ESP32 Smart Thermostat: Temperature Control and Code

BuildWhat You Will Build

GoalsLearning Objectives

• Wire and test the DHT22 temperature sensor on GPIO4 before connecting the relay circuit.
• Map each signal to a named ESP32 GPIO and keep the code constants readable.
• Control a relay module from GPIO26 using clear threshold and relay-polarity constants instead of hidden magic numbers.
• Use Serial Monitor as an engineering tool, not only as a success message.
• Identify power, wiring, and timing faults using a repeatable test checklist.

PartsComponents List

• PartESP32 DevKit boardDHT22 and relay thermostat controller
• PartDHT22 sensor moduleGPIO4 room temperature input
• PartRelay moduleGPIO26 control for a low-voltage fan
• PartSSD1306 OLED displayOptional setpoint display
• PartBreadboard and wiresPrototype wiring

PartsBill of Materials

🔗Wiring

Wire the DHT22 temperature sensor first, verify readings on GPIO4, then connect the relay module control input on GPIO26 for a separate low-voltage fan or indicator load.

1. 1

   Unplug USB and keep the relay load disconnected.

2. 2

   Connect a 3-pin DHT22 module VCC to 3.3 V, GND to GND, and DATA or OUT to GPIO4.

3. 3

   If you use a bare 4-pin DHT22 sensor, add about a 10 k ohm pull-up resistor from DATA to 3.3 V and leave the NC pin disconnected.

4. 4

   Connect relay VCC to 5 V if your relay module requires it, GND to ESP32 GND, and IN to GPIO26.

5. 5

   Connect only a separate low-voltage test fan or indicator load to the relay contacts during learning.

6. 6

   Power the ESP32 and watch Serial Monitor before connecting any moving fan.

GPIO Mapping

Circuit Explanation

Engineering Explanation

⌨️Code

Copy into Arduino IDE. Install any libraries noted in the component guides first.

#include "DHT.h"

#define DHTPIN 4
#define DHTTYPE DHT22
#define RELAY_PIN 26

const int RELAY_ON = HIGH;   // Change to LOW for an active-LOW relay module.
const int RELAY_OFF = LOW;   // Change to HIGH for an active-LOW relay module.

const float FAN_ON_C = 27.0;
const float FAN_OFF_C = 25.5;
bool fanOn = false;
DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(115200);
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, RELAY_OFF);
  dht.begin();
  Serial.println("Smart thermostat ready");
}

void loop() {
  float tempC = dht.readTemperature();
  if (isnan(tempC)) {
    Serial.println("Temperature read failed");
    digitalWrite(RELAY_PIN, RELAY_OFF);
    delay(2000);
    return;
  }

  if (!fanOn && tempC >= FAN_ON_C) fanOn = true;
  if (fanOn && tempC <= FAN_OFF_C) fanOn = false;

  digitalWrite(RELAY_PIN, fanOn ? RELAY_ON : RELAY_OFF);
  Serial.print("Temp C: "); Serial.print(tempC, 1);
  Serial.print(" | Fan: "); Serial.println(fanOn ? "ON" : "OFF");
  delay(2000);
}

Code Explanation

PhotosBuild Photos

• Photo Breadboard overviewShow the ESP32, module placement, and power rails clearly.
• Photo Close-up wiringCapture each GPIO wire so beginners can compare their build.
• Photo Working outputShow the Serial Monitor, display, robot, pump, or lock state after the code runs.