DC motor speed control

[blueonblack]

My current project is going to incorporate a 5V DC motor, and I would like to be able to adjust the speed. I had planned to use a simple rheostat, but now I am reading that this is not a good idea. I've found several purpose-made DC motor speed controls on the market (evidently supporting the notion that a rheostat should not be used), but they all require at least 9V.

Can anyone with more electrical experience than I confirm that a simple rheostat will not work in this application and/or suggest another method? I don't really HAVE to have speed adjustment but it would be much better with it. Thanks in advance!
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[Eclecticos]

http://img262.imageshack.us/img262/3465/speedcontrol515et2.jpg
5-15v DC Motor Speed Controller. Here (http://www.electronics123.com/s.nl/it.A/id.439/.f)

[blueonblack]

Outstanding!! +rep I'll have to look into the size, as I am pressed for room in this area, but this just might work. Thanks! :banana:
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XL175 (http://www.cyclechaos.com/wiki/Honda_XL175)