December 2004, Issue 173

Light-to-Frequency Conversion (Part 1)
TSL230R-Based Pulse Oximeter

DESIGN PARAMETERS

A heart rate in the vicinity of 70 beats per minute (bpm) is considered normal for an adult. A newborn’s heart rate is typically around 120 bpm. Your heart rate slows to approximately 50 bpm as you enter your golden years. When exercising, your heart rate may double. (Sustained exercising need only elevate the normal heart rate by roughly an additional 50% to be effective.) Accounting for all of this data, I’d limit what could be considered good readings to, say, 50 to 200 bpm.

Figure 3 shows the circuit I used for experimenting with this project. It may be overkill for the end product, but I can have the hardware serial port on the microcontroller output some data for analysis. I’ll consider using a smaller device when I don’t need to log any data. Although it’s possible to drive an LED directly from the microcontroller’s I/O, any change in voltage will have a different effect on the current through each LED because the LEDs have different drops. I chose to use constant current drivers for the two LEDs. This automatically takes into account the different drops for the red and IR LEDs.

The TSL230R and LEDs are a sensor unit connected to the electronics with a 10-conductor ribbon cable (see Photo 1). Figure 3 shows how it’s split. This allows the sensor to connect to various prototype circuits.

I found a piece of plastic conduit that fit over my finger after I slotted it. By slotting all but a 0.25²”, it acts like a clothespin and holds on firmly to my finger without being uncomfortable. The TSL230R sensor is glued into a square hole placed on one side of the conduit. The red and IR LEDs are forced into two drill holes directly across the diameter from the sensor. Square pin headers make all the connections easier.

SMT and flex circuitry would be perfect for this application. I did not experiment with mounting the LEDs and sensor on the same side of the conduit. Although this becomes more of a reflective illumination, it avoids having wires span two moving objects, which is a potential mechanical point of failure.

Because I used a red LED and an IR LED, the circuit can actually measure the oxygen content of your blood in addition to your heart rate. To measure a heart rate, you must calculate the time between the maximum (or minimum) excursions of the AC portion of the light absorption output. Both the red LED and the IR LED can provide the light source for the TSL230R. However, the hemoglobin in red blood cells picks up oxygen molecules in the lungs and becomes a brighter shade of red, which will absorb less red light.

Figure 4 shows the difference in light absorption between oxygenated and deoxygenated blood at various wavelengths. Notice that for infrared there is little difference in the absorption. At lower wavelengths (especially the red region), there is a significant difference. You can calculate the level of oxygen by comparing the absorption outputs of each light source separately.