Pulse Oximeter

Overview

A fingertip pulse oximeter is a clip-on device that estimates arterial blood oxygen saturation (SpO2) and pulse rate without drawing blood. The user slides a fingertip into the spring-loaded Housing, which clamps shut so the finger sits between an LED emitter and a detector. Within a few seconds the Display Module shows oxygen saturation as a percentage, the pulse rate in beats per minute, and a small plethysmograph bar that tracks each heartbeat.

The optical core is the Optical Sensor, a transmission sensor with the Red/IR LED Emitter on one side of the finger chamber and the Photodiode Detector facing it across the finger. Light that survives the trip through the tissue lands on the detector, and the Main Board turns that signal into a saturation reading. Two AAA cells in the Battery Compartment power the unit, and the Piezo Buzzer beeps with each detected pulse and sounds an alarm when saturation drops below a set threshold.

How it works

Oxygenated and deoxygenated hemoglobin absorb red and infrared light differently. Oxyhemoglobin absorbs more infrared (940 nm) and lets more red (660 nm) pass, while deoxyhemoglobin does the opposite. The LED Driver switches the two LEDs on alternately at high speed, and the photodiode current for each wavelength is amplified and sampled by the Analog-to-Digital Converter. This is the practical application of the Beer-Lambert law, which relates how much light a medium absorbs to the concentration of the absorbing substance and the path length.

The trick that isolates arterial blood is the pulse itself. With every heartbeat the arteries swell, lengthening the optical path through arterial blood, so the absorbed light has a small pulsing (AC) component riding on a large steady (DC) component from skin, bone, venous blood, and tissue. The firmware on the Main Board extracts the AC/DC ratio for both wavelengths and forms the ratio-of-ratios, which maps through an empirical calibration curve to SpO2. The same pulsatile waveform yields the pulse rate and drives the plethysmograph bar. Because the method needs a pulse, low perfusion, motion, dark nail polish, or strong ambient light degrade the reading, which is why the Housing uses a cushioned chamber to block stray light and hold the finger still.