The spec's for the motor have no details. It says 140mA but does not say what it is doing (No load? Working hard or stalled?).
I suspect when the motor spins with no load it draws only 140mA. It probably runs well doing some work with a current of 1A and when it starts or when it is stalled it draws 6A.

 

PWM is sort of the state of the art for this chore, yes. But the right pot, if you could find one cheap enough, would work fine, as would the voltage regulator I mentioned. If you could live with just a "low" and "high" setting, you could even put a lightbulb or such in series with the motor and use a switch to short across that. Switch open would deliver less power to the motor, closing the switch would deliver full power.

Lots of ways to skin the cat.

The ghetto lightbulb method amuses me actually. I'm going to spend a couple hours tonight trying to figure out if I can make a PWM and go shopping for parts in the morning. If anybody's interested I'll post whatever I ended up building.

 

Last question before I try to build a 555 timer:

My wattage is dependent upon my amperage, which since my voltage is constant, is inversely proportional to my resistance, right? So if I have the resistance on the pot cranked all the way up, say to 100kohm, the amperage would be tiny, so the wattage would be tiny too. As long as I didn't turn the resistance too far down, shouldn't I be able to control the speed, with the caveat that I could destroy the pot if I turned the resistance down too far?

 

But the pot would be dissipating all that current. Those 6 AA batteries would be depleted fairly quickly. PWM will allow you to run the motor slowly with out draining the battery as much as you would with a pot.

 

If you using the NE555 PWM to control the motor speed, just don't worry about the Variable Resistor(Pot), it won't get hurt or damage, because the current flowing through the Pot can be hold up by itself.

You can see the Figure 2. Block Diagram at page 2, and the Figure 3. Equivalent schematic at page 3.
http://www.datasheetcatalog.org/datasheet/philips/NE_SA_SE555_C_2.pdf

 

I'm going to give this one a try and see if I can knock a test version together with an LED today: http://www.easterngeek.com/2008/06/simple-and-dirty-pulse-width-modulation.html

Is there anything that would keep this from working with reversible motors if I put the DPDT switches across the wires coming from 7 and 8?

 

As long as I didn't turn the resistance too far down, shouldn't I be able to control the speed, with the caveat that I could destroy the pot if I turned the resistance down too far?

Yes, in theory, but you won't have a useful range of control in that case. Worst case scenario for the pot is when the pot and the motor are both drawing the same power. So if you want half-power to be within the control range for the motor, your pot must see this worst case. Power in both the motor and the pot is less at higher resistances, but the motor is forced into a narrow and low power range. And finally, you always have to assume somebody is going to turn the knob all the way. A corollary of Murphy's law.

A pot is not a black box, either. Inside, going to low resistance means all the current is flowing through a very short portion of the resistive material. So, maybe, as a whole, the power dissipated in the pot is not excessive but now you're concentrating all that power into one tiny region. It's going to blow. A pot beefy enough to avoid this is, as you've found, not cheap.

One big advantage of PWM is that you don't lose torque like you do if you try to control a motor by voltage.

 

Everybody, thank you for all the information you've given me so far. I'm not really into your hobby/profession, but you've helped a great deal. You've probably given 7 CE students a letter grade from what you've posted in this thread.

 

Print from, I believe, a ALL ELECTRONICS kit. PWM is slight variation from one you are considering, but I prefer yours with pot wiper at pin 3. If you are stuck with RS,N-ch field effect might be IRF 510, there are ones with lower RDS. With more complication, over current protection can be added.

 

I built this one and it dims a 12V LED very smoothly using a 9V battery as a power source, but when I replace the LED with one of those motors, there isn't enough power to even turn the shaft.

I'm not sure where all my current is going. The motor takes right off if I just touch it to the 9V.

 

I believe you will need a motor driver, pref. a N ch MOSFET.

 

There is some disagreement about how much current the discharge pin can handle, and it likely varies from manufacturer to another. Apparently you can sink a bit more current by tying the output and the discharge together. Of course you still can't dissipate more power.

 

wayneh, thanks for bringing up discherge connection to motor, time for new glasses. Only once have I seen a spec on discharge current, maybe Poly- Pak, @ 200 mA. It's always made me nervous to see 1000 μF connected directly to pin 7. In most cases motor current is for no load, so need to plan for worst trancient of 6 A.
What is the gauge of Cat 5 cable, & how many feet. Might a V boost ckt. be in order??
Cat5, 24 ga, .02 ohm /ft., .o2 in dia., 4 pr twisted.

 

Finally....added an n-channel MOSFET from Radio Shack and we now have smooth speed control for one motor.

 

It didn't happen without pictures and a schematic.

 

Sorry, took me a while to upload pictures. We had a couple of false starts due to short circuits, but it's working great now. For some reason the fifth battery from the left in the battery box gets hot when the batteries get low. Until that happens, it works exactly as expected, so we've resigned ourselves to changing batteries frequently for Tuesday's trials.

 

Your schematic has the Mosfet shorting the power supply.
The Mosfet is not driving the motor.
Pin 7 of the 555 is driving the motor and is killing the 555.

 

Maybe this will help explain Audioguru's comments.

 

So Bernard, what would I do with pin 7 in that circuit?

 

Leave pin 7, blank, it servs no purpose in this ckt. ' Would add a snubber across motor terminals, say .1 μF in series with 1 Ω, 1/2 W.