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Teardown: Pulse Oximeter

Pulse oximeters attempt to non-invasively determine the percentage of oxygenated hemoglobin in circulation in the blood. We open one up to find out what is inside.

The pulse oximeter purchased for this article is a $25 unit branded AccURate. Outwardly, it appears to be similar, if not identical, to other units currently on the market sold by FaceLake, Santa Medical, and many others. If you purchase one for medical use, please focus on the FDA approval status before considering price or whether or not it has other features, such as a two-color display or Bluetooth.



Disassembly

The hard-and-soft-plastic unit holds a finger between an upper housing and lower housing that are connected by a pair of asymmetrical pivot hinges that connect a hole on the top with a slot on the bottom. The bottom housing contains batteries and a LED; the top housing contains a light sensor, a microprocessor, and the display.
To open the top housing, gently pry along the seam to separate the three mechanical hooks of the cover from the recesses on the upper housing.



To separate the top housing from the bottom housing, remove the glued-in-place hinge pins. Insert a small flat object under the edge of the hinge pins and gently lift. Springs are removed by pushing them towards the far end of the unit (to the right in the above picture).
There isn't much to take apart in the lower housing. There is a small plastic cover that can be pried loose to reveal the battery connectors and solder points for four wires. Gently pull the soft plastic away from the hard plastic to reveal the rest of the circuit board and the LED.



See the video below to watch the disassembly.




ComponentDescriptionCostMore Information
NANO102LC2AN32-bit Microcontroller$2Technical Reference Manual
TSL237LFLight-to-Frequency Converter$4Datasheet
PDI-E833-ND*Bidirectional LED<$1Datasheet
01531*64x128 Bicolor OLED Display$4Datasheet
*  Positive identification is impossible due to lack of identifying marks. These parts have identical functionality to the parts inside the device.

What's Happening Underneath Your Finger?

A bi-color LED is present in the lower housing. A microcontroller generates a positive pulse that is immediately followed by a negative pulse. This is accomplished by means of a digital switch IC that is controlled by the microcontroller.
Visible red light of ~650 nm is emitted by the LED during one pulse, and infrared light of ~950 nm is emitted by the LED during the other. These short pulses are only separated by a few milliseconds so, to the human eye, the light appears as though it is continuously emitted and your finger glows faintly red.


Oscilloscope capture of bi-directional LED power supply

What's Happening in Your Finger?

Infrared and visible light from the LED are transmitted into your finger where the light is absorbed by the hemoglobin and oxyhemoglobin in your blood. The oxygen saturation percentage is calculated by a microcontroller based on the levels of absorbance of the light from the LED.
This paper explains the equations and theory used to determine the oxygen saturation percentage and this video from the New England Journal of Medicine explains how Pulse Oximeters work.

What's Happening Above Your Finger?

The TSL237LF

Above your finger is the TSL237, a light-intensity-to-frequency converter that outputs a square wave whose frequency is directly proportional to the intensity of light that shines on the photodetector. The TSL237 is just one of the sensors that AMS-TAOS produces. They have a complete line of optical sensors that includes light, color, proximity, and gesture detection.
The frequency output of the TSL237 is such that several dozen cycles can occur during each pulse of the LED. In the pulse-oximeter circuit, the TSL237 would have a changing frequency output. On my workbench, the frequency is influenced by ambient lighting that outshines the light from the LED.


The white trace shows the power supplied to the bi-color LED and the yellow is the signal generated by the light-to-frequency converter in artificial lighting
 

The frequency of pulses is determined by the amount of light that reaches the sensor
 
The output of the TSL237 is passed to the microcontroller which counts or times pulses while it controls the LED in the lower housing. The microcontroller then calculates the oxygen saturation percentage based on those values.

 

The NANO102LC2AN Microcontroller

This inexpensive microcontroller is built around the ARM Cortex-M0, a 32-bit microprocessor with a meager 56 instructions in its set. The microprocessor features all manner of common chip-to-chip communication, has a 4 kB flash memory loader for In System Programming (ISP), and can operate at voltages as low as 1.8V with its built-in low drop out regulator.
The microcontroller controls the state of the LED, counts pulses of the light sensor, calculates the oxygen saturation, and controls the display.


NANO102LC2AN overview. Graphic courtesy of Nuvoton
 

The Yellow Blue 128x64 OLED Display

There is a two-color OLED display in use in this device from an unknown manufacturer. There are few traces visible on the circuit board that join the microcontroller and display, which suggests serial (as opposed to parallel) control. However, without probing, it is possible that there are traces hidden on a non-visible layer of the circuit board.

Miscellaneous

The circuit board also has a few miscellaneous resistors, capacitors, and a crystal to provide timing for the microcontroller.

Conclusion

Finger-style pulse oximeters are a non-invasive and inexpensive addition to hospital rooms and are inexpensive enough for the home medical kit. Before you purchase one, have a discussion with your doctor to determine if you have a condition that requires one and then have your health-care provider teach you how to use it.

A low reading might be a sign that someone experiencing a heart attack needs supplemental oxygen, or it could simply mean that the person wearing the device has cold hands or fingernails with polish, and the fright is enough to cause a heart attack. Lastly, if you are ever uncertain about the well-being of someone and are concerned they are not getting enough oxygen due to a health condition, call emergency services immediately.

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