Driving small DC motors with very low speeds

I need to drive a small DC motor (the general kind used in toys and such), at very low speeds - below 50 rpm, perhaps. It won't be needing a lot of torque, since it will only be holding a small mirror. I'm rather inexperienced with these devices, so, is it possible to drive the motor directly at these speeds without a gearbox (perhaps with a simple 555-based PWM driver, or a voltage regulator)? The simpler the better.

Reply to
Lisandro Pin
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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.

Reply to
mlw

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

Reply to
cadcoke3

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

Wayne

Reply to
Wayne Lundberg

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!

Reply to
Lisandro Pin

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

Reply to
Gordon McComb

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.

Reply to
Randy M. Dumse

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

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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.

Reply to
Lisandro Pin

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

Reply to
cadcoke3

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

Reply to
blueeyedpop

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

blueeyedp>

Reply to
Gordon McComb

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

Reply to
Alex Wenger

The "laserpod"

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is basically a disco ball in a can.

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

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John Nagle

Reply to
John Nagle

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!

Reply to
Lisandro Pin

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Reply to
zwgearbox

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