First test for the display on the arm of the diver, it shows the amount of oxygen.
The display consists of two red, three yellow and seven green LEDs each with 150 Ω resistors.

The meaning of the colors:
green – enough oxygen
yellow – the oxygen content is critical
red – not enough oxygen, the diver has to press a button to activate the other tank on his back. At the end blink the the last red LED as a kind of alarm.