Photoplethysmograph sensor based on light-to-frequency converter

This paper describes a low cost high sensitivity photoplethysmograph sensor based on a light-to-frequency converter. A working prototype, consisting of a probe, a data-processing unit, a PC real-time data display with graphical user interface (GUI), is built. For the probe, a light-to-frequency converter and an IR LED are integrated into the center button of a three buttons PC mouse, with the left and right buttons operating as the IR LED switch. The volume of blood circulation through the finger modulates the light intensity of the reflected IR beam from the finger bone. The light-to-frequency converter converts the light intensity changes into a pulse train of corresponding frequencies, which are accumulated in a counter. A micro-controller then reads the counter value at 200 Hz and transmits it to the PC for further analysis. Optocouplers are used in the communication path between the system and the PC to provide electrical isolation between the user and the PC power supply. The resulting waveforms are validated through comparison with a commercial system.