DC Motor Speed Control: var. V+ or PWM?

I think PWM is the way to go. It will vary the voltage continuously from 0V to 12V to the motor. You could have the LM555 timer drive a MOSFET.

Do you think that dinky little motor will behave consistently at slow speeds? Like 40 -50 rpm? I found a few circuits with a IRF511 mosfet and a CMOS chip for the timer for motor power. The speeding up/slowing down to two different speeds is fine, sonically it's actually pretty cool. From what little I know about PWM it seems that this would work and I have some 555 IC's around somewhere. Do you (or anybody else) know a good source of information about expected rpm ranges from such circuits?

Some DC motors can be (loosely) controlled down to about 2% of rated speed. Your results will depend on the nominal speed rating of the motor.

If you start with a motor rated for 18000 RPM, GOOD LUCK trying to run it consistently below 180 RPM. If you can find one rated 1200 RPM or less, you have a much better chance of it working smoothly down to 30 RPM. I used to have an old reel2reel tape deck motor that was rated 400 RPM and could run smoothly at 10 RPM.

A larger motor is NOT needed for this application. You only need enough power to overcome bearing friction. No other work is being done here.

I actually use the CD40106 hex Schmitt inverter for almost all of my PWM circuits! I like them because they operate on so little current, the pin layout is easy to remember and they are very simple ICs, there are six inverters in one chip for convenience, and they produce good square waves.

Seems like you should be able to do what you want with the motor you have. If not, maybe a gear reduction motor would provide better speed control from 30-300 rpm. Problem with gears is that they can generate noise.

For PWM controller circuit, view this post and the entire thread...

http://cr4.globalspec.com/comment/434157/Re-potentiometer-on-12-volt-soldering-iron

I actually found a little kit for a PWM (after I picked up a bunch of mosfets, ICs, caps, etc.) that runs at 12 V with a 555 timer. I can get some decent motor speeds from it, but it makes the motor really noisy at low speeds. When I ran the motor at low speeds with a the voltage var. circuit, it was quiet even at about 40 RPM! When I swap out the cap that adjusts the frequency, I get different noises! Sometimes a groan, sometimes that high pitched whine that you hear from that ghastly tool at the dentist. Same when I swap motors. Should I try some circuits that run at lower voltages? Or keep experimenting with different cap/resistor(pot) combinations?

You probably need to up the basic frequency till you don't hear it anymore. eg.Its above your hearing range, try 20KHz as a start.....

But do not get too upset if some noise is generated......get used to it.....

Andy is correct. Try adjusting the fequency to just above the audible band and see how it performs.

Keeping the pot value the same and reducing the capacitor value by a factor of 10 should raise the freqency by a factor of 10. Even if you have "golden" ears, the whine should disappear when you get above 20 kHz.

FYI: It is fairly easy to build PWM circuits up to about 20 kHz. Above this point (up to about 100 kHz), proper drive of the MOSFET gate becomes critical and you really want an oscilloscope to verify or fine-tune the circuit opertion. Above 100 kHz, you need to use proper circuit layout tecniques and good test equipment.

It's coming together. I found that two or three .001uf caps in series to set the frequency will run a quiet motor. Now I've got to play with some other resistances as my "fast" speed is a little slow. Should I draw & post the current schematic? I think I'm on the right path now so it's probably just a matter of experimentation.

Sounds good, I'm definitely going to order some mosfets, CMOS & other recommended parts and start breadboarding this stuff. Did I make it clear that it was the baffle that was gumming up the works? When I hook up an identical motor without the baffle the circuit in the schem works fine. Anyhow, I hope the PWM works well, those gearboxes are too expensive and noisy.

If the motor is suitably geared down with a rubber belt over two pulleys, or even a T% toothed belt maybe and there is enough weight in the loudspeaker assy to even out any speed differences you should be fine.

By the way, PWM is the way to go, keeping torque up while controlling the speed well....There are many circuits on the web with just a handfull of components that will do this for you.....just reducing voltage ups the current and reduces torque....not good.

PWM is much more reliable but lookout for noise from the pulses getting into your amps!