Arduino & Touchdesigner
You can connect your arduino to TD to send & receive data

Various tutorials connecting TD & Arduino (no firmata)


Using Firmata
What is Firmata? 
 Firmata is a protocol for communicating with microcontrollers (like Arduino) from software on a computer, using a standardized messaging format over serial communication (usually USB). It allows you to control and read from Arduino pins in real-time , without needing to constantly upload new Arduino sketches. 
 
 
 Firmata is like a translator that is programmed on the Arduino. 
 
 
 You upload a Firmata sketch once to the Arduino. 
 
 
 Then, you can control the Arduino live from TouchDesigner (or other software like Isadora, Max/MSP or Processing) without reprogramming it. 
 
 Be aware that is is resource heavier the programming your Arduino with specific code for the goal you want to achieve, Nice for prototyping though!