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Mobile phone users are rapidly switching over to smartphones as the devices provide not only mobile communications, but also powerful information processing, display, and storage abilities. It is expected that smartphone adoption will increase continuously through 2017 (“Smartphone Users,” 2014). In past few years various smartphone centric ubiquitous sensing applications have emerged, which use a smartphone in conjunction with external sensors for data acquisition, processing, display, communication, and storage (“square Inc,”2015; “Breathometer,” 2015; “MoboSens,” 2015). In order to implement a smartphone based ubiquitous sensing application, two major technical problems must be solved:
The USB and the 3.5mm audio interfaces are two common hardware interfaces between smartphones and external sensors. The USB interface has inherent abilities to power and communicate with external sensors. The research in Black et al. (2009) leverages smartphone’s processing and interface capabilities with a microcontroller attached to an oximeter probe to create low-cost health data possessing features, like respiratory and pulse rate calculator, gestational date calculator, formulary/drug dose calculator, drip rate calculator, and drug reminder alarm. The USB interface of a smartphone is used to establish a connection with the microcontroller, and the oximeter probe is attached to one of the Analogue/Digital ports of the microcontroller.
However, not all smartphones have an USB interface: iPhones use a proprietary data interface. It means that the external sensors designed for iPhones and iPads cannot work with other types of smartphones, and vice versa. As a result, the convenience of ubiquitous sensing applications is diminished, their market value is limited, and their production and maintenance cost becomes higher. Fortunately, all smartphones have a common 3.5mm audio interface that can provide wide enough bandwidth to support the communication between external sensors and smartphones. A group of sensing applications use the audio interface to establish data communications between smartphone and external sensor (“square Inc,”2015; “Breathometer,” 2015; “MoboSens,” 2015). Square Inc. developed a device which converts a smartphone into a mobile POS machine, accepting credit card payments (“square Inc,”2015). Breathometer Inc. developed a device to transforms a smartphone into a breathalyzer to estimate your blood alcohol level (“Brreathometer,” 2015). A research group at University of Illinois developed a nitrate concentration measurement which allows citizens to monitor water quality with a smartphone (“MoboSens,” 2015).
However, because the earphone channels in the 3.5mm audio interface only allow alternative current (AC) signal being sent out from a smartphone, the interface cannot provide direct current (DC) power to external sensors. In some applications using the audio interface, an external battery is added to power external sensors (“Breathometer,” 2015), which not only increases the size and cost of external devices, but also hurts user experience as the battery must be switched periodically. From the structure of the 3.5mm audio interface for smartphones shown in Figure 1, we can see that the audio interface has four connectors: the connectors A and B are respectively the left and the right earphone channels, which send audio signal out from a smartphone; the connector C is the common/ground; and the connector D is the microphone channel, which sends audio signal into a smartphone.
Figure 1. Structure of the 3.5mm audio interface for smartphones