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Slowing down an electric motor? Answered

I am on radio shacks website looking at a 1.5-3VCD hobby motor.   The specs say up to 8,300 RPM at no load.   If I added resistors to this, would it slow it down?  I want it very slow, like 1 rotation every 3 seconds. lol  




Best Answer 8 years ago

 For the motor you are looking at, the easiest way to make it turn that slow would be with a PWM (Pulse Width Modulation) controller. Luckily you're working with very little current here so an LM555 timer IC will work work just fine for this. You can build the whole thing for under $5.00 (under $10 if you get your parts from The Shack).

This circuit will do nicely.


I'd like to see that run a motor at 1/400th of base speed.


Hey! I recognize that schematic posted by Marsh! It's from a nice tutorial done by the fine folks at the Dallas Personal Robotics Group! You can find a tutorial for how to build that PWM circuit at: http://www.dprg.org/tutorials/2005-11a/index.html

 I'm a complete newb at electronics, and even I was able to make one work! In fact, I combined that with a geared down motor to make a variable speed disco ball! 

You can see my video at:

That should work for whatever you are doing.

 i think you could use an electric clock that has hands and then gear it down with pulley attached to the second hand .note the pulley on the clock needs to be smaller than the device your running has.

For slow speeds, stepper motors can be quite useful. You wouldn't need a very high torque one to turn something that light, and it may be easier than working out pulse width modulation (PWM). You'd need the stepper motor and a stepper motor driver circuit, which you could build or buy.

OR you could use Pulse Width Modulation as others suggested.

OR you could get a gear box to change the motor from high speed to low speed.

We could advise you better if we knew what you were using it for, do you need to keep track of how many revolutions it makes? Or know the exact speed?

Its just for a little lighted display I was thinking of making.   Ever see a snack cake display in stores? Sometimes they have a slow spinning  sign on top.  That's what I need.  Perhaps I'll look around the grocery stores and see if I can run into a distributor and ask them if they can get me one.  

I dont need it for an experiment or anything.  I dont need to know the revolutions or the speed.  I also dont want to pay 10 dollars for one.  I also have limited space so it has to be small.

It won't work that way.  Google "gear motor" and look for one that has 1 rpm or less.  Sometimes you find them surplus for $10 or less.

No way, without a gearbox.

Using a resistor on that motor would be no good at all.  You'd lose all power if you tried to slow it down that much. Probably the best bet would be to get a small motor / gearbox kit like THIS.  You don't have Maplin in the US, but your local friendly model shop will pretty certainly have something suitable.

first try it without a resistor, and see what that gives you

Is this for a disco ball?  Or for some kind of tacky advertising?

The question is important.  Obviously a disco ball would be a more appropriate and noble use of the technology.


if you just add a resistor then you will get virtually no torque.

Try PWM, you can just use a 555 timer in astable mode driving a transistor driving the motor with a protection diode in parallel

Thanks for the reply guyfrom7up, unfortunately I have no idea what that means.   I took radio and tv repair for 1 year in votech back in the early 80s but I remember almost nothing about it.  I cant even remember the resistor color code. lol.

Anyway, this motor doesnt have to have much torque.  I just needed it to spin something that weighs about 3 ounces.  

To follow up with guyfrom7up, you might try PWM. It is one of the more difficult of speed control mechanisms, though it offers probably the finest control.

If you underpower the motor, you're likely to stall it, which left long enough will burn out the motor (in addition to your power source).

Also, you'd be best using a gear reduction set or a motor with a built-in reduction set. A simple hobby motor will be nearly impossible to run at that speed.  Good luck mate!

Reducing voltage (which is what the resistor would be doing for you) will reduce speed, but also reduce torque, perhaps to the point where the motor just stops.

I would suggest that you either use gears or belts to slow it down, or get a motor which already has a suitable gear train. This is not the motor you're looking for.