Z Axis Optical Endstop for Geeetech Prusa I3 Pro

Update 1/28/18 I added a retainer to secure the stop flag in its holder. I relied on a friction fit on an earlier version and when I replaced it, I found there wasn't anything to secure the flag. You can mount it either way in the holder, depending upon whether you are using a printed flag or a thinner one made out of plastic or card-stock.
Added 5/29/18. Originally I used a printed flag that was painted at the end. After a long while, I started getting inconsistent Z homing. I believe