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Build a Smart Thermostat with ESP32, Home Assistant, and Relays

Build a Smart Thermostat with ESP32 and Home Assistant Using Relay and Sensors

Ever wanted to ditch your clunky old thermostat for a DIY smart version that integrates seamlessly with Home Assistant? I did—and after a few weekends of tinkering (and a couple of fried relays), I built a reliable ESP32-based smart thermostat. Here’s how you can too.

Why Build a DIY Smart Thermostat?

Commercial smart thermostats are great, but they often come with limitations: proprietary ecosystems, cloud dependencies, or lack of customization. With an ESP32, a relay, and a temperature sensor, you can build a thermostat that’s:

  • Local-first: No cloud required.
  • Fully customizable: Tweak the logic to fit your needs.
  • Cheap: Under $30 for most setups.

What You’ll Need

Hardware

  1. ESP32 Dev Board (like this one) – The brains of the operation.
  2. DS18B20 Temperature Sensor (or DHT22) – For accurate temperature readings.
  3. 5V Relay Module (single-channel) – To control your HVAC system.
  4. Breadboard & Jumper Wires – For prototyping.
  5. USB Cable – To flash the ESP32.
  6. Resistors (4.7KΩ for DS18B20, 10KΩ for DHT22).

Software

  1. Home Assistant – For automation and dashboards.
  2. ESPHome – To program the ESP32 easily.
  3. PlatformIO or Arduino IDE – Optional if you prefer manual coding.

Step 1: Wiring the Hardware

Circuit Diagram

ESP32 thermostat wiring diagram

  1. Connect the Temperature Sensor:

    • DS18B20:
      • VCC → ESP32 3.3V
      • GND → ESP32 GND
      • DATA → GPIO4 (with 4.7KΩ pull-up resistor)
    • DHT22:
      • Similar, but use a 10KΩ resistor.

  2. Connect the Relay:

    • IN → GPIO5 (or any free pin)
    • VCC → ESP32 5V
    • GND → ESP32 GND

  3. Power the ESP32 via USB (or a 5V power supply if deployed).

Step 2: Flashing the ESP32 with ESPHome

ESPHome makes this ridiculously easy. Here’s the configuration:

# thermostat.yaml
esphome:
  name: esp32-thermostat
  platform: ESP32
  board: esp32dev

wifi:
  ssid: "YOUR_WIFI_SSID"
  password: "YOUR_WIFI_PASSWORD"

# Enable logging and OTA updates
logger:
ota:

# DS18B20 Sensor
sensor:
  - platform: dallas
    address: 0xYOUR_SENSOR_ADDRESS  # Run `esphome logs` to find this
    name: "Living Room Temperature"
    accuracy_decimals: 1

# Relay Output
switch:
  - platform: gpio
    pin: GPIO5
    name: "HVAC Relay"
    id: hvac_relay

# Thermostat Logic (PID Control)
climate:
  - platform: thermostat
    name: "Living Room Thermostat"
    sensor: living_room_temperature
    default_target_temperature: 21.0
    heat_output: hvac_relay

Flash it via USB:

esphome run thermostat.yaml

Step 3: Integrating with Home Assistant

  1. Auto-Discovery: If your Home Assistant is on the same network, the thermostat should appear automatically.

  2. Manual Setup (if needed):

    # configuration.yaml
climate:
  - platform: mqtt
    name: "Living Room Thermostat"
    current_temperature_topic: "esp32-thermostat/sensor/temperature/state"
    temperature_command_topic: "esp32-thermostat/climate/thermostat/target_temperature"

  3. Create a Dashboard:

    • Use Lovelace to add a thermostat card.
    • Set up automations (e.g., turn off when no one’s home).

Troubleshooting Tips

Problem: Temperature Readings Are Wrong

  • Fix: Check the sensor wiring. DS18B20s are fussy about pull-up resistors.

Problem: Relay Doesn’t Toggle

  • Fix: Ensure the ESP32’s GPIO pin matches your YAML config. Test with a simple switch.turn_on service call.

Problem: Home Assistant Doesn’t See the Device

  • Fix: Restart ESPHome and check your Wi-Fi credentials.

Going Further

  1. Add More Sensors: Humidity (DHT22) or motion detection (PIR) for occupancy-based control.
  2. Multi-Zone Control: Use multiple ESP32s for different rooms.
  3. Energy Monitoring: Integrate a smart plug to track HVAC usage.

FAQ

Q: Can I use a different microcontroller like ESP8266?

A: Yes, but ESP32 is recommended for better stability and GPIO options.

Q: How do I calibrate the temperature sensor?

A: Use a known-good thermometer and adjust offsets in ESPHome.

Q: Is this safe for my HVAC system?

A: Yes, but double-check relay ratings (most handle 10A at 240V).

That’s it! Now you’ve got a DIY smart thermostat that’s as powerful as a Nest but without the subscription fees. For more ESP32 projects, check out my guide on monitoring home temperature with ESP32 and Xiaomi sensors.