How to Build a Personal Smart Thermostat with ESP8266

Old ones don’t learn. Smart ones phone home to some cloud server. If you’re privacy-conscious or just want full control, that’s a problem.

So I built my own Wi-Fi thermostat — no Alexa, no vendor app, no cloud. Just an ESP8266 microcontroller, a DHT sensor, a web interface, and full local control. It logs temperature, controls heating or cooling, and runs on 5V USB. All in a weekend.

Environment Setup

Hardware

ESP8266 NodeMCU (cheap, Wi-Fi-enabled microcontroller)
DHT22 temperature + humidity sensor
IR LED (for dumb ACs) or Relay Module (for heaters/HVAC)
5V USB power supply
Breadboard + jumper wires

Software

Arduino IDE
ESP8266 board package
DHTlib (for sensor reading)
LittleFS (for storing logs and UI)
Optional: WiFiManager for OTA updates and fallback config mode

Tested on both Windows and Ubuntu. No extra dependencies beyond Arduino defaults.

Main Content

Wiring

DHT22:
- Data → D4
- VCC → 3.3V
- GND → GND
IR LED or Relay Module:
- Control pin → D5
ESP8266:
- Powered via micro USB

Firmware Logic

Reads temperature and humidity every 5 seconds
Hosts a local web server on 192.168.x.x
UI lets you:
- View current temperature
- Adjust a slider for target temp
- Trigger manual override
Control logic:
- If current < target: turn heater ON
- If current > target: turn heater OFF or start cooling
Logs all data to onboard LittleFS
Optional: Serve data as JSON API

Web UI

Built-in HTML served directly from the ESP8266.

Includes:

Temperature readout
Setpoint slider (saved in flash)
Manual override buttons
Lightweight, no frameworks or CDN dependencies
Mobile responsive

Soft AP Fallback

If Wi-Fi config fails, it starts its own Wi-Fi AP
Connect via your phone and enter new credentials
Works on any 2.4GHz network

Modes of Operation

IR Mode

Use IRremoteESP8266 library
Send NEC codes to dumb ACs
Can mimic original remote with no extra hardware

Relay Mode

Ideal for electric heaters or HVAC plugs
Simple ON/OFF control via GPIO

Testing Results

Stable temperature logging every 5 seconds
UI loads in under 1 second
DHT22 is accurate within ±0.5°C
Relay and IR response within ~3 seconds

Use Cases

Home automation (custom climate control)
Server closet thermal management
Climate-controlled RVs or storage
Off-grid solar heating controller

Best Practices

Store setpoint in flash only when changed
Debounce temperature readings to avoid false triggers
Rotate logs weekly to prevent storage overflow
Secure the UI with basic auth or PIN access
Limit Wi-Fi reconnect attempts to save power
Enable OTA updates for easier debugging
Use a heat-resistant enclosure for safety
Add a physical reset button for config wipe
Power via UPS if connected to heating appliances

Conclusion

This DIY smart thermostat checks all the boxes:

No cloud
Local logging
Instant web UI
Works in any Wi-Fi environment
Fully hackable

Whether you’re building for your home, lab, or van — it works. And no subscriptions, ever.

Want to expand it? Hook it into MQTT for smart home automation, or add ML logic to auto-learn your schedule. For now, this setup gives you all the control with none of the vendor noise.

If your thermostat sucks or talks to third parties — build your own.