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?
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:
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.
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...)
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!
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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.)
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.