stuck on halflife 1

[spin03]

i am on the level blast pit. i am at a part where there is a huge green monster occupying a room. i went through the path to get to the room where there is a switch to turn on oxygen and fuel so that i can activate laser beams to destroy the monster. i switched them on, and went to the main control room, but the power switch was off. i have the fuel and oxygen on, but i dont know how to turn on the power. can anybody help me?

 

[beam]

There are two doors in the pit with the monster, you already went through one to turn on the fuel now find the other door in the pit and you will find the power!