Smart Classroom Welcome Monitor, a Arduino project

This smart classroom monitor displays real-time temperature and humidity readings on an LED matrix while alerting when the room gets too warm. Built around an Arduino Uno, DHT11 sensor, and MAX7219 LED matrix display, it continuously monitors classroom conditions and triggers a red LED warning light if temperature exceeds a configurable threshold.

The guide includes a complete wiring diagram showing breadboard layout, a full parts list with recommended suppliers, Arduino firmware with configurable thresholds and scroll speed, and step-by-step assembly instructions. Readers will learn how to wire SPI communication to the LED matrix, integrate analog sensor readings, and implement real-time environmental monitoring on a microcontroller.

Wiring diagram

Gather all the parts

Qty	Component
1	DHT11 Digital temperature and humidity sensor (lower accuracy than DHT22)
1	LED Red Standard 3mm/5mm through-hole LED. A current-limiting series resistor is added automatically.
1	Resistor 220 Ω Through-hole resistor (current-limiting in series with an LED)
1	MAX7219 LED Matrix Display Serial LED matrix / 7-segment display driver IC module. Drives one 8x8 LED matrix or up to eight 7-segment digits. Cascadable for multi-digit or multi-matrix displays. Uses a 5V supply with segment current set by an external RSET resistor. Communicates with DIN, CLK, and LOAD/CS; when powered at 5V the datasheet logic-high threshold is 3.5V, so use level shifting for strict 3.3V hosts.

Assemble it in 5 steps

1. Breadboard power rails

Connect the UNO's 5V pin to the breadboard's red (+) rail and GND to the blue (−) rail. This powers all components from the board's USB supply.

Double-check the polarity — 5V to red, GND to blue.

2. Wire the MAX7219 LED Matrix

Plug the MAX7219 module into the breadboard. Connect: • VCC → 5V rail • GND → GND rail • DIN → UNO D11 • CLK → UNO D13 • CS → UNO D8

DIN, CLK and CS are the only three signal wires needed.
If you have more than one panel, daisy-chain DOUT of the first into DIN of the second and change MAX_DEVICES to 2 in the firmware.

3. Wire the DHT11 sensor

Plug the DHT11 module into the breadboard. Connect: • VCC (or +) → 5V rail • GND (or −) → GND rail • DATA (or S/OUT) → UNO D2

The Elegoo kit DHT11 is a 3-pin module — the middle pin is DATA.
Leave at least 1 cm clearance from heat sources for accurate readings.

4. Wire the red LED and resistor

Insert the 220 Ω resistor between UNO D3 and the LED's ANODE (longer leg). Connect the LED's CATHODE (shorter leg) to GND rail.

The longer leg of the LED is the anode (+); the flat side of the LED body marks the cathode (−).
Never connect an LED directly to a GPIO pin without the resistor — it will burn out.

5. Power up and verify

Plug the UNO into USB. The LED matrix should start scrolling temperature and humidity within 2 seconds of power-on. The red LED lights up automatically when the temperature exceeds 27 °C.

Open Serial Monitor at 9600 baud to see live sensor readings.
If the matrix is too dim or too bright, change the setIntensity(4) value (0–15) in the firmware and redeploy.

Review all connections

1. Connections between "dht11" and "Arduino"

Function	dht11	Arduino
`power`	VCC	5V
`ground`	GND	GND
`data`	DATA	GPIO 2

2. Connections between "r1" and "Arduino"

Function	r1	Arduino
`digital`	P1	GPIO 3
`digital`	P2 → LED ANODE	EXT

3. Connections between "red_led" and "Arduino"

Function	red_led	Arduino
`ground`	GND	GND

4. Connections between "matrix" and "Arduino"

Function	matrix	Arduino
`power`	VCC	5V
`ground`	GND	GND
`spi`	DIN	GPIO 11
`spi`	CLK	GPIO 13
`spi`	CS	GPIO 8

Deploy the firmware

src/main.cppOpen in Schematik

#include <Arduino.h>
#include <DHT.h>
#include <MD_Parola.h>
#include <MD_MAX72xx.h>
#include <SPI.h>

// ── Pin definitions ───────────────────────────────────────────────
#define DHT_PIN         2
#define LED_PIN         3
#define MATRIX_DIN_PIN  11
#define MATRIX_CLK_PIN  13
#define MATRIX_CS_PIN   8

// ── Config ────────────────────────────────────────────────────────
#define DHTTYPE         DHT11
#define MAX_DEVICES     1          // Change to 2, 3 … for more panels
#define SCROLL_SPEED    120        // Lower = faster scroll (ms per step)
#define HOT_THRESHOLD   27.0f      // °C — red LED turns on above this
#define SENSOR_INTERVAL 3000UL     // ms between sensor reads

// ── Objects ───────────────────────────────────────────────────────

// Forward declarations
void readSensor();

DHT dht(DHT_PIN, DHTTYPE);
MD_Parola matrix = MD_Parola(MD_MAX72XX::FC16_HW, MATRIX_CS_PIN, MAX_DEVICES);

// ── State ─────────────────────────────────────────────────────────
char displayBuf[32];
float lastTemp = 0.0f;
float lastHum  = 0.0f;
unsigned long lastSensorRead = 0;

// ── Helpers ───────────────────────────────────────────────────────
void readSensor() {
    float t = dht.readTemperature();
    float h = dht.readHumidity();
    if (!isnan(t)) lastTemp = t;
    if (!isnan(h)) lastHum  = h;

    // Red LED
    digitalWrite(LED_PIN, lastTemp > HOT_THRESHOLD ? HIGH : LOW);

    // Build display string  e.g.  "Temp:24C  Hum:55%"
    snprintf(displayBuf, sizeof(displayBuf),
             "Temp:%dC  Hum:%d%%",
             (int)round(lastTemp), (int)round(lastHum));
}

void setup() {
    Serial.begin(9600);
    pinMode(LED_PIN, OUTPUT);
    digitalWrite(LED_PIN, LOW);

    dht.begin();

    matrix.begin();
    matrix.setIntensity(4);          // 0 (dim) – 15 (bright)
    matrix.setTextAlignment(PA_LEFT);

    // First sensor read
    delay(2000);                     // DHT11 warm-up
    readSensor();

    // Kick off first scroll
    matrix.displayScroll(displayBuf, PA_LEFT, PA_SCROLL_LEFT, SCROLL_SPEED);

    Serial.println(F("Smart Classroom ready."));
}

void loop() {
    // Update display on each scroll completion
    if (matrix.displayAnimate()) {

        // Time to refresh sensor?
        unsigned long now = millis();
        if (now - lastSensorRead >= SENSOR_INTERVAL) {
            lastSensorRead = now;
            readSensor();
            Serial.print(F("Temp: ")); Serial.print(lastTemp);
            Serial.print(F(" C  Hum: ")); Serial.print(lastHum);
            Serial.println(F(" %"));
        }

        // Restart the scroll with the (possibly updated) buffer
        matrix.displayScroll(displayBuf, PA_LEFT, PA_SCROLL_LEFT, SCROLL_SPEED);
    }
}