heltec-wireless-tracker-docs/docs/examples/heart-rate-monitor.md

Monitor Heart Rate on SnapEmu with Wireless Tracker

Overview

This project enables real-time heart rate monitoring using a wireless tracker connected to a heart rate sensor. Data transmits via LoRaWAN protocol to the SnapEmu platform for visualization and analysis.

Hardware Components Required

Component	Purpose
Heltec Wireless Tracker	Main controller + LoRa TX
HT-M7603 Indoor LoRa Gateway	LoRaWAN network gateway
MAX30102 Heart Rate Sensor	Pulse oximetry sensor
DuPont Wires	Connections

Hardware Configuration

Pin Connections

The MAX30102 sensor connects to the Heltec Wireless Tracker using I2C:

MAX30102 Pin	Wireless Tracker Pin	Description
VCC	3V3	Power (3.3V)
GND	GND	Ground
SDA	GPIO45	I2C Data
SCL	GPIO46	I2C Clock

Note: GPIO45 and GPIO46 are recommended as they don't have conflicting built-in functions.

Software Setup

Step 1: Configure HT-M7603 Gateway

Configure your LoRa gateway connection to SnapEmu platform:

• Follow official Heltec documentation

Step 2: Register Tracker Node

Register your wireless tracker device on SnapEmu:

• Follow node device connection guidelines

Step 3: Install Libraries

Required Arduino libraries:

• Wire.h (built-in)
• MAX30102_PulseOximeter.h
• LoRaWan_APP.h (Heltec library)

Step 4: Sensor Test Code

First, test the sensor standalone:

#include <Wire.h>
#include "MAX30102_PulseOximeter.h"

#define REPORTING_PERIOD_MS 1000
PulseOximeter pox;
uint32_t tsLastReport = 0;

void onBeatDetected() {
    Serial.println("Beat detected!");
}

void setup() {
    Serial.begin(115200);
    Wire.begin(45, 46);  // SDA=GPIO45, SCL=GPIO46
    Serial.print("Initializing MAX30102..");
    delay(3000);

    if (!pox.begin()) {
        Serial.println("FAILED");
        for(;;);
    } else {
        Serial.println("SUCCESS");
    }

    pox.setIRLedCurrent(MAX30102_LED_CURR_7_6MA);
    pox.setOnBeatDetectedCallback(onBeatDetected);
}

void loop() {
    pox.update();

    if (millis() - tsLastReport > REPORTING_PERIOD_MS) {
        float heartRate = pox.getHeartRate();
        float spo2 = pox.getSpO2();

        if (heartRate > 0) {
            Serial.print("Heart rate: ");
            Serial.print(heartRate);
            Serial.print(" bpm / ");
        } else {
            Serial.print("Heart rate: N/A / ");
        }

        if (spo2 > 0) {
            Serial.print("SpO2: ");
            Serial.print(spo2);
            Serial.println(" %");
        } else {
            Serial.println("SpO2: N/A");
        }

        tsLastReport = millis();
    }
}

Step 5: Full LoRaWAN Code

Complete code with LoRaWAN transmission:

#include <Wire.h>
#include "MAX30102_PulseOximeter.h"
#include "LoRaWan_APP.h"

#define REPORTING_PERIOD_MS 1000

// OTAA Parameters - Replace with your values from SnapEmu
uint8_t devEui[] = { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xA8 };
uint8_t appEui[] = { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
uint8_t appKey[] = { 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88,
                     0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88 };

// LoRaWAN Configuration
uint16_t userChannelsMask[6] = { 0x00FF, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000 };
LoRaMacRegion_t loraWanRegion = ACTIVE_REGION;
DeviceClass_t loraWanClass = CLASS_A;
uint32_t appTxDutyCycle = 15000;  // 15 second transmit interval
bool overTheAirActivation = true;
bool loraWanAdr = true;
bool isTxConfirmed = true;
uint8_t appPort = 2;
uint8_t confirmedNbTrials = 4;

PulseOximeter pox;
uint32_t tsLastReport = 0;

static void prepareTxFrame(uint8_t port) {
    pox.update();
    float heartRate = pox.getHeartRate();
    float spo2 = pox.getSpO2();
    unsigned char *puc;

    appDataSize = 0;

    // Packet header
    appData[appDataSize++] = 0x04;
    appData[appDataSize++] = 0x00;
    appData[appDataSize++] = 0x0A;
    appData[appDataSize++] = 0x02;

    // Heart rate (4 bytes float)
    puc = (unsigned char *)(&heartRate);
    appData[appDataSize++] = puc[0];
    appData[appDataSize++] = puc[1];
    appData[appDataSize++] = puc[2];
    appData[appDataSize++] = puc[3];

    // Separator
    appData[appDataSize++] = 0x12;

    // SpO2 (4 bytes float)
    puc = (unsigned char *)(&spo2);
    appData[appDataSize++] = puc[0];
    appData[appDataSize++] = puc[1];
    appData[appDataSize++] = puc[2];
    appData[appDataSize++] = puc[3];

    Serial.print("TX: HR=");
    Serial.print(heartRate);
    Serial.print(" SpO2=");
    Serial.println(spo2);
}

void onBeatDetected() {
    Serial.println("♥ Beat!");
}

void setup() {
    Serial.begin(115200);
    Wire.begin(45, 46);
    Mcu.begin(HELTEC_BOARD, SLOW_CLK_TPYE);

    Serial.print("Initializing MAX30102..");
    delay(3000);

    if (!pox.begin()) {
        Serial.println("FAILED");
        for(;;);
    }
    Serial.println("SUCCESS");

    pox.setIRLedCurrent(MAX30102_LED_CURR_27_1MA);
    pox.setOnBeatDetectedCallback(onBeatDetected);

    LoRaWAN.init(loraWanClass, loraWanRegion);
    LoRaWAN.setDefaultDR(3);
}

void loop() {
    pox.update();

    // Local display
    if (millis() - tsLastReport > REPORTING_PERIOD_MS) {
        float heartRate = pox.getHeartRate();
        float spo2 = pox.getSpO2();

        Serial.print("HR: ");
        Serial.print(heartRate > 0 ? String(heartRate) : "N/A");
        Serial.print(" bpm | SpO2: ");
        Serial.print(spo2 > 0 ? String(spo2) : "N/A");
        Serial.println("%");

        tsLastReport = millis();
    }

    // LoRaWAN state machine
    switch (deviceState) {
        case DEVICE_STATE_INIT:
        case DEVICE_STATE_JOIN:
            LoRaWAN.join();
            break;
        case DEVICE_STATE_SEND:
            prepareTxFrame(appPort);
            LoRaWAN.send();
            deviceState = DEVICE_STATE_CYCLE;
            break;
        case DEVICE_STATE_CYCLE:
            txDutyCycleTime = appTxDutyCycle + randr(-APP_TX_DUTYCYCLE_RND, APP_TX_DUTYCYCLE_RND);
            LoRaWAN.cycle(txDutyCycleTime);
            deviceState = DEVICE_STATE_SLEEP;
            break;
        case DEVICE_STATE_SLEEP:
            LoRaWAN.sleep(loraWanClass);
            break;
        default:
            deviceState = DEVICE_STATE_INIT;
            break;
    }
}

Data Decoding on SnapEmu

Custom decoder function for the payload format:

function Decoder(bytes, port) {
    var decoded = {};

    if (bytes.length >= 13) {
        // Extract heart rate (bytes 4-7)
        var hrBytes = new Uint8Array([bytes[4], bytes[5], bytes[6], bytes[7]]);
        var hrView = new DataView(hrBytes.buffer);
        decoded.heartRate = hrView.getFloat32(0, true);

        // Extract SpO2 (bytes 9-12)
        var spo2Bytes = new Uint8Array([bytes[9], bytes[10], bytes[11], bytes[12]]);
        var spo2View = new DataView(spo2Bytes.buffer);
        decoded.spo2 = spo2View.getFloat32(0, true);
    }

    return decoded;
}

Platform Visualization

SnapEmu displays:

• Real-time heart rate graph
• SpO2 level monitoring
• Historical data (up to 1 month)
• Mobile app + web interface

Troubleshooting

Issue	Solution
No heartbeat detected	Ensure finger is properly placed, adjust LED current
Erratic readings	Clean sensor, reduce movement, check power supply
LoRa join failed	Verify gateway connection, check credentials
Data not showing on platform	Check decoder function, verify port number

References

• SnapEmu Documentation
• Heltec Sensor Decoding
• Original project by ashley15 on Hackster.io

License: GPL3+