Servo + photocell

hello I'm calling a help because I do not understand. I created a sketch that works well with a small servomotor but when I change to a more powerful servo (TowerPro MG995) nothing works. I tried several basic sketch and impossible to operate the servo (2 servos tested).I also tried an independent power supply for the servo.

Cordially

Start by reading this post and following the directions.

I also tried an independent power supply for the servo.

Probably not adequate or not properly separated from the arduino.