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.
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.
LED T-shirt @ work
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.