The anatomy of a headphone jack

Most of us live under the simple algorithm : Plug a headphone into the jack and move on with life. But the beauty of it is what happens during those instances when you insert the jack and the mobile/laptop recognizes the device to be indeed a headphone jack.

Most modern smart-phones and laptops detect a headphone using the following simple principle :

Establish a potential difference between the mic and the ground ( ~ 2- 3 V ) and observe the resistance. If its high, its air and probably nothing has been inserted. If its really low, then a headphone jack has been inserted.

And the fact that a potential difference is constantly being given between the mic and the ground allows us to plug in a led and light it up .

Corollary 😉 :

All that the phone is looking for low resistance value. You can very easily fool the phone to think that an aluminum foil is a headphone jack.

Older headphone jacks

This answer by Rick on stackexchange answers this question so accurately :

” Headphone jacks have extra contacts inside, which act as switches. The the drawing below, pins 4 and 5 are intended for sensing that the plug was inserted. They are not intended for audio signal. When the plug is not present, the switche, which are formed by 2 & 4 and 3 & 5, are closed.

When the plug is inserted, these switches are open. The plug flexes 2 and 3 slightly, and they break contact with 4 and 5. You could insert a 3.5mm plastic rod [a dummy] into the jack, which will open the contacts, and the phone might think that earphones are plugged in. ”

 

Headphone jack plugged in or not ? (Software end)

In a previous post , we talked in depth about the /dev/input directory in Linux.  This video talks about how the computer knows whether a headphone jack has been plugged in or not from a software point of view.

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The ultimate guide to using LEDs with headphone jack

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

Beats phenomenon

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.

 

 

 

Preparing a headphone jack for hacking

This is one of the most frequently asked questions regarding our project on the headphone jack : I have an old headphone, how do I configure it to do all the stuff that you feature on your blog ? This post will be a pictorial DIY edition of it.

One of the first steps is to procure an old headphone ( in working condition or otherwise).

And then cut off the mic and the rest of the earphones with it. ( But don’t throw it away! ) The reason why we do this is because when you want to hack into a device, its a boon to have accessibility to the Input/Output ports.

When you strip open the wire that you have, in the case of a TRRS headphone jack you will find 4 wires (Left,Right,Mic and ground) and with a TRS (Left, Right and Ground). The next step is to attach female jumper wires to them so we can plug in anything we want.

These wires are not your conventional “plug and play” type i.e If you take these wires  and plug them into anything it won’t work. This is because they have a non-conductive plastic-like coating in them that prevents the wires from shorting.

Therefore soldering them to the jumper wire is a bit tricky. But in our experience it helps to preheat the wires before soldering and also to wrap the wire in a braid fashion for longer life.

And similarly you solder the rest of the wires as well. Now in order to find out which wire corresponds to what, connect a speaker between the Left/Right and the ground, plug it into your computer and start playing tones.

If you are asking where am I going to find a speaker ? Well just use the speaker from your headphone that you stripped off in step 1 and solder two wires to its terminals like the picture above and you are all set.

 

 

The headphone jack meets an actual spark gap!

So, we had the opportunity to test out the headphone jack with an actual spark gap and it was absolutely wonderful. Check it out:

With the data we can actualy find out the frequency of the spark occurrence. In our case it turned out to be ~ 34 – 36 Hz. And since this is in the Audible range we can actually hear this (somehow we missed this when making the video)

Audio file : GoogleDrive

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Part – I – Lightning detector with a simple headphone jack

Part – II – Detecting switching ON/OFF of Tube Light using headphone jack

Part – III – Cigarette lighter spark detection using headphone jack

 

DIY: Obstacle detector with Audible feedback (IR sensor + Headphone jack + 555 timer)

We were inspired by the buzzer that you find in mobile and laptop showrooms – the ones that produce this annoying high frequency tone if you fiddle a ‘little too much’ with the displayed product.

We use a 555 timer in its Astable mode to produce the frequency tone and couple it with a digital IR sensor module. We do this by connecting the output pin and the ground parallel to R2 in the figure.

And as a result when there are no objects in the vicinity, the system produces a high frequency tone, but when an object is introduced the sound dies out.  This is attributed to the change in resistance value.

Here is another variation of the same:

Cigarette Lighter spark detection using headphone jack

This is part-III of the post series on detecting electromagnetic waves using headphone jack. In this we capture the EMW that emanate from the spark gap junction on a lighter. This is analogous to our gas lighter experiment but conducted at a smaller scale.

We say that this is analogous to the Gas lighter experiment because the waveforms obtained are extremely similar in nature.

 

Check out:

Part – I – Lightning detector using headphone jack 

Part – II  – Detecting switching ON/OFF tubelight with a headphone jack

 

Detect switching ON/OFF of Tube-light with a simple headphone jack

Now in our previous post, we established that the headphone jack can be used to successfully detect electromagnetic disturbances ( Sparks and Lightning ). In this post, we explore the interaction of the tube light with the headphone jack.

Here is a close up of the peak that you saw in the video:

Nowadays of course, we have moved on Non-flickering tubelights and it is highly unlikely that we might be able to detect the peculiar peaks that you see on this. But to be sure we are currently working on this and we will keep you guys posted on the results.

Also if we could incorporate this into a IoT network, things will go crazy!!!

 

*   Image Source

Don’t do this! – Headphone jack

Technically, one should not give more than 3V/3.3V (according to the Android Documentation) between the ground and the left/right/mic pin of the headphone jack. But there is this innate curiosity to know what would happen if one was crazy enough to do so.

We request you to not try this at home since doing this can damage your sound card! But nevertheless here is what would happen:

Testing basic headphone jack functions (Android)

In this post we will be trying to emulate basic functions as prescribed in the Android Documentation.  In order to understand the functions we first need to understand the circuit layout of the headphone jack which is as follows:

Now with this circuit in hand you can perform a series of tasks. This has been summarized in the table below.

This post will primarily focus on the Function A since that is the one that is commonplace in all mobile phones with a headphone jack. In the following video, we demonstrate how to emulate the following functions using a headphone jack

• Play/Pause
• Open Google Voice
• Next song
• Radio
• Emulating Google voice

Every speaker is also a microphone!

Sometimes it comes as a surprise as a people to people when I tell them that every speaker is also a microphone. This is true because in a speaker you send in electrical signals to change the way a speaker cone moves. This in turn produces various sounds.

With the same setup, if one provides a mechanical vibration to the diaphragm, this will generate audio signals corresponding to that mechanical vibration.

We have made a video demonstrating this using a headphone jack and a phone. In order to loop the sound from the mic to the speaker in the video, we use the following command on Linux:

pactl load-module module-loopback latency_msec=1