This Max patch and Arduino sketch together allow to make RGB leds pulse (or do whatever yout want, just edit the source code!) in sync with Ableton Live's transport and tempo.
The arduino sketch is adapted from adafruit. Follow Adafruit's guide and use Adafruit WS2801 Led pixel library to set up the RGB lights.
Then upload tempo.ino to your arduino, put a tempo.amxd on a midi track in Ableton Live, connect your arduino to the computer USB port and fire up Play in Live!
Several parameters are exposed to control the device from a Live midi clip (change rate, fire everything up, shutdown everything...)
A plugphasor~ object sends a pulse synchronised with Live.
After a few tricks, this results in sending a character on a serial port;
The arduino program monitors the serial port and does different things with the leds depending on
the received character.
On my computer, the Arduino serial port is b, it may be a for you, just change this directly in the Max patch (serial object).