Simple pulse oximeter for wearable monitors

This design example is a new pulse oximeter design that is very simple to use and powerful. With its excellent performance, it can even be used as a standalone device to monitor heart rate and blood oxygen saturation.

The core components of the system include an ultra-bright red LED (KA-3528SURC), an infrared LED (VSMB3940X01-GS08), and a photodiode (VBP104SR) that is equally sensitive to both wavelengths of light.

The LT6003 operational amplifier is the basic building block of the entire system and is used in several amplifier stages of the circuit. In Figure 1, IC1 acts as a transimpedance amplifier, converting the current generated by the photodiode into a voltage. This stage provides high gain, allowing the sensor to be used in almost every part of the human body. Op amp IC2 acts as an inverting amplifier with a gain of 30.

Figure 1: Pulse oximeter.

The negative input of comparator IC4 is connected to the correction signal using a peak detector circuit. Components such as IC3, D1, and C6 are used to detect and hold the maximum voltage of the input signal. R7 and R10 are used to discharge the C6 capacitor. This circuit is used as a reference voltage, and even the weak pulse generated by the sudden change of the sensor's position on the human body is detected.

The circuit is equipped with a high-pass filter and two low-pass filters to filter out unwanted interference signals caused by external light changes or AC flashlight flickering. The frequencies of the high-pass filter and the first low-pass filter are set to 0.86Hz and 159Hz. The other terminals of the high-pass filter and the low-pass filter are not grounded, but connected to a 1V reference voltage to increase the bias of the detection signal for subsequent processing. The reference voltage is generated by LM4040 and a voltage divider (R15, R16). After passing through IC2, the second low-pass filter with a frequency set to 5.9Hz processes the signal to further filter out other unwanted interference signals (see Figure 2 and Figure 3).

Figure 2: The voltage output of the COMP node (CH1) and the pulse node (CH2) over time.

Figure 3: Signals at the output node.

This article does not discuss the pulse reading process, which can be done by an MCU with an ADC. The MCU is used to control the LED, detect the signal, and convert the signal into blood sample saturation. The blood oxygen saturation can be calculated even with a narrow bandpass filter. When the red LED is lit, the ADC starts to detect the signal. After two or three pulses, the infrared LED can be lit at the same time as the red LED. The MCU uses the formula S=VR/VIR for calculation, where the voltage is the peak-to-peak reading and S represents the value in the tissue blood oxygen saturation (StO2) calibration table.