simple. i need a DC motor that can change speed (via a POT??). it should have something similar to rotary encoder that returns the square wave whose frequency varies with the motor speed.
instead of making one, which involves the troublesome of mechanical precision, i wish to buy sth similar.
I have built some devices exactly as you describe for some projects of my own. If you become interested in building one, I'll help you through it, just PM me.