Heart rate and blood oxygen measurements provide an important indication of an individual's fitness and medical condition. A widely-used device known as a pulse oximeter uses an optical method to continuously and non-invasively make these measurements. The method is based on the fact that absorption of red and near-infrared (NIR) light differs significantly between oxyhemoglobin and its deoxygenated form. Heart rate can be measured by a simpler optical sensor that uses a single light source to detect the change of blood volume at the fingertip for each heartbeat. This method is called photoplethysmography (PPG).

The goal of this project is to create, test, and demonstrate such a PPG sensor. The hands-on practical experience to be gained will complement and extend the syllabus of the third-year electronics course, PHY335. Our current PPG sensor contains a NIR LED (850 nm peak wavelength) and NIR photodiode mounted side-by-side to detect light reflected from a finger tip or other tissue. (We have also experimented with red light and a Si photodiode in a transmission geometry.) The output of the photodiode consists of a relatively large DC (steady) component and a much smaller AC component at the pulse frequency. The electronic circuit must block the DC and highly amplify the AC signal in a narrow frequency range between about 1 and 3 Hz (60 and 180 bpm). So far we have used a commercial pre-amplifier (PAR Model 113) and displayed its output on an oscilloscope, but we hope to later replace this with an purpose-built operational-amplifier circuit with optimized band-pass filters that could drive an LED pulse indicator or a digital pulse rate display.

One unexpected result from our experiments so far is that some individuals have a readily observable pulse while for others (who typically report poor circulation) no pulse can be detected.