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Would it be possible to power a small LED light using the headset jack of a regular smartphone, and if so, how would you connect the LED light with an audio jack. How would such an implement look like. I'm not very versed in electrical terminology so simplified explanations are appreciated.

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    $\begingroup$ No, this won't be possible with most cellphones. The electrical power used to power headphones is on the order of ~1 mW while an LED needs ~1 W. In addition, the circuitry connected to the headphone jack is designed to produce signals in the frequency range of 100 Hz to 20 kHz, but powering an LED would need a DC power supply. $\endgroup$ – Chris Mueller Jul 21 '15 at 15:06
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    $\begingroup$ You can find some related questions here on SO and here on EE.SE. $\endgroup$ – Air Jul 22 '15 at 16:36
  • $\begingroup$ It heavily depends on volume outputtable by the phone. I've been successfully powering a DC motor (with a diode in series) from a headphone jack in a PC sound card; a tiny LED will take much less. And it's a diode, so it won't have any problems dealing with AC supply. You may not get superb brightness, but you'll easily get visible light. $\endgroup$ – SF. Dec 16 '18 at 19:48
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This is a very interesting question. I would say, yes and no. The headphone jack is not designed for this at all, and you won't get much power out of it.

My preliminary research indicates that an iPhone 4 headphone jack may support up to 60 mW of power output. This, however would be AC power, like sound.

So, I think you could put a full bridge rectifier in to make the signal only in the positive direction, then smooth the peaks of the sine wave to a dc signal (approximately) with a capacitor. The you have a DC source. Then it's as simple as running an LED off of a battery. The thing is, you're not going to have much power. Otherwise, I would still have to say it is technically possible.

By the way, to turn it on you would basically just be playing a sine wave beep. To turn it off, just stop playing the sound.

Can I ask what you were hoping to do with this LED? I don't think you could make a very good flashlight with it, if that's what you mean.

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  • $\begingroup$ Me and a couple of friends are building a small, minimalistic but hopefully functional microscope that can be attached to the back of a smartphone. We're going to 3D print it in the next weeks. We needed to know if powering the LED was possible, because we need a light-source for the microscope. The headphone jack seemed like a good idea because it was close to the camera, but if that doesn't work, we'll need to use the usb/power jack. Thanks for the great answer! $\endgroup$ – Dion Jul 26 '15 at 8:47
  • $\begingroup$ I would probably recommend using the USB to power it, and having a hardware switch to turn the LED on and off. I think there will be a lot less variance from phone to phone that way. But glad I could help. $\endgroup$ – Dan Jul 27 '15 at 16:47
  • $\begingroup$ This being a Light Emitting Diode, instead of rectifier that will introduce a bunch of losses, either just connect it and accept 50% loss, or connect two, back-to-front in parallel. $\endgroup$ – SF. Dec 16 '18 at 19:50
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Another option to consider is that many phones have a high intensity LED built in near the camera lens which works as a flash. It may be possible to use mirrors or a light pipe to make use of this.

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