[Help] Ambient generic

Hi everybody  :)

I want to make a Zombie Mode map at counter strike source and I'm planning to add some sounds that make it more realistic.

I'm using Hammer Editor and I have the following question.

Well, I use ambient_generic entitiy to place a sound.

I want the sound to play every 15 seconds.

So i made a logic_case and a logic_timer, I went to link the outputs but in the outputs

of logic_timer it has no option for specific case play. It has only pickrandomly.

Well i don't want randomly!  >:( I want specific time and specific case play!

So... any ideas on how to play the sound i want every 15 seconds?

Edit:Found a way...

Lock it please =]

And not randomly starting and stopping?