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DIY pulse oximeter Lately, our electronic markets have been flooded with pulse oximeter probes based on the MAX30100 IC for as low as INR250 (about $2) apiece, such as the RCWL-0530. I purchased a few from a local supplier, but I found that the readings were awfully wrong! After reading the MAX30100 data sheet and doing some internet research, I discovered that these lowcost oximeters have two main problems: poor regulation of the 1.8V supply and incorrect I2C pull-up resistor values. The MAX30100 is an I2C infrared measuring device. Ideally, the board should supply 3.3V for the infrared LED driver and 1.8V for the control & measurement circuitry. The I2C and interrupter pins need to be pulled up to 3.3V via 4.7kW resistors, but many implementations only pull them up to the 1.8V rail. In this case, they will not work with the I2C bus of an Arduino or ESP32 micro. The MAX30100 has a temperature sensor for oxygen reading correction, but it cannot be read separately, and the sensor has a power-down sleep state which is generally not used. I don’t know why. Check the voltage on either side of the 3-pin (SOT-23) regulator with a voltmeter. You should get readings of 3.3V and 1.8V. So far, so good. But the three 4.7kW resistors are connected to the SCL, SDA & INT pins from the +1.8V rail. This prevents us from siliconchip.com.au getting the correct measurements for this device, even though the software shows success. The adjacent image shows where you can cut the track and run a short length of small-diameter solid-core insulated wire to fix this (see github. com/oxullo/Arduino-MAX30100/ issues/51). We can now connect it to the ESP32 microcontroller module, as shown in the circuit diagram. I have added a DS18B20 temperature sensor since, as mentioned above, we can’t query the temperature sensor on the oximeter module. The resulting probe measures the oxygen level and temperature from a finger and uploads it to ‘the cloud’ at www.thingspeak.com The ESP32 has been programmed to support multiple WiFi SSID and password combinations. It will connect to whichever is available at that moment. The LED at GPIO12 will blink briefly to indicate that the data has been uploaded to the cloud server. After uploading the data, the micro goes into deep sleep mode for 20 seconds, then it wakes up and repeats the process. During sleep mode, the IR led of the MAX30100 sensor switches off and the total power consumption goes down to 4.2mA. During measurement, the current is 160mA. One 26650 3.7V Li-ion cell of around 3000mAh can sustain this for weeks non-stop. When attaching the sensor to a Australia’s electronics magazine On this RCWL-0530 module, the track marked in yellow must be cut, and then solder a piece of wire between the two locations marked in red. This connects the I2C pull-ups to the correct 3.3V rather than 1.8V supply. finger, ensure that the area which makes contact is clean and without oil, ink or grease. It’s better to clean it with alcohol beforehand. If the body contact is not perfect, the device will hang. To solve that, the micro will restart after 25 seconds. It will also restart if it is unable to upload data. The Arduino sketch to load onto the ESP32 is available for download from siliconchip.com.au/Shop/6/5860 You will need to open a free account at www.thingspeak.com and modify the API key in the software to match the one you are supplied with before it will upload data. Sample data is visible on my Thingspeak channel at www.thingspeak. com/channels/1203838 Bera Somnath, Vindhyanagar, India. ($100) July 2021  63