Why The Best Way Into Your Customer’s iPhone Is The Headphone Jack

Saturated with apps and social utilities, the iOS ecosystem is beginning to see more third-party hardware than ever–especially the kind with a minijack.

Why The Best Way Into Your Customer’s iPhone Is The Headphone Jack
[Image: Flickr user Robert S. Donovan]

New York-based Kinsa Health makes something simple: a mobile phone-connected digital thermometer. Nothing thrilling there. Not even brand new. Why it is especially interesting: Their device connects with the phone via the headphone jack, an increasingly common way to tie peripherals to phones–and not just for credit card readers.


The problem? Jacking in is tricky, made more so by differences in iOS and Android, and everyone is building their hardware solutions from scratch. There’s no common standard for peripherals on mobile phones.

“When we thought about the product, the problem we were trying to solve was as follows: How do we create a truly cross-platform product that is even lower cost than the non-connected analog?” says Singh. “This is where the headphone jack came in. The headphone jack is on every smartphone out there. If you try to use any other connection points–the Apple 30-pin connector–that’s only for one platform, so it’s not truly cross-platform. If you use mini-USB, those work on many Android devices and many other devices, but they don’t really work on Apple devices. So the headphone jack made a lot of sense.”

However, piggybacking off the universal port headphone jack comes with a few engineering problems. There are challenges with automatic gain control. There are potential power issues. And there are software issues on the iOS versus the Android system, since they implement audio settings very differently.

“How do you turn off the sensing software that connects to your headphones that does the noise reduction? How do you make sure you’re not changing the signal that we’re sending back into the phone? How do I pull enough power from the phone to actually power the device, and do that when each of the hardware manufacturers have it set up differently?” asks Singh.

Here’s a primer on the fundamentals of building for the hardware jack.

Turning Information Into Sound (And Back)

No matter what your headphone-connected device can do functionally, all the information it passes back and forth with the phone must be translated the same way as audio: Tones and key presses are the Morse code of headphone jack hardware development.


Singh explains, “There are multiple different approaches to using the headphone jack. The most obvious approach would be to use a Morse code signal where you have beeps. Beep beep beep. Or alternatively you could have different tones. You can use the tones or frequency to actually accomplish the objective.” That’s the most obvious method, he says.

“What we decided to do was to send an audio signal out of the headphone jack [and] allow it to be changed by, in our case, a temperature sensitive resistor called a thermistor, and allow it to be channeled back into the phone,” Singh says. “So what we’re really doing is we’re processing an audio, an analog signal. We are interpreting that analog signal and the changes in that analog signal to determine a very precise temperature reading.”

Doing this translation accurately requires calibration. “We just basically try, in very simplistic terms, to read the settings on your hardware. You can detect the hardware parameters to make sure the signal you’re detecting across different devices is the same,” letting his engineers adjust for variations in device by calibrating to each model.

Other people are using sonic technology in different ways, across different industries. AliveCor heart monitors convert ECG signals into ultrasonic FM sound signals, transmitted through an audio transmitter to the microphone of the iPhone where it is processed and displayed on screen.

Nerf Cyberhoops uses a different type of sonic technology and “contacts” the iPad or iPhone “by playing inaudible sounds that the device playing the app picks up on.” This saves power when compared to methods such as Bluetooth and Wi-Fi.

How Can You Pull Power From The Phone To Power The Device Via The Jack?

Another big challenge in building headphone jack peripherals is powering the device. Since you have a battery in the phone itself, can you somehow draw enough power from the headphone jack to make it run?


“In our device we don’t have power issues because we don’t need power,” says Singh–a function of the way he chose ultimately to design the thermometer. “We’re simply changing the audio signal. However, other devices have power issues.”

Singh offers some advice for people going this route. “There’s a third party that’s written something called The Hi-Jack technique. It came out of the University of Michigan. That is one method to try to draw some power from the headphone jack.”

The availability of power is a big advantage over Bluetooth and Wi-Fi. But perhaps the headphone jack’s biggest advantage is its low cost.

“Wi-Fi is very expensive. It requires heavy componentry. It’s high cost. Low-energy Bluetooth is marketed and positioned as a very, very low-cost alternative and it is compared to Wi-Fi–but it’s still not low-cost enough today. Using the headphone jack is ultra, ultra low cost.

There are also questions of standards and ubiquity across devices. Bluetooth LE, for instance, isn’t built into mobile phone models older than the Android 4.3. It gets more complicated in hardware where developers are constantly faced with the challenge of developing for every single model out there.

“Hardware guys think about not only Android versus iOS but they think about every single different type of phone that’s out there because they’re all manufactured slightly differently. It’s much, much more fragmented than you think,” says Singh. “It’s the intersection of both of those together that impacts how you think about hardware connectivity.” As he points out, “every phone has a headphone jack.”