Driving small DC motors with very low speeds

Depending on your current requirements and precision, you could probably do
it with a number of passive components and a few opamps.

The voltage at the terminals of a motor (assuming a current source is
driving the motor) represents the back EMF of the motor. A perminent magnet
motor will generate electricity when turned. Just because you are powering
it, doesn't mean it ceases to be a generator.

Using (assuming low current) use a 2N2222 transistor and an opamp to create
a variable current supply to drive the motor.

Using an opamp and a few other parts, read the voltage off the back of the
motor, this will be the speed feedback.

Now, design a comparitor circuit that takes the input from the speed
feedback and an adjustment potentiometer. (probably 10K between power and
ground, or something.) Feed the output of this circuit to the current
source that drives the motor.

I'm not sure that the back EMF of the motor will be enough to really do this
circuit, you may need to use a simple integrated motor gearbox. 50rpm is
VERY SLOW.

  No, you can't do it without some sort of speed reduction device.  But
there are choices in addition to a gear box; such as a belt, or a wheel
on the motor rotating against the mirror platform.

Joe Dunfee

The problem is i don't have a lot of space to work with - my other
choice was a small stepper motor salvaged from old junk like floppy
drives. I'll check the wheel idea though, thanks!

These toy motors are very ineffecient and crudely made. You can feel the
coarseness of their armatures by manually totating the shaft. As another
poster mentioned, small R/C servo motors will allow for quite slow
movement. If these motors need to be driven continuously you can modify
the servo for this (they usually turn only about 180 degrees max), but
keep in mind that the modification isn't as easy with the really small
micro and mini servos out there.

-- Gordon

I guess you aren't wanting simple, but cheap perhaps? Then may I suggest
you don't even need a motor. Voice coil technology can position a mirror
very percisely, consists of a coil and a magnet and a feedback control
system. If you are looking for pivoting a mirror, hinge one side, and
push/pull the otherside with a voice coil.

--
Randy M. Dumse
www.newmicros.com
Caution: Objects in mirror are more confused than they appear.

Well, simple and cheap would do just fine ;) I just need a small mirror
(or cristal, still don't know) rotating each x seconds to create a
lighting effect, a-la-Laserpod (www.laserpod.org). It also has to be
(relatively) small, so my first thought was to use a small DC motor,
which i feared wouldn't turn at all at those speeds.

For laser caustics, I suggest a gear reduces AC sysncronous motor. You will
want something that doesn't jerk aroud. If variable speed is necessary, then
a DC gear reduced motor is good.

For producing lissajous patterns with 2 or 3 DC motors ala the wobbly mirror
method, then a motor driver circuit and a PWM generator is a nifty thing.

Mike

Looks like this is Star Trek Atavachron stuff -- semi-random light
patterns on the inside of a transluscent shell. Years ago did this with
a simple AC display motor -- $3. An SCR  (or relay) can turn it on and
off.

For lissajous patterns I direct interested parties to my earlier book,
Laser Cookbook, available at most public libraries. Has a section on
mounting mirrors to a voice-coil, too, if you want that effect.

-- Gordon


blueeyedpop wrote:

    The "laserpod" (www.laserpod.com) is basically a disco ball
in a can.

    Try in "sci.engineering.lighting".  Or get cheap, small
gearmotors here:

    http://www.hobbyengineering.com/SectionM.html

                John Nagle

  While a stepper motor can turn that slow... I am not certain it will
be smooth enough for you.  Perhaps a small geared system like the
servos already mentioned would do. Or perhaps one of the bit-charger
type of tiny R/C cars.  In particular, the tanks have slower geared
down motors.

Joe Dunfee

Hi,

cadcoke3@yahoo.com wrote:

you can drive the stepper motor with ac direct comming out the
transformator,
the second winding connected with a suitable condensator. A common 200
steps stepper motor gives you then a slow and stable motion....

-Blueloop

I just tried this and works very well - the movement is not *perfectly*
smooth (of course), but it's hard to notice it. I'll try to get a
gearbox motor for cheap; if not, this will do just fine. Thanks!

RC gear is now so cheap that you can buy a complete kit with two servos for
around $40.

Wayne