Hijacking power and bandwidth from the mobile phone's audio interface

We endow the mobile phone with a low-cost, open interface that can parasitically power external peripherals, and transfer data to and from them, using analog, digital, and serial signaling, using only the existing headset audio port. This interface, called HiJack, allows the mobile phone to easily integrate with a range of external sensors, opening the door to new phone-centric sensing applications. In this paper, we characterize the signaling and power delivery capability of the audio jack, design circuits and software to transfer data and harvest energy, and evaluate the performance of our designs. We also use the mobile phone's audio channel to create a layered communications stack that supports low-level, analog signaling and high-level, multiplexed data communications with external devices. Our design supports a single, bi-directional communications channel at a data rate of 8.82 kbps over a Manchester-encoded serial stream, using just a few discrete components and the hardware peripherals found in almost any microcontroller. Our harvester delivers 7.4 mW to a load with 47% efficiency using components that cost $2.34 in 10K volume. Integrating the pieces, we present a combined system for delivering data and power over audio, and demonstrate its use by turning an iPhone into an inexpensive oscilloscope.