Time clauses

Future with time expressions (after, as soon as, before, till, when, while ...)

The time clauses in the English language are introduced by conjunctions such as

 after, as soon as, before, till, until, when, whenever, while 

or time expressions such as the minute, the moment etc.

We do not use the future tense (will) in a time clause to describe future activities (in this respect, it it similar to if clauses).

Compare:
I'll come back home and I'll do it. x I'll do it when I come back home. (when I come is the time clause)
You will push this button and the door will open. x As soon as you push this button the door will open.
Don't stand up. First I'll tell you. x Don't stand up till (until) I tell you.
You'll need my car. Take it. x Whenever you need my car you can take it.
You'll tidy up the house and I'll do the shopping. x You'll tidy up the house while I do the shopping.
You will drop the bomb and it will explode. x The moment you drop the bomb it will explode.

Similarly, other future forms also change to the present simple tense.

He is going to leave. The room will be empty. x As soon as he leaves the room will be empty.

We are moving next week. Then we'll call you. x When we move next week we'll call you.

If we describe an action that is happening at the same time as another future action (the two activities are simultaneous), we use the present continuous tense in time clauses.
We are going to cut the grass. You'll pick the apples. x While we are cutting the grass you'll pick the apples.

Source: http://www.e-grammar.org/time-clauses/