(edit: picture didn't show up last time)

The idea here is that the motor shaft presses against the outside of the wheel. The wheel is something with a rubber outside, like a fat rubber band over cardboard or an o-ring between laser cut acrylic disks. The shaft actually goes all the way through to the other side (only showing half here). The red block has a slot not a hole, so the shaft slides up until the top of the wheel hits the motor shaft. The motor is connected to the red block, or to a sheet or PCB connected to that block. A PCB behind the motor also holds the light sensor, pointed at the phone screen. 

https://www.tinkercad.com/things/2JTX6x4Dj0k-dazzling-lappi/edit