9. Input Devices

This week we had to:

• Probe an input device’s analog levels and digital signals

MAX30102 Pulse Oximeter and Heart Rate Sensor

First, we tried a new sensor in the Fablab: the MAX30102 Pulse Oximeter and Heart Rate Sensor.

We found an great web page to get us started.

Its functioning is interesting. It is equipped with two LEDs: one red emitting at 660nm and one IR at 880nm. These LEDs pulse their light through the skin of the finger (for example) and a photodetector detects the amount of light reflected. For pulse detection, it is detecting the frequency of the wave resulting from the pumping of the blood through the blood vessels. As hemoglobin absorbs IR, more or less IR is reflected according to the pressure in the vessel.

For measuring oximetry it is measuring the difference between the absorbtions of hemoglobin with and without oxygen.

On the web page , we found some example of code enabling us to measure the heart rate:

#include <Wire.h>
#include "MAX30105.h"
#include "heartRate.h"

MAX30105 particleSensor;

const byte RATE_SIZE = 4; //Increase this for more averaging. 4 is good.
byte rates[RATE_SIZE]; //Array of heart rates
byte rateSpot = 0;
long lastBeat = 0; //Time at which the last beat occurred

float beatsPerMinute;
int beatAvg;

void setup() {
  Serial.begin(115200);
  Serial.println("Initializing...");

  // Initialize sensor
  if (!particleSensor.begin(Wire, I2C_SPEED_FAST)) {
    Serial.println("MAX30102 was not found. Please check wiring/power. ");
    while (1);
  }
  Serial.println("Place your index finger on the sensor with steady pressure.");

  particleSensor.setup(); //Configure sensor with default settings
  particleSensor.setPulseAmplitudeRed(0x0A); //Turn Red LED to low to indicate sensor is running
  particleSensor.setPulseAmplitudeGreen(0); //Turn off Green LED
}

void loop() {
  long irValue = particleSensor.getIR();

  if (checkForBeat(irValue) == true) {
    //We sensed a beat!
    long delta = millis() - lastBeat;
    lastBeat = millis();

    beatsPerMinute = 60 / (delta / 1000.0);

    if (beatsPerMinute < 255 && beatsPerMinute > 20) {
      rates[rateSpot++] = (byte)beatsPerMinute; //Store this reading in the array
      rateSpot %= RATE_SIZE; //Wrap variable

      //Take average of readings
      beatAvg = 0;
      for (byte x = 0 ; x < RATE_SIZE ; x++)
        beatAvg += rates[x];
      beatAvg /= RATE_SIZE;
    }
  }

  Serial.print("IR=");
  Serial.print(irValue);
  Serial.print(", BPM=");
  Serial.print(beatsPerMinute);
  Serial.print(", Avg BPM=");
  Serial.print(beatAvg);

  if (irValue < 50000)
    Serial.print(" No finger?");

  Serial.println();
}

We managed to read the sensor values using the console of Arduino IDE:

Analog IR distance sensor

The first distance sensor we tried was the GP2Y0A02YK. It is an Infra Red distance sensor that works by comparing the angle of arrival of the reflected beam. It also modulates the emitted IR signal in order to discriminate between the sensor beam and the background radiations. We used the datasheet to understand how to wire the sensor to our power supply and the oscilloscope. We made sure to have common ground between the power supply and the oscilloscope.

We then tested it by detecting our hand at different distances:

Using the oscilloscope, we monitored the analog signal from the sensor. As it is analog, it varies between 0V and +5V. At very close distance, the voltage rises with the distance and after about 20cm, it starts decreasing with the distance. That’s why the sensor has a range of 20cm to 150cm.

Digital ultrasonic distance sensor

The US-100 Y401 ultrasonic sensor is a digital sensor. It generates a pulse signal on the Echo output, where the pulse duration corresponds to the distance to the object.

Goal: To check the change in the output signal depending on the distance to the object.

• Connected VCC to 5V, and GND to ground.

• By generating a signal for Trig using the signal generator, we observed the pulses on Echo with an oscilloscope.

These are the devices we used and the result on the oscilloscope when varying the distance. The wider the pulse, the wider the pulse, the longer the distance until the ultrasonic wave hits an obstacle:

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Last update: March 26, 2025