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I am trying to do this, light a LED from the headphone wire(mobile) but it seems it gives ac in there, I don't have much idea, please help.

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    \$\begingroup\$ I have actually wondered the same myself. Both answers below state "No", but none provides any quantitive data. I would suspect IF this is possible, you will need some special "song" to play during the experiment. For example, you can get Audacity and create an all-max-values song, then upload it and play it, to get your DC offset. \$\endgroup\$ – Vorac Dec 13 '15 at 10:06
  • \$\begingroup\$ The answer is "no". There is insufficient voltage to drive a LED. You will need a power source \$\endgroup\$ – Mahendra Gunawardena Dec 13 '15 at 10:28
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    \$\begingroup\$ Yes it can. There are ir led that can be used to control a camera from a standard headphone jack. And see electronics.stackexchange.com/questions/26785/… \$\endgroup\$ – Passerby Dec 13 '15 at 21:16
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If you must light your led without an external supply you could consider a small audio stepup transformer.Perhaps something out of a dead transistor radio.Otherwise you could build a diode pump that say quadrupled the voltage.Use shottkey diodes for your diode pump because they waste less voltage .Use a red led for your experiment because they need less volts than the other colors.

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Yes, audio signals are AC. Headphone voltages will be too low to light an LED.

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  • \$\begingroup\$ So, is there any way that i can work around that? \$\endgroup\$ – Hrishikesh Dec 13 '15 at 9:26
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    \$\begingroup\$ @HrishikeshBarman Without some sort of additional power supply - no. \$\endgroup\$ – Rohan Dec 13 '15 at 9:45
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I don't really know for sure, but I would be checking out the mic wire. It could have bias current designed to power electret mics. Easy enough to check - just measure the mic to GND voltage, and then to be sure put in 1k - 10k resistor there, and measure voltage again. From these two measurements you should be able to determine open circuit voltage of that source, and internal resistance.

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I assume you by headphone wire you mean the 3.5mm jack of your phone.

First consider lighting a LED. To light a LED you need a DC voltage and a few mA of current. Your audio jack isn't built to source current or provide DC voltage. It just transmits AC signal to your headphones.There is no way in which you will be able to light a LED from your jack.

You cannot use it for a purpose which it is not intended to serve.

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    \$\begingroup\$ Phone audio jacks are designed to source some current, they do after all power your headphones! In fact, I suspect that the jack may even be able to source enough current to drive a small, low power LED. In this case, I think it will be the small signal voltage which will be the limiting factor and not the current sourcing capability. \$\endgroup\$ – Rohan Dec 13 '15 at 9:43

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