[tdicola]

Very cool, have you thought about using a RGB color sensor (like a TCS34725) to detect the color of the ribbon instead of finding an IR homing strip? That might help with slip since you know the order of all the colors and could move exactly until the right one is being displayed.

[mattiasgunneras]

Yes! we actually prototyped that.. didn't have great success unfortunately, because of space constraints in our particular application, we have to have our sensors look down a 1/8" passage (passed the motor) down onto the ribbons. I feel like I tested every single optical sensor from digi-key before we finally had to settle with an IR sensor. We use a pretty strong IR LED and bouncing that off of a reflector on the ribbon. We simply could not separate the signal from noise trying to read the actual color of the ribbon. Moving the sensor off the PCB (closer to the ribbon) would have worked, but the cost increase for 6400 pixels.. yeah, couldn't do it.

[kevin_thibedeau]

You could have printed a quadrature pattern on the back for not much more complexity. Just one additional sensor per strip. Even that wouldn't be an absolute necessity since you know which direction the rotation is going. A single series of bars with one skipped or widened for home would suffice. Then you'd be able to compensate more readily for slippage and stretching.

[modeless]

Any reason why you didn't use a camera looking at the front of the display? That way you'd only need one sensor and the ribbons wouldn't need to be instrumented in any way.