Eggstatic 2 – How to decorate Easter eggs with a laser and photochromic paint

After one year we are back eggain to inspire you how to decorate Easter eggs in a true STEAM fashion. Last time we showed stroboscopic patterns that animate when eggs rotate (YouTube: Eggstatic – stroboscopic patterns for Easter eggs). This year we took it a bit far and built a device that can create and change the patterns on the egg dynamically. YouTube: Eggstatic 2 – laser drawing stroboscopic patterns on an egg covered in photochromic paint
The apparatus creates stroboscopic patterns using a laser on an egg covered by photochromic paint. Such paint temporarily changes the color (from white to violet in our case) when illuminated by UV light. Here we use a violet laser from a blu-ray drive and a scanning unit from a laser printer. In the laser printer this unit projects the printing image line by line on a rotating photoconductive drum, but here it projects various patterns line by line on the rotating egg.
Those patterns are carefully designed to create an illusion of animation when captured by a camera (or illuminated using a stroboscopic light). You see exactly what the camera captures---no aftereffects were used. This device is a kind of zoetrope (an old device used for animation), but it allows to change the pattern dynamically. 
Technical details
The heart of the device is the laser scanning unit cannibalized from an old laser printer. From the original unit we used only the motor with the driver and optics. We replaced the original diode by a violet laser diode from blu-ray drive. The unit sweeps the laser beam using a hexagonal mirror that rotates at the constant speed of 160 Hz, which gives the scanning frequency approximately 1kHz.
The patterns are generated in Matlab and sent to the control board, which is based on a Discovery kit STM32 F4. The board controls the laser and the stepper motor with an egg. To synchronize modulation of the laser beam with its position, the beam hits a phototransistor once every sweep.
Sad story about laser diodes
We managed to burn a laser diode we had for this project originally when replacing its driver (modules from China often include a boost converter that is not suitable for fast modulation of the light). Therefore we cannibalized a laser diode from an old blu-ray drive. It worked great, but not having a datasheet, it burned after some time as well. Unfortunately, it was right before the filming. The fastest way to get a new diode was to buy a new blu-ray drive and take the diode from it. Otherwise, the video would have been postponed for the next year.
Video: Eggstatic 2 – laser drawing stroboscopic patterns on an egg covered in photochromic paint
Image: Eggstatic 2 - aparatus
Image: Eggstatic 2 - detail
Image: Eggstatic 2 - stand
Image: Eggstatic 2 - pattern 1
Image: Eggstatic 2 - pattern 2
Image: Eggstatic 2 - in dark
Image: Eggstatic 2 - development