In this paper we describe the further development of a low-cost smartphone-based oximeter that requires no intermediate microcontroller, interfacing the sensor directly to the phone (Figure 1). By leveraging the full capabilities of the phone in this fashion, the total cost of the new device is reduced to that of the finger probe itself, and all supporting infrastructure is inherent to the host mobile phone. A clinical oximeter finger probe can be manufactured for almost two orders of magnitude less than the price of the not-for-profit Lifebox oximeter, thus potentially giving the Phone Oximeter significant global reach.Figure 1.Principle of the low-cost smartphone oximeter. An oximeter finger sensor with two light emitting diodes and a photodiode is interfaced to a smartphone running a software pulse oximeter application.

Any viable implementation of a clinical sensor that relies on consumer electronics must have an effective way of verifying performance across different devices. We present an automatic simulator-based test system that can be used to systematically examine the entire clinically relevant range of operation of the low-cost smartphone oximeter and validate the system across many different smartphone hardware versions.2.?Experimental Setup2.1. Sensor InterfaceA conventional oximeter sensor contains two LEDs for actuation and a photodiode for detection (Figure 1). The audio interface of any phone or smartphone is well suited to drive such a sensor.

The audio interface has a high-current output capable of driving the low impedance load of the LEDs and a high-gain input designed to interface to a high-impedance Junction-gate Field Effect Transistor (JFET) electret microphone pre-amplifier, equally suitable for amplifying the photodiode signal.The sensor LEDs of the audio-based smartphone oximeter are driven directly by the speaker output of the phone (Figure 2). The LEDs are wired in reverse polarity to facilitate alternating activation at opposite polarities of a driving signal. With the peak-to-peak amplitude of the speaker output larger than the forward voltage threshold of the LEDs, this can be accomplished by sending a suitable audio signal to the speaker output. The forward voltage thresholds of the red and infrared diodes Brefeldin_A are approximately 1.3 and 1.8V, respectively. The Apple iOS family of mobile devices (iPhone, iPod Touch, iPad and iPad Mini) was found to generate sufficient output voltages to perform clinical measurements.Figure 2.Schematic interface of a low-cost smartphone oximeter. The LEDs are driven by the headset speaker output and the photodiode signal is amplified by a line-powered JFET amplifier before being detected by the microphone.