At my annual checkup recently I was curious just how that little pulse oximeter they clipped on my fingertip worked. How can they measure blood oxygen without taking a blood sample?
The hemoglobin in red blood cells binds oxygen to iron to carry oxygen throughout the body. As the oxygen is discharged the color of the red cells changes. Oximeters use two LEDs—flashing at alternative intervals—and a photodetector to measure the relative absorption of red (600-700 nm) and infrared (800-940 nm) light passed through a patient’s finger or earlobe. During a heartbeat blood volume increases and this AC component of the photodetector's current is used to calculate the pulse rate as well as the absorption of oxy- and deoxy-hemoglobin.
The Renesas RL78/L12 is an ideal candidate to use in a pulse oximeter. With integrated ADCs, DACs, and an LCD display controller, the RL78/L12 requires a minimum of external active components to implement this application. The current from the photodetector needs to be converted to a voltage and then amplified before being passed to the MCU’s ADC. A Renesas RNA519xx Reset IC detects supply voltage and resets all logic circuits; however it may be possible to omit this chip and rely on the MCU’s power-on reset (POR) functionality. Finally add a battery, display, and a handful of passive components and your BOM is complete.
In addition to a high level of integration the major advantage of the RL78/L12 is its ultra-low-power profile. In this application there is no reason why the MCU can’t remain in Stop mode (with RAM retention) almost all of the time, wakened with the push of a button. In Stop mode the chip draws only 0.23 μA, which assures extremely long life for the CR2032 coin cell that powers the unit you see in the photo. In Run mode the RL78/L12 draws 62.5 μA/MHz, and you can surely scale back the clock well short of the 24 MHz of which the chip is capable. The LEDs and LCD will draw far more than the MCU.
Finally, with up to 32 KB of program flash the RL78/L12 can easily store the modest amount of code required for this relatively straightforward application.
With a rapidly aging, health-conscious, and tech-savvy population the market for portable home healthcare devices such as pulse oximeters is booming. The Renesas RL78/L1x family of MCUs is well suited to these applications.