I'm working on an iPhone accessory that consists of a temperature sensor and LED. I want to power the accessory through the iPhone headphone jack, but I'm concerned that the user can't use headphones while the headphone jack is occupied by the accessory. I am considering Project HiJack to create build this. (http://www.seeedstudio.com/depot/hijack-development-pack-p-865.html)

Does anyone know if it is possible to have headphone jack power an accessory, provide some application data, and be used to reroute the audio data into a second jack built into the accessory as well?

Maybe a RCA splitter adapter is a solution? Would a splitter adapter interfere with HiJack and function properly with such a set up?


  • \$\begingroup\$ I think that's what radio tuners do, but most of them are probably powered by the iPhone's USB port. \$\endgroup\$ – mimipc Jul 31 '13 at 18:13
  • \$\begingroup\$ Will the iPhone port even provide enough electricity for both, even if you changed the frequency or whatever? Might be easier and cheaper to get a solar panel and a battery, not to mention that you'd probably have more power supplied, and it's FREE! \$\endgroup\$ – Anonymous Penguin Jul 31 '13 at 19:11
  • \$\begingroup\$ I'ts sad that apple dropped the 30 pin connector there you had 5v, line out, usb, firewire, video out, line in and more. \$\endgroup\$ – Djkrugger Jul 31 '13 at 19:58

This is from the schematic of the HiJack. The Right Audio Channel is used to harvest a voltage through a transformer, a mosfet bridge and a diode. This is done by:

The HiJack energy harvester can supply 7.4 mW to a load with 47% power conversion efficiency when driven by a 22 kHz tone from the output from a single audio channel on the iPhone 3GS headset port

The left channel is used as data input into the HiJack's microcontroller.

enter image description here

The power harvesting audio (right), data input (left) and data output (mic) is handled by a software library you would include in your custom built ios app (or with hijack's default apps)

Because of this, you cannot use audio and use the hijack at the same time. It's unknown if you can patch through (or in parallel as a 3.5mm splitter would do) without having a negative impact on either the headphones AND/OR the hijack, when either or is being used.

For the most part, someone using a temperature sensor module probably isn't interested at listening to music at the same time.

An idea, requiring much work. Modify the Hijack library to only use the power pin (right channel) and not the data input pin (left channel). Add this to your app, which must include both the sensor code, and a music player code. The music player would only use the left channel. You would tie the lft channel from the iphone out to both the right and left of a second jack. You can still use the mic in to read the sensor. Downside? No data input from the app to the sensor section.


simulate this circuit – Schematic created using CircuitLab

Please ignore the abuse of standard schematic symbols.

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The HiJack system uses the headphone jack to provide power to and exchange data with your accessory. As a result, the headphone jack is not available to play music at the same time.

With HiJack, the iPhone communicates with the accessory at 8.82 kbps. I suppose that it would be possible to write your iPhone app in such a way as to multiplex some audio along with your accessory data, then have a Digital-to-Analog converter on your accessory process the audio data and send it to a speaker or a headphone jack, but you won't have enough bandwidth to get a decent sound quality for music.

HiJack is also limited in the amount of power that is provided through the jack (7.4 mW). It would be difficult to power your circuitry and a headphone without external power.

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  • \$\begingroup\$ For your second paragraph: But, would it supply enough power to power all the digital-to-analog parts, the headphone, and the electrical components? It only has 2.8v and 7.4mW. Not that much. Plus, you'd have to modify the firmware of the iPhone. \$\endgroup\$ – Anonymous Penguin Jul 31 '13 at 19:16
  • \$\begingroup\$ You are right, there isn't much power there at all for driving a speaker, although you could provide external power. I don't know much about iPhone programming, so I don't know how hard it would be to get iPhone to send audio to your app that you could reprocess. It might be more trouble than it is worth. \$\endgroup\$ – Ben Miller - Remember Monica Jul 31 '13 at 19:23

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