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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Lisandro Pin wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Lisandro Pin wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Lisandro Pin 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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi,
snipped-for-privacy@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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
RC gear is now so cheap that you can buy a complete kit with two servos for around $40.
Wayne

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
China Shenzhen ZHAOWEI Machinery & Electronics Co. Ltd engages in designing , manufacturing and marketing all kinds of electric motors. They are mainly suitable for the following applications: smart home application used in sm art kitchen and laundry, medical instrument for personal care, smart E-tran smission applied in automobile, industry automation applied in telecommunic ation and a great variety of plastic/metal planetary gearbox in different s izes. In order to develop the oversea market, we are current seeking new partne rs around the world to create a bright future together. ZhaoWei is a right choice and excellent partnership with sincere services.
Company: Shenzhen ZHAOWEI Machinery & Electronics Co., Ltd URL: http://www.zwgearbox.com/ Contact: Annie Liu Tel?+86-755-27322652 Fax?+86-755-27323949 E-mail: snipped-for-privacy@zwgearbox.com Add: Blk. 18, Longwangmiao Industry Park, Fuyong Tn., Bao?an Dist., Shenzhen 518103, Guangdong, China
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.