24-hour clock, and glue the disk to the hour hand.

    Cheers!
    Rich
     
  8. Jasen Betts

    Jasen Betts Guest

    use a cheap battery powered clock mechanism and 14:1 or 2:1 gearing on the
    hour hand.
     
  9. ehsjr

    ehsjr Guest


    A 30 degree stepper motor with 100:1 external gearing would
    give you .3 degrees per step. Fire it once every 504 seconds.
    Doubtful you'll see the motion - .3 degrees is only 1/20
    of the distance the minute hand moves in one minute.
    You might even be able to get away with a 1.8 degree stepper
    and no gearing, fired 200 times in 7 days, or once every
    ((7*24*60)/200)*60 seconds.

    Ed
     
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