We controlled an interactive Christmas tree on the lubricating tram

Last year we have built an interactive Christmas tree for the entrance hall of Czech Technical University in Prague at Karlovo namesti, this year we were asked by journalists from a technological on-line magazine Technet to put our tree on board of the service tram, so that even wider audience would enjoy it. This tram lubricates the rails all around Prague to minimize noise and is equipped with a camera streaming live video on the Internet. The installation was done in cooperation with Prague Public Transit Co. and Czech Slow TV. You can see the video stream and control the tree here

The Christmas tree is decorated by a string of individually addressable RGB LEDs (WS2811) controlled by Raspberry PI 2. Blinking of the lights is far from random. We were pursuing a suitable way to enable several people to control lights on the Christmas tree simultaneously and we came out with an idea of using a mathematical model of a dynamical system that everybody can influence. Specifically, we implemented a discrete model of the wave equation (describing for example series of masses and springs) for R, G and B channel separately. Users can select the color and the place on the string and send a force pulse that creates a wave traveling along the string. The force pulses and waves from different users interfere giving rise to a colorful fluctuating light show.

“Isn’t it too much work for such a side project?” we asked ourselves quite often, but we believe it was worth our sweat and tears. Even though the system was prepared from the last year, we had to rebuild it substantially. For example, we rode the tram back and forth in a tram depot with oscilloscope to investigate noise on the power line caused by motor controllers. We had to prepare the hardware for outdoor environment and cold. The software was adjusted so that we could keep the connection to it alive, it would withstand frequent turning off and on again and we had to change it also because our original solution designed for the entrance hall would not handle the number of visitors all around the Czech Republic.

The authors of the interactive Christmas tree Jiří Zemánek and Martin Gurtner would like to thank (besides their tolerant life partners) to all colleagues from the Department of Control Engineering who helped them to successfully finish the project. Namely for IT support to Aleš Kapica and Martin Samek, for expert Linux advices to Pavel Píša, for the development of the web interface to Filip Kirschner, for support on mechanic side to Ladislav Čmělík and thanks belong also to our bosses Zdeněk Hurák and Michael Šebek. 

Video: Interaktivní stromek na mazací tramvaji
Video: Lubricating tram with Christmas tree - night flight above Vltava in Prague
Video: Interactive Christmas tree - lights based on wave equation
Image: Mazacka stromek 2
Image: Mazacka stromek 3
Image: Mazacka stromek 4
Image: Mazacka stromek 5
Image: Mazacka 6
Image: Mazacka stromek 1