LED T-Shirt

During the last couple of months I’ve been working on a fun side-project with my friend Shlomoh Oseary. For a long time I wanted to make a T-shirt with an equalizer display on it that will light up in correspondence with surrounding sounds and music, and once I had a buddy excited about this idea too we started working.

We decided to use E-textile dedicated components. Arduino Lilypad with its 8 MHz Atmega processor seemed suitable for the task. Now we had to understand how will will drive the LEDs. The naive approach of connecting each LED to ground and to one of the Lilypad’s outputs would limit the number of LEDs we can drive this way. After searching a bit we found that what we want is to build a LED matrix. The principle in a LED matrix is that all the LEDs in the same row or column are connected. In our case all the minus legs of the LEDs in the same column are shorted and all the plus legs of the LEDs in the same row are shorted. To light up a LED we need to feed positive voltage to the corresponding row and short to ground the corresponding column. To light up multiple LEDs our LED matrix driver code  loops over all the rows and columns and constantly lights up each LED that is required to be turned on for a fraction of a second thus achieving the effect of those LED being constantly turned on.

Testing the microphone and the FFT calculation

Each column of the LED-matrix represents a frequency range with lower frequencies on the right. The more energy is sensed in a certain bin – the more LEDs in this column will be turned on. To find the energy for each frequency range we used FFT over a window of  128 samples. The sampling frequency was chosen to be 4000 Hz providing according to Nyquist theorem coverage for tones up to 2000 Hz. A predefined threshold (which we need to calibrate) is subtracted from the calculated energy to filter out small fluctuations and the outcome is mapped to the number of rows of the LED matrix to represent an energy level.
We used an existing FFT implementation for Arduino from http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1286718155.
There is still a final touch missing to the algorithm which is applying a low-pass filter to clean frequencies higher then 2000 Hz from the recorded signal prior to FFT calculation.

Connecting the electret microphone and the power supply to the Lilypad.

LED T-shirt @ work

When beauty and electronics meet… (Julia Shteingart modeling)


Project’s code (except for FFT implementation which can be downloaded using the link above and the TimerOne library which can be downloaded from Arduino site) is available through SVN under



To Shlomoh’s mom for sewing.

3D on Android (Geekcon 2011)

I’ve recently participated in Geekcon 2011. It is similar to Garage Geeks but the thing is that people actually build stuff during the two and a half days of staying there.
My friends, Ofer Gadish and Gil Shai from Acceloweb and I worked on displaying stereoscopic 3-D images on an Android device. Those were exciting three days of sleeping very little, coding a lot, soldering, drinking beer and having a lot of fun.

When we initially discussed the idea we thought about using 3-D glasses controlled by Bluetooth but we realized that in the short time we had we would probably not be able to study the control protocol and also figure out how we directly control the Bluetooth transmitter of the mobile device, if it is at all possible to do it on such a low level from a user application.

Instead we have chosen to control the glasses through the audio jack output of the mobile phone. We found another pair of glasses controlled using quite an old VESA standard. The glasses are supplied with 3-pin mini-DIN receptacle. The idea is very simple: high voltage means logical “1” means opening the left eye and low voltage means logical “0” means opening the right eye.

To supply ground, +5V and an accurate square wave synchronization signal to the glasses we’ve done some soldering and connected the mini-DIN to an Arduino, that was in turn receiving the output from the mobile’s audio jack.


The Android software was a bit of a mess, reaching a switching rate of 60 Hz wasn’t very simple considering the slow performance which I don’t exactly know what to attribute to, whether it is a slow refresh rate of the display or the technique we used to draw the images (although we accessed the canvas directly, bypassing any higher level APIs for displaying a picture). On Saturday afternoon we had it running, with some glitches occurring every couple of seconds, but giving some feeling of 3-D depth. Or was it our exhausted imagination after not sleeping too much during this crazy and awesome weekend?