DIY Smart Thermostat: Build an AI-Powered Climate Controller for Under $60

Why Build Your Own Smart Thermostat?

Commercial smart thermostats like Nest and Ecobee cost $200–$300 and phone home to company servers. They profile your habits, harvest usage data, and lock you into cloud fees. A DIY unit built on open-source software costs under sixty dollars, learns your schedule with on-device AI, and gives you absolute control of your data. The bill of materials:

• Raspberry Pi Zero 2 W – $15
• 1-Wire DS18B20 waterproof temperature probe – $4
• CJMCU-811 CCS811 CO₂ + TVOC sensor – $8
• KY-019 4-channel relay module – $6
• 24 V AC HVAC transformer – $10
• Breadboard, jumpers, enclosure – $10
• Micro-SD card (≥16 GB) – $8

Total: ~$61. Hardly any soldering is required, and the entire stack fits inside a standard thermostat wall-box.

Before You Touch the Wires

Turn off power at the breaker panel. HVAC control lines carry 24 V AC, low but still dangerous in a confined metal box. Label the existing wires (R, G, Y, W, C) with masking tape before removing the old thermostat.

Wire Up the Hardware

1. Power: Connect the 24 V transformer to R (24 V hot) and C (return). This powers the relay board and charges the Pi through its GPIO 5 V pin, eliminating wall-warts.
2. Relay Outputs:
   • Relay 1 – Y (cooling)
   • Relay 2 – W (heating)
   • Relay 3 – G (fan)
3. Sensors: Plug DS18B20 into 1-Wire via GPIO 4 (data), 3.3 V (power), and GND. Connect CCS811 (I²C SDA/SCL) for air-quality–driven ventilation scheduling.
4. Indicator LEDs: Optional status RGB LED on GPIO 18, 19, 20 shows heat/cold demand wirelessly across the room.

Using a screw-down terminal block keeps everything secure and serviceable. Take photos as you work; they save hours of troubleshooting later.

Flashing the SD Card

Download Raspberry Pi OS Lite (64-bit). Enable SSH by writing an empty file named ssh to the boot partition before first boot. Create a wpa_supplicant.conf with your Wi-Fi credentials; the Pi will connect automatically.

From GPIO to Thermostat Logic

We’ll run Home Assistant Core on the Pi. It takes < 2 GB of RAM yet packs full HASS hardware automation. Install with the official script (curl -sSL https://get.home-assistant.io).

Add sensors to configuration.yaml:

sensor:
  - platform: 1wire
    names:
      28-xxxxxxxxxxxx: living_room_temp
  - platform: template
    sensors:
      ccs811_co2:
        value_template: "{{ states('sensor.ccs811_co2') }}"
        unit_of_measurement: "ppm"

The DS18B20 driver loads automatically; CCS811 needs the adafruit-circuitpython-ccs811 library via pip.

Teaching the AI Your Routine

We attach a lightweight neural net (tinyML PID model) that runs entirely on the Pi Zero. It treats indoor temperature, humidity, and occupancy as inputs and outputs heating/cooling set-points every 30 seconds. Training is fully local; it begins with a conservative schedule and refines itself daily.

1. Capture 7 days of raw data (/config/history).
2. Use hass-exporter to dump a dataset.json with minute-granularity readings.
3. Train the model with TensorFlow Lite’s microcontroller workflow on your laptop (takes 60 seconds).
4. Copy the .tflite file back to the Pi at /config/models/.

Total turnaround: 15 min per week, entirely optional. Skipping the training keeps the thermostat in classic programmable mode but still benefitting from web-based control.

Creating a Hass-Free Construction Model

Under Home Assistant, add a Generic Thermostat card:

climate:
  - platform: generic_thermostat
    name: DIY Smart Thermostat
    heater: switch.relay_heat
    cooler: switch.relay_cool
    fan: switch.relay_fan
    target_sensor: sensor.living_room_temp
    min_temp: 10
    max_temp: 30

You now have a conventional web interface and Google Home / Alexa integration via the default Home Assistant cloud (optional), all without nanoseconds of external telemetry.

Ethernet Backup for Wi-Fi Fails

Add a PoE Splitter ($12) to supply both network and power in one cable. If Wi-Fi glitches during a firmware update, you can still SSH in from the unmanaged switch in your wiring closet.

Air-Quality Aware Ventilation

Pump fresh air only when CO₂ > 1000 ppm. Create an automation:

automation:
  - id: co2_fan_boost
    trigger:
      platform: numeric_state
      entity_id: sensor.ccs811_co2
      above: 1000
    action:
      - service: switch.turn_on
        entity_id: switch.relay_fan

Average energy saved: 8 % according to a 2023 Lawrence Berkeley National Laboratory field study—without breaking privacy.

Voice & Away Mode with In-House AI

Install Rhasspy 3, a fully off-line voice assistant. A cheap USB microphone captures commands. Custom sentences file sentences.ini:

[SetTemperature]
temperature = 20..26
set (living room|) temperature to (
  [the] temperature 
)

Rhasspy returns JSON to Home Assistant; the thermostat adjusts instantly. When your phone’s MAC address leaves Wi-Fi (no cloud ping required), presence detection kicks the system into Eco mode (heat to 63 °F / 17 °C, cooling to 80 °F / 27 °C).

Failsafe & Freezing Protection

Raspberry Pi boot failures should not paralyze heating. A hardware watchdog relay (TC358748) cuts in after 10 min without 3 V3 logic and restores basic faux-stat operation: ALWAYS on heat below 50 °F (10 °C). This part costs $5 and ties to the same enclosure.

Controlling From Anywhere While Staying Private

We avoid opening ports. Home Assistant’s WireGuard add-on gives a single encrypted UDP tunnel that every phone and laptop can use. Configure a split-DNS so you never have to remember IP addresses. Total setup time, five minutes per client.

Hardware Calibration Tips

1. Place DS18B20 in the return-air path, not on an exterior wall.
2. Offset CCS811 CO₂ readings by −50 ppm based on fresh-air streaming test.
3. Wrap all relay terminals with heat-shrink to quiet emi clicks from nearby Wi-Fi access point.
4. Add a 10 k resistor between DS18B20 data line and 3.3 V (required for accurate communication).

Power Budget Analysis

The Pi Zero 2 W idles at 110 mA @ 5 V (0.55 W), relay coils add 250 mA per active relay, and the average total draw across a 24-hour cycle is 2.1 W. Over a year that’s about 18 kWh, or $2.70 at U.S. average rates—one-tenth the idle load of a Nest.

Extra Features for the Curious

• Window Open Alert: Sharp temperature drop + CO₂ drop triggers a push notice.
• Vacation Mode Scheduler: one YAML file enables email report of runtimes; integrate with your email filter to shut HVAC off automatically when IFTTT vacation calendar event ends.
• DS18B20 Spike Detector: ignores sensor glitches (>3 °C in a minute) with a simple median filter in Node-RED.
• In-home Extremes Diary: log all heat-pump starts for you to forward to HVAC service as evidence of efficiency loss after a decade.

Budget vs. Payback

With conservative 10 % heating/cooling savings on a typical 2,000 ft² U.S. house, the unit pays for itself in one heating season. The experience level required: the same as wiring up a ceiling fan.

When Things Go Wrong

1. “No DS18B20 device found” — Check pull-up resistor and connect Pin 4 to 3.3 V (not 5 V!).
2. Relay stuck on — Swap a spare 4-channel module; they are ubiquitous.
3. Overshoot/undershoot 1 °F events — Change kp: 2.0, ki: 0.05, kd: 1.0 in the tinyML model JSON.
4. Corrupt SD after 6 months — Over-provision 1 GB, set fsck.mode=force in /boot/cmdline.txt.