The robot is composed of 3 parts:

Receiver  
4 photodiodes and a  LM324N comparator
or in my case, I finally chose 4 photoresistors with an arduino nano to manage the speed of wheels as I explain below.

Motor controler
I chose a L298N bridge which can regulate the speed of both engine.

A chassis
I chose a two wheels chassis it's cheap but it's ok for this PoC but it could be more accurate with 4 wheels

Principle
The screen of the smartphone is split in 4 parts to control both directions of each wheels.
White screen => photodiode ok => engine ON like in my first version.
In my second version, the speed regulation can be done with a color from Black to White thanks to Photoresistors instead of photodiodes.
The smartphone is sending information about compass value to the server through wifi.
The command is currently done manually through a server on a computer with a keyboard, As the photorestiror is gradual, an analogic joystick could be linked with this speed regulation


The robot could be more automous :
My goal would be to use compass to maintain a destination, or use the accelerometer to detect collision/ There are a lot to do on this software part because compass sensor is not very fast. so it could be difficult to calibrate.

In my latest version, I added a periscope, it's fine but I just take photos, no broadcast the video.  I also added a sonar to avoid collisions. Adding this kind of sensor increase the price but it's nice to see what we can improve with simple components.

I hope this project could inspire yours by reusing old phones.
don't hesitate to send me your comments or feedback