This design uses a MPXV7002DP for the pressure transducer and includes Web Graphing, LCD Display. I will add alarms and email but this is a starting point.

I place the device between the output of the laser and the fan (at our maker space) to take the exhaust air outside. We were having a problem with people not turning on the fan when the laser was operational or leaving the fan on (and sucking the maker space air through the laser) when the laser was turned off. Hopefully, this post will help you understand the project.

Project Overview

This project creates a network-connected differential pressure monitor with:

• ±2 kPa measurement range
• 5-minute sampling
• 5-hour historical graph (60 points)
• High & Low alarm thresholds
• Physical zero-calibration button
• Web-based calibration initiation button
• 16×2 LCD display
• WiFi auto configuration portal
• Email alert via Gmail SMTP – planned
• EEPROM storage of configuration
• Connected via USB cable for power

System Features

Measurement

• Reads analog output from MPXV7002DP
• Voltage scaled safely for ESP32
• 64-sample averaging (with the highest and lowest thown out for noise reduction)
• Calibration offset stored in EEPROM

Web Interface

• Chart.- line graph
• Threshold configuration
• Email address configuration – planned
• Zero calibration button
• Alarm status display – planned

LCD Display Interface

• Line 1: IP Address
• Line 2: Pressure or ALARM message

Physical Requirements

Main Components

Electrical Design

Sensor Output Scaling

MPXV7002DP sensor output:

• 0.5V to 4.5V
• Center = 2.5V (0 kPa)

Wiring Diagram (Simplified)

Resistors are actually 20KΩ and 11KΩ, because that is what I had lying around. The graphic has an illustration for a pressure sensor that was already in Fritzing.

Firmware

Required Libraries

Install:

• WiFiManager
• WebServer
• LiquidCrystal_I2C

ESP32 code

Not all the code is included here but it should give you a good idea of what I am doing.

main

#include <Arduino.h>
#include <LiquidCrystal_I2C.h>
#include "secrets.h"
#include <softpause.h>
#include <WiFi.h>
#include "PressureTransducer.h"
#include <WebServer.h>
#include <Preferences.h>
// #include <ESP_Mail_Client.h>

// ===================== PINS & CONSTANTS =====================
#define ADC_PIN 34
#define ZERO_BUTTON 25
#define ALARM_PIN 26

const int HISTORYLENGTH = 60; // Store 60 samples of history at 1 sample per 5 minutes

// ===================== GLOBALS =====================
LiquidCrystal_I2C lcd(0x27, 16, 2);
PressureTransducer transducer(ADC_PIN);
WebServer server(80);

// AsyncWebServer server(80);
Preferences prefs;
// SMTPSession smtp;

float pressureHistory[HISTORYLENGTH];
float calibrationOffset = 0;
float calibrationFanOn = 0;
float fanThreshold = .05;
float highThreshold = 1.0;
float lowThreshold = -1.0;
bool alarmActive = false;
bool emailPending = false;
float lastPressure = 0;
unsigned long lastSampleTime = 0;
unsigned long lastHistoryTime = 0;
float maxPressure = -999.0;  // Track highest pressure since last calibration
float minPressure = 999.0;   // Track lowest pressure since last calibration

// Button hold detection variables
unsigned long buttonPressTime = 0;
bool buttonHeldNotified = false;
const unsigned long BUTTON_HOLD_TIME = 5000; // 5 seconds in milliseconds

// ===================== CORE LOGIC =====================

void saveSettings() {
  prefs.begin("monitor", false);
  prefs.putFloat("calOffset", calibrationOffset);
  prefs.putFloat("fanOn", calibrationFanOn);
  prefs.putFloat("maxP", maxPressure);
  prefs.putFloat("minP", minPressure);
  prefs.end();
}

void loadSettings() {
  prefs.begin("monitor", true);
  calibrationOffset = prefs.getFloat("calOffset", 0.0);
  calibrationFanOn = prefs.getFloat("fanOn", 0.0);
  maxPressure = prefs.getFloat("maxP", -999.0);
  minPressure = prefs.getFloat("minP", 999.0);
  prefs.end();
}

void calibrateSensor()
{
  // Reset min/max on calibration
  maxPressure = -999.0;
  minPressure = 999.0;
  
  // setup up calibdration offset
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.println("Turn off the fan.");
  lcd.setCursor(0, 1);
  lcd.println("Press the button.");
  softPause(2000); // give user time to read instructions
  while (digitalRead(ZERO_BUTTON) == HIGH)
  {
    // wait for button press
    softPause(100);
  }
  // setup the base pressure
  calibrationOffset = transducer.readPressure();
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.println("Baseline set:   ");
  lcd.setCursor(0, 1);
  lcd.println(calibrationOffset, 2);
  lcd.print(" kPa   ");
  Serial.println("Baseline: " + String(calibrationOffset) + " kPa");
  softPause(1000);
  calibrationFanOn = 0;
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.println("Turn on the fan.");
  while (calibrationFanOn == 0)
  {
    lcd.setCursor(0, 1);
    lcd.println("Press the button.");
    while (digitalRead(ZERO_BUTTON) == HIGH)
    {
      // wait for button press
      softPause(100);
    }
    float tempPressure = transducer.readPressure();
    Serial.println("Delta Pressure: " + String(tempPressure+fanThreshold) + " kPa");
    Serial.println("calibrationOffset Pressure: " + String(calibrationOffset) + " kPa");
    if (tempPressure+fanThreshold >= calibrationOffset)
    {
      lcd.setCursor(0, 0);
      lcd.println("Check fan!      ");
    }
    else
    {
      calibrationFanOn = tempPressure;
    }
  }
  saveSettings();
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("IP: " + WiFi.localIP().toString());
  lcd.println(calibrationFanOn, 2);
  lcd.print(" kPa   ");
}

// Web server handlers
void handleRoot() {
  String html = "<html><head><title>Pressure Monitor</title><style>";
  html += "body { font-family: Arial; margin: 20px; }";
  html += "h1 { color: #333; }";
  html += "p { font-size: 18px; }";
  html += "button { padding: 10px 20px; font-size: 16px; background-color: #4CAF50; color: white; border: none; border-radius: 4px; cursor: pointer; }";
  html += "button:hover { background-color: #45a049; }";
  html += ".stats { background-color: #f0f0f0; padding: 15px; border-radius: 4px; margin: 20px 0; }";
  html += ".chart-container { width: 100%; max-width: 800px; margin: 20px auto; }";
  html += "</style></head><body>";
  html += "<h1>Pressure Monitor</h1>";
  html += "<div class='stats'>";
  html += "<p>Current Pressure: <strong id='currentPressure'>" + String(lastPressure, 2) + "</strong> kPa</p>";
  html += "<p>Max Pressure: <strong id='maxPressure'>" + String(maxPressure, 2) + "</strong> kPa</p>";
  html += "<p>Min Pressure: <strong id='minPressure'>" + String(minPressure, 2) + "</strong> kPa</p>";
  html += "<p>Time since power-up: <strong id='uptime'>Calculating...</strong></p>";
  html += "</div>";
  html += "<p>Calibration Offset: <strong id='calOffset'>" + String(calibrationOffset, 2) + "</strong> kPa</p>";
  html += "<p>Fan Threshold: <strong id='fanThreshold'>" + String(calibrationFanOn, 2) + "</strong> kPa</p>";
  html += "</div>"; // close stats
  html += "<div class='chart-container'><canvas id='pressureChart' width='800' height='300'></canvas></div>";
  html += "<form action='/calibrate' method='POST'>";
  html += "<button type='submit'>Calibrate</button>";
  html += "</form>";
  html += "https://cdn.jsdelivr.net/npm/chart.js";
  html += "<script>";
  html += "const ctx = document.getElementById('pressureChart').getContext('2d');";
  html += "const pressureChart = new Chart(ctx, {";
  html += "  type: 'line',";
  html += "  data: {";
  html += "    labels: [],";
  html += "    datasets: [{";
  html += "      label: 'Pressure (kPa)',";
  html += "      data: [],";
  html += "      fill: false,";
  html += "      borderColor: 'rgba(33,150,243,1)',";
  html += "      backgroundColor: 'rgba(33,150,243,0.5)',";
  html += "      tension: 0.2";
  html += "    }]";
  html += "  },";
  html += "  options: {";
  html += "    responsive: true,";
  html += "    scales: {";
  html += "      x: { title: { display: true, text: 'Sample Index (updated every 5 minutes)' } },";
  html += "      y: { title: { display: true, text: 'Pressure (kPa)' } }";
  html += "    }";
  html += "  }";
  html += "});";

  html += "function updateData() {";
  html += "  fetch('/data')";
  html += "    .then(response => response.json())";
  html += "    .then(data => {";
  html += "      document.getElementById('currentPressure').innerText = data.current.toFixed(2);";
  html += "      document.getElementById('maxPressure').innerText = data.max.toFixed(2);";
  html += "      document.getElementById('minPressure').innerText = data.min.toFixed(2);";
  html += "      document.getElementById('uptime').innerText = data.uptime;";
  html += "      document.getElementById('calOffset').innerText = data.calOffset.toFixed(2);";
  html += "      document.getElementById('fanThreshold').innerText = data.fanThreshold.toFixed(2);";
  html += "      const history = data.history || [];";
  html += "      const now = new Date();";
  html += "      pressureChart.data.labels = history.map((_, idx) => {";
  html += "        const minutesAgo = (history.length - 1 - idx) * 5;";
  html += "        const labelTime = new Date(now.getTime() - minutesAgo * 60000);";
  html += "        const hh = String(labelTime.getHours()).padStart(2, '0');";
  html += "        const mm = String(labelTime.getMinutes()).padStart(2, '0');";
  html += "        return `${hh}:${mm}`;";
  html += "      });";
  html += "      pressureChart.data.datasets[0].data = history;";
  html += "      pressureChart.update();";
  html += "    });";
  html += "}";
  html += "updateData();";
  html += "setInterval(updateData, 5000);";
  html += "</script>";
  html += "</body></html>";
  server.send(200, "text/html", html);
}

void handleData() {
  unsigned long uptimeMillis = millis();
  int hours = uptimeMillis / 3600000UL;
  int minutes = (uptimeMillis % 3600000UL) / 60000UL;
  int seconds = (uptimeMillis % 60000UL) / 1000UL;
  String uptimeStr = String(hours) + ":" + (minutes < 10 ? "0" : "") + String(minutes) + ":" + (seconds < 10 ? "0" : "") + String(seconds);
  String json = "{";
  json += "\"current\":" + String(lastPressure, 3) + ",";
  json += "\"max\":" + String(maxPressure, 3) + ",";
  json += "\"min\":" + String(minPressure, 3) + ",";
  json += "\"uptime\":\"" + uptimeStr + "\",";
  json += "\"calOffset\":" + String(calibrationOffset, 3) + ",";
  json += "\"fanThreshold\":" + String(calibrationFanOn, 3) + ",";
  json += "\"history\": [";
  for (int i = 0; i < HISTORYLENGTH; i++) {
    json += String(pressureHistory[i], 3);
    if (i < HISTORYLENGTH - 1) json += ",";
  }
  json += "]";
  json += "}";
  server.send(200, "application/json", json);
}

void handleCalibrate() {
  calibrateSensor();
  String response = "<html><head><title>Calibration Complete</title><style>";
  response += "body { font-family: Arial; margin: 20px; }";
  response += "h1 { color: #4CAF50; }";
  response += "a { display: inline-block; margin-top: 20px; padding: 10px 20px; background-color: #008CBA; color: white; text-decoration: none; border-radius: 4px; }";
  response += "a:hover { background-color: #007399; }";
  response += "</style></head><body>";
  response += "<h1>Calibration Completed</h1>";
  response += "<p>Calibration offset: " + String(calibrationOffset, 2) + " kPa</p>";
  response += "<p>Fan threshold: " + String(calibrationFanOn, 2) + " kPa</p>";
  response += "<p>Min/Max pressures have been reset.</p>";
  response += "<a href='/'>Back to Home</a>";
  response += "</body></html>";
  server.send(200, "text/html", response);
}

void setup() {
  Serial.begin(115200);
  // define devices
  pinMode(ZERO_BUTTON, INPUT_PULLUP);
  //pinMode(ALARM_PIN, OUTPUT);
  lcd.init(); 
  lcd.backlight();
  lcd.setCursor(0,0);
  lcd.print("Startup...");
  //wifi setup
  WiFi.mode(WIFI_STA);
  WiFi.begin(SSID, PASSWORD);
  int loop=0;
  while (WiFi.status() != WL_CONNECTED) {
        lcd.clear();
        lcd.setCursor(0,loop);
        lcd.println("Connecting...");
        softPause(200);
        loop +=1;
        if (loop>1) loop=0;
    }
  // Connected!
  lcd.clear();
  lcd.setCursor(0,0);
  lcd.print("IP: " + WiFi.localIP().toString());
  lcd.setCursor(0,1);
  lcd.print("Connected");
  // setup ADC
  analogReadResolution(12);              // 0–4095
  analogSetPinAttenuation(ADC_PIN , ADC_11db);
  loadSettings();
  if (calibrationOffset == 0.0 && calibrationFanOn == 0.0) {
    // No calibration data, run calibration - this should only happen on first run or if settings are erased, so we can block here until it's done without impacting the user experience
    calibrateSensor();
  } else {
    lcd.clear();
    lcd.setCursor(0,0);
    lcd.print("IP: " + WiFi.localIP().toString());
    lcd.setCursor(0,1);
    lcd.print("Loaded: ");
    lcd.print(calibrationFanOn, 2);
    lcd.print(" kPa");
    softPause(2000);
  }
 
  // Set up web server routes
  server.on("/", handleRoot);
  server.on("/data", handleData);
  server.on("/calibrate", HTTP_POST, handleCalibrate);
  server.begin();
}

const unsigned long SAMPLE_INTERVAL = 1000;    // 1 second
const unsigned long HISTORY_INTERVAL = 300000; // 5 minutes

void loop() {
  server.handleClient();  // Handle web server requests
  
  // Monitor button for 5-second hold to trigger calibration
  bool buttonPressed = (digitalRead(ZERO_BUTTON) == LOW); // Button is pressed when LOW (INPUT_PULLUP)
  
  if (buttonPressed) {
    // Button is currently being held
    if (buttonPressTime == 0) {
      // Button just pressed, record the time
      buttonPressTime = millis();
      buttonHeldNotified = false;
    } else if (!buttonHeldNotified && (millis() - buttonPressTime >= BUTTON_HOLD_TIME)) {
      // Button has been held for 5 seconds
      buttonHeldNotified = true;
      lcd.clear();
      lcd.setCursor(0, 0);
      lcd.println("Calibrating...");
      Serial.println("ZERO_BUTTON held for 5 seconds - starting calibration");
      calibrateSensor();
    }
  } else {
    // Button is not pressed
    buttonPressTime = 0;
    buttonHeldNotified = false;
  }
  
  // take a sample every second 
  if (millis() - lastSampleTime > SAMPLE_INTERVAL) {
     lastSampleTime = millis();
     lastPressure = transducer.readPressure();
     lcd.setCursor(0, 1);
     lcd.println("Last: " + String(lastPressure, 2) + " kPa   ");
     // Track min and max pressure
     if (lastPressure > maxPressure) {
       maxPressure = lastPressure;
       saveSettings();
     }
     if (lastPressure < minPressure) {
       minPressure = lastPressure;
       saveSettings();
     }
   }
   // update history every 5 minutes
   if (millis() - lastHistoryTime > HISTORY_INTERVAL) {
     lastHistoryTime = millis();
     for (int i = 0; i < HISTORYLENGTH - 1; i++) {
       pressureHistory[i] = pressureHistory[i + 1];
     }
     pressureHistory[HISTORYLENGTH - 1] = lastPressure;
   }

}

Pressure transducer class

#include "PressureTransducer.h"
PressureTransducer::PressureTransducer(int pin) : adcPin(pin) {
    pinMode(adcPin, INPUT);
}
#define NUMBER_OF_SAMPLES 64
float PressureTransducer::readPressure() {
    float samples[NUMBER_OF_SAMPLES];
    float minSample = 4095.0;
    float maxSample = 0.0;
    float sum = 0.0;
    for (int i = 0; i < NUMBER_OF_SAMPLES; i++) {
        float temp = analogRead(adcPin);
        samples[i] = temp;
        if (temp < minSample) minSample = temp;
        if (temp > maxSample) maxSample = temp;
    }
    // Sum all except min and max
    for (int i = 0; i < NUMBER_OF_SAMPLES; i++) {
        if (samples[i] != minSample && samples[i] != maxSample) {
            sum += samples[i];
        }
    }
    float avg = sum / (NUMBER_OF_SAMPLES - 2);
    float adcVoltage = (avg / 4095.0) * 3.3;
    float sensorVoltage = adcVoltage * 1.55;
    float pressure_kPa = (sensorVoltage - 2.5);
    return pressure_kPa;
}

Web page functions

1. Page loads with initial values
2. Chart renders 60 stored samples (gathered every 5 minutes)
3. Calibration initiation button displayed
4. Every 5 seconds the web page:
   • Loads data
   • Updates graph

No page refresh required.

Calibration Procedure

There are three ways where a calibration is initiated:

1. System sees that there is no preference data defined
2. Web page calibration button pressed
3. User holds the device Zero button for 5 seconds

When the calibration is initiated, all previous data is lost and the user is asked to:

1. Turn off fan
2. Press Zero button
3. Turn on fan
4. Press Zero button
5. System auto-adjusts offset and starts tracking
6. IP address is displayed in line 0 of the LCD

Housing design

Attaches between the laser smoke exhaust and the exhaust hose to the fan and outside.

Sensor air intake hose is attached to the air flow via Bowden tube – that I had lying around.

When it is all assembled with the LCD housing, it looks like:

Here is a picture of the current software/hardware in production:

Showing the current IP address (so you can look at the web page) and the last pressure reading. Normally, when the fan is on, it is reading between -0.10 and -0.30. If the fan is off, the pressure value is dependent on the difference between the outside barometric pressure and the air pressure inside the maker space.