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I took an IR LED transmitter, and soldered it to a mono audio jack (positive pin to the left channel, negative to mass). Now, the LED is quite small, and I'm pretty sure that the phone audio output voltage is enough to power it on.

If I connect a multimeter to the jack and put it on the diode check mode, the LED turns on (I can see it glow violet using a camera). However, if I connect the jack to my phone, turn the volume at 100% and play a WAV file, the LED doesn't light.

I have searched the internet, and found that some other people had my same idea, however most of them used a stereo jack and soldered the LED pins to the Right and Left channels of the jack connector, and left mass unused.

I don't understand why is that, and why my LED doesn't work. Did I miss something?

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  • \$\begingroup\$ instructables.com/id/DIY-Infrared-transmitter-for-iPhone-iPod/… Most of them I found are using both right and left signals because audio jacks are AC voltage. I don't think you get enough power for the LED off of a single track. You also want to use a specific audio track that's made for this purpose if you aren't already. \$\endgroup\$
    – I. Wolfe
    Commented Jan 23, 2015 at 15:00

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http://www.instructables.com/id/DIY-Infrared-transmitter-for-iPhone-iPod/?ALLSTEPS This explains it pretty well but I'll expand a bit.

Since the audio signal is AC voltage it has negative and positive portions of the wave. By tying the grounds together and using the left and right signals as the +/- for the LED, the LED sees the voltage difference between the channels. If one channel is positive max and the other is negative max, this gives you more voltage than a single channel to ground. You use two LEDs so when one is reverse biased the other comes on (forward biased). You use a specific audio file in order to make sure when one channel is high the other is low (one is just the reverse wave of the other). WAV file obviously to avoid the losses

You are just not getting enough voltage off of a single channel to ground. The LED would only be on half the time (not an issue by itself with high frequency), and the positive half is probably not high enough to meet the LED forward voltage requirement.

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if you're still interested in this I have worked on it for weeks and finally got it to work. Like in the instructable you will need 2 LEDS, but the negatives both need to be wired to the ground. And then one of the positives goes to the left and one goes the the right channel. Then you should have each channel pumping out 19,000 hertz while alternating to add up to 38,000 hertz. Just ask if you need anymore details

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  • \$\begingroup\$ You can't add frequencies like that. \$\endgroup\$
    – Transistor
    Commented Sep 9, 2016 at 10:21
  • \$\begingroup\$ If two LEDs are driven by 19 kHz waveforms in antiphase, so each LED emits a pulse during the positive half cycle of its drive waveform, then yes, the resulting total optical output from the two LEDs will be a 38 kHz signal. That doesn't make this a good answer to the question though. \$\endgroup\$
    – nekomatic
    Commented Sep 9, 2016 at 11:01

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