Triggering transistor via Iphone's audio output jack

[cmd1024]

I want to trigger a transistor using iPhone's audio output jack. My goal is to turn off an LED(which is fed by an external +5V battery) by sending an audio signal from iPhone's output jack. At the beginning i thought it is easy but i encounter many problems. I tried to do it with a transistor but it seems the iPhone audio output is too weak even for a transistor.

I am confused every time i find different ideas. Let me be more precise, here are the facts:

1-) The output voltage of iPhone is very low, something like 300 mV

2-) The output is an audio signal

And here are the problems:

1-) How to amplify this voltage? I found some different ideas after some googling such as:

a-Using transformers. This guys says one needs micro-transformer

http://web.eecs.umich.edu/~prabal/pubs/papers/kuo10hijack-islped.pdf

b- Using capacitor. Here one guy is suggesting using a very simple circuit. I tried it didn't work:

Power LED from headphone jack

c- Using op-amp

Can you assist me a little bit about this?

2-) Do we need to rectify the audio signal?

Here is the circuit i was trying at the beginning. I thought it is simple. But the transistor is not triggering with the audio signal:
http://imageshack.us/photo/my-images/52/signcae.jpg/

[Hero999]

Your LED circuit already has a 5V supply and you want it to turn off, not on when the audio is present, so the transformer idea won't work.

My first question is why are you doing this?

Secondly, how long does the LED need to remain off for when audio is present?

You need more than one transistor to get enough amplification.

A simpler approach would be a comparator or logic gates.