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-transformerhttp://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/