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Circuit to light LED if there is any audio

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sleman

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Trying to come up with a circuit that would light up an led constantly if there's any audio at all coming out of a headphone jack of a mobile phone. I found several circuits that flicker the led based on the ac voltage but I need the led to either be on when there's audio or off if there's no audio. I tested with a multimeter and the peak ac voltage coming out of the phone is 0.6v.
 
Define audio, do you mean people talking or music? Music is not too hard but speech would be harder there can be long pauses occasionally. Your multimeter more than likely is reporting the wrong voltage unless it's a true RMS meter rated for the full audio spectrum, most meters are calibrated for 60hz AC only so the results you got measuring AC voltage aren't valid.

If you can describe a little more about exactly what kind of audio you need this to switch on for it'd be easier to make suggestions.
 
Per what Sceadwian covers audio isn't a full time on / off affair. If for example you were to view the audio signal on a scope you would not see a on or off waveform, rather a constantly changing waveform of frequency and amplitude. With that in mind you may want to think about building a retriggerable one shot triggered by the incoming audio and set a RC time of a second or two. When a trigger is detected the output would go high and remain high as long as subsequent triggers kept coming along. If no triggers came along when the one shot RC time timed out the input would go low. You would have to screw around with the trigger levels and input signal conditioning but something along those lines might work for you. Maybe a 555 timer configured as the one shot?

Also per Sceadwian, the more accurate info that you can provide the better a shot at a solution.

Ron
 
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Reloadron got into the specifics quiet nicely. Exactly what you're listening for will determine how you tune the circuit and the specific methods you use. Even human beings get listening/speaking precedence wrong frequently. You don't want your LED to go on when no one is talking or the output is not what you would consider 'on' but you want it to not be on when you think it should be off as well. This is a common problem in electronics which is clarifying intent and finding out a practical method.
 
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