It sorts skittles by color. Not the most useful thing but may be a fun build particularly if you are interested in doing something moderately challenging with a micro-controller like an Arduino.

I just wanted to make a color candy sorter design unique to others I have seen online. The core design is: 

(1) The top disk (collector) with 4 holes catches skittles and indexes each to the color sensor and then to a drop position.

(2) The bottom disk (dropper) with 1 hole moves to the incoming collector skittles drop position.

(3) When the collector and dropper reach the drop position their respective holes are aligned and the skittle falls into bin.

CAD - https://cad.onshape.com/documents/09872778ce77623bb9aff94d/w/a6595dfbfa589d02b9f12ed1/e/cfe6d77ad2fd95653cd68591?renderMode=0&uiState=66211b67abacf2208c301964

Program - https://github.com/FourBasic/Skittle-Sorter

Printing - Nothing special. 2 walls / 20% infill 0.4mm nozzle. I will probably add some .3mf print part plates with the print orientations and a profile at some point.

*** DISCLAIMER ***

(1) I have built this and it works well but there are certainly things that can be made better. I have a list of improvements that could be made in the attached manual. The only real outstanding issue is the skittles have a tendency to bridge across the hole in the main bowl and you have to stir the skittles to free up the jam. I think the bowl hole just needs to be bigger. I invite you to remix or design improved parts but I have no plans to implement these improvements myself as skittle sortation isn't something I want to spend my time perfecting. I am currently working on a screw sorter which is way more useful and I plan on trying to perfect that instead.

(2) I made the design around some non-standard parts I had laying around. If you don't have the exact steppers, color sensor or photo-interrupter you will have to remix to make your parts fit, obviously. If you have a different stepper driver you'll probably have to rework some of the program.

(3) I am creating an assembly manual for the mechanical parts only. I am only including some basic information on the Arduino program and the wiring so you should be comfortable with, for example, wiring the color sensor to whichever IO pins you want and changing the color sensor IO pin definitions in the program. You should feel comfortable filling in a few gaps here and there if something doesn't quite work as my intention isn't to spell out everything from start to finish in high detail.

 Non printed parts 

Assortment of M2 ranging from 5mm to 15mm and nuts
Assortment of M2.6 ranging from 5mm to 15mm and nuts
x32 3.2x1.6mm disc magnets (Optional)
Arduino Uno
x1 OSEPP Motor and Servo Shield - https://www.osepp.com/electronic-modules/shields/49-motor-servo-shield
x1 OSEPP Color Sensor - https://www.osepp.com/electronic-modules/sensor-modules/58-color-sensor-module
x2 KP35FM2-035 Stepper Motor - https://secure.sayal.com/STORE2/View_SHOP.php?SKU=162138
x1 Power supply 12V 2-3A - https://secure.sayal.com/STORE4/prodetails.php?SKU=247397
x1 Photo-interrupter - https://secure.sayal.com/STORE4/prodetails.php?SKU=262362
+ Wires of you choosing