Lighting up an LED using the headphone jack is probably one of most easiest tasks. Take the mic of the TRRS pin ( or left in a TRS ) and connect it to the shorter end of the LED. Take the ground and connect it to the longer end of the led and you are good to go !
A potential difference of ~3V exists between the two pins that is sufficient to light up a LED. Why is there a potential difference in the first place ? Well, this will answered in great detail in one of our post on the Anatomy of a headphone jack.
Now no one wants to stop with just lightning up a LED, so let’s improvise..
Controlling LED brightness
Like we said: A potential difference of ~3V exists between the two pins that is sufficient to light up a LED. By controlling the volume, we can reduce this potential difference and thereby dim the LED.
You can control 2 LEDs (min.) with a single headphone jack
With a simple headphone jack, one is capable of controlling 2 LEDs at the very minimum. Take 2 LEDs and connect them both to the Left/Right in the configuration shown below:
If one plays a square wave through the Left/Right then the first LED would light during the positive half of the cycle and the second one during the negative half. This is because LEDs are conductive only in one direction.
You can watch a demonstration of this in the following video.
And as a bonus, we did a frequency sweep from 1 – 30 Hz (Square Wave) and here is how that looks:
Hang on a second!
If you can do that, then you play songs and also visually witness Beats phenomenon right? Absolutely!
Visualizing songs using LED
Headphone jack as a switch
In all our above setups, we connected the jack directly to the LED. But one might need the LED to be brighter. So, to do that we had to bring in a operational amplifier ( LM324 ). This can be powered using a OTG (On-the-go) cable or using an Arduino.
Now using this we can use the headphone to perform switching operations. And this is what we demonstrate in the following series of videos:
Schematics/Circuit diagrams will be uploaded soon! Thank you.