Hi,

don't use tape motor they don't gear inside. Since you seem to be good at reclaiming the motors, Try to salvage a remote control audio volum control motor, yet from some cheapy Chinese stereo (or your neighbour). That motor is fairly low speed and has the gear drive to give you good torque at low speed. OR why don't you try open up some good battery driven toy like car or anything, they are all with 6 to 9 Volts gear driven monsters.

To increase its rpm you can raise a couple of volts.

But mind you one thing as Steve has already said, whatever is the final voltage at the required rpm, just regulate the power supply at the voltage level.

The above solution will help you get rid of any encoding too I.E. no micro as well.

Or even if you are hell bent to use an encoder, just take the interrption timing (between two sluts) and use it as base value to compare, you do not need to do full programming to calculte the RPM to show on your billboard.

Regards

Babar