Linear Taper

I have modeled this one for one of my electronics projects a year ago. The mechanism has a small DC motor which drives a belt, thus moves a slider. In the inside there's a carbon stripe which has a total resistance of 10k Ohms. The slider has a contact patch touching the stripe. As the slider moves the resistance is varied. You can you the feedback from the resistor to calculate the position of the slider... This is really simple and pretty useful small device and someone might need the CAD for it. It cost about 10£ in the UK, but probably you can build one now when you have the idea :P