Good idea Kai. It works especially well if the current gain of the transistor is quite large. And it certainly takes less components than the design with the LM317. Here is a schematic presentation of Kai's idea.



With this design there will be a small voltage variation across the motor as the load on the motor changes. The LM317 design compensates this out via a regulation feedback which this circuit does not have. However, with a high gain transistor this effect will be minimized.

Cheapest working idea so far !!

Michael Karas