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Jonathan Bennett’s 2019 Hackaday project used a Raspberry Pi 3 B+, the original 7-inch Raspberry Pi touchscreen, relay outputs and MCP9808 temperature sensors to create a local touchscreen thermostat.
The idea remains technically possible, but it is not a drop-in, beginner-safe thermostat replacement. The original project is best understood as an educational architecture: a Raspberry Pi reads temperatures, operates isolated switching hardware, records HVAC performance and presents a local control panel. A responsible 2026 version would need modern software, stronger fault handling, secure networking and professional validation of the HVAC interface.
What the original project built
The project solved more than temperature control. It combined a thermostat, a local home-automation dashboard and an HVAC data logger.
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- Heating, cooling, automatic and off modes.
- Temperature readings from multiple locations.
- Historical graphs for room, outdoor and equipment data.
- HVAC duty-cycle tracking to help investigate insulation and efficiency.
- Integration with other house controls, including a garage-door button.
The original hardware consisted of a Raspberry Pi 3 Model B+, the official 7-inch Raspberry Pi display, a SainSmart four-channel mechanical relay module and Adafruit MCP9808 I²C temperature sensors. The installation used a three-gang wall box, 3D-printed mounting hardware and network boot infrastructure.
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- Fully Compatible with Raspberry Pi 5/ 4B / 3B+ / 3B / 3A+ / 2B. (No HDMI port, not compatible with any other device.)
- Supports for Raspbian OS 2 points to zoom the page(old version), for Ubuntu/Kali/Win10 IoT (single-touch only). Support backlight brightness adjustment.
- Easy to use, no configuration required, plug and play (for new and configuration unchanged raspberry pi systems). Instructions was provided.
This is a historical reconstruction, not a current, tested installation guide. The original code, display, operating system assumptions and GPIO libraries date from 2019.
How the thermostat interface works
In a conventional North American low-voltage HVAC system, a thermostat commonly controls calls for heat, cooling and fan by switching a 24-volt control circuit. The commonly encountered labels are:
| Label | Typical function |
|---|---|
| R | 24-volt AC supply |
| W | Heat call |
| Y | Cooling call |
| G | Fan call |
| C | Common connection, when provided |
A basic thermostat can call for heat by closing a contact between the supply and the heat-call circuit. Bennett’s article uses this simplified contact-closure model, with the Raspberry Pi operating relays in place of thermostat contacts.
That description must not be treated as universal wiring advice. Heat pumps, multi-stage equipment, dual-fuel systems, hydronic boilers, millivolt systems, communicating HVAC equipment and zone-control panels may use different control methods. Before replacing an existing thermostat, identify the equipment, control voltage, terminals, stages, required timing and manufacturer-approved interface. A qualified HVAC or electrical professional should validate the design.
Conceptual system architecture
Temperature sensors ── I²C ──> Raspberry Pi
Raspberry Pi GPIO ───────────> Isolated relay interface
Relay contacts ──────────────> HVAC control inputs
Raspberry Pi display link ───> Touchscreen
Local web service ────────────> Chromium kiosk interface
This is an architectural diagram, not a wiring instruction. The relay interface must be suitable for the control voltage and current, electrically isolated where required, enclosed appropriately and designed so that booting, rebooting or software failure cannot create a dangerous operating state.
The I²C mistake that made the project fail
The most useful troubleshooting lesson in the article concerns the I²C bus. The author connected the touchscreen’s I²C-related pins in addition to using its ribbon connection. On the Pi A+ and B+ arrangement described in the article, the display already had an I²C path through the ribbon connection. The extra wiring unintentionally bridged buses.
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- 5-inch 800*480 resolution capacitive touch screen, IPS type, good viewing angle.
- The MIPI DSI interface directly outputs, plug and play, no driver installation required.
- As a touchscreen monitor, compatible with Raspberry Pi 5 / 4B / 3B+ / 3B / 3A+ / 2B / 1B+ / 1A+. (No HDMI. Not compatible with any other devices.)
- Supports for Raspbian OS 2 points to zoom the page(old version), for Ubuntu/Kali/Win10 IoT (single-touch only). Support PWM backlight brightness adjustment.
- Easy to use -> No configuration required (for new and configuration unchanged systems). Provide detailed usage documentation.
The symptoms were unusually revealing:
- The temperature sensor returned 0°C.
- The touchscreen stopped responding.
i2cdetectappeared to find a device at every address.
The documented fix was to use the display ribbon cable and connect only the display power connections appropriate to that hardware. The separate display I²C pins should not also be connected when the dedicated display connection already carries the required signals.
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The original software design
The project split its functions across several lightweight components:
- A Flask service read sensors and controlled relay outputs.
- HTTP endpoints such as
/enable/<pin>,/disable/<pin>and/temp/<sensor>exposed control and readings. - Temperature, humidity, CPU temperature, equipment state, outdoor temperature and duty cycle were stored with RRDTool.
- A PHP/HTML interface displayed controls and graphs.
- Chromium ran the interface fullscreen on the touchscreen.
- Settings were written to a JSON file.
The article shows an MCP9808 at I²C address 0x18, uses the historical smbus and RPi.GPIO stack, and reads the Pi temperature with vcgencmd measure_temp. It also shows GPIO outputs including BCM GPIO 17, 18 and 27.
Those details are useful for understanding the 2019 architecture, but the examples should not be copied unchanged into a permanent 2026 installation. Confirm the Python version, GPIO library, I²C permissions, sensor support and operating-system tools on the selected board. The service should also be supervised rather than launched casually from a terminal.
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Temperature placement matters
An MCP9808 mounted beside a warm Raspberry Pi can measure the computer rather than the room. The original article specifically warns about separating the sensor from the Pi’s heat.
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For a useful room reading, place the sensor away from the processor, display electronics, power regulators, direct sunlight, supply vents and drafts. Multiple sensors also require deliberate address configuration and a plan for disconnected, stale or implausible readings.
Hysteresis prevents rapid switching
The controller runs approximately once per minute. In the heating example, it starts heating when the measured temperature is 2°F below the target and stops when it reaches 2°F above the target. That creates a total four-degree span around the set point, or a ±2°F band.
The article represents the setting with:
"heater-width": 2,
"ac-width": 2The naming can be confusing because the code value and the prose describe the span differently. The important concept is hysteresis: the start threshold and stop threshold are separated so normal sensor noise does not repeatedly toggle a relay.
Hysteresis alone is not complete compressor protection. A modern controller should also consider:
- Minimum compressor-off time.
- Minimum run time.
- Separate heating and cooling deadbands.
- Heat-pump reversing-valve and auxiliary-heat sequencing.
- Safe behavior after a reboot.
- Sensor timeouts and invalid readings.
- Watchdog and service-failure behavior.
A one-minute loop is not proof that one-minute cycling is safe for every HVAC system. Equipment-specific protections and manufacturer requirements take priority.
Relay behavior and failure states
The original software uses active-low relay control: outputs are initialized high and the relevant relay is enabled by driving its GPIO pin low. A modern implementation must verify this behavior rather than assume it.
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Before connecting HVAC equipment, establish:
- Whether the board is active-low or active-high.
- Whether contacts are normally open or normally closed.
- What happens while the Pi is booting.
- What happens if a GPIO becomes an input.
- Whether the contact ratings suit the HVAC control circuit.
- Whether the relay board provides adequate isolation, creepage and suppression.
- What state is selected after a software crash, sensor failure or power restoration.
A generic four-channel relay board is not automatically HVAC-certified. The control electronics should be separated from building wiring, installed in an appropriate enclosure and tested with the equipment disconnected before any live commissioning.
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Why the display cannot simply be swapped
The original project used the first-generation official Raspberry Pi 7-inch touchscreen. That is not the same product as the current Raspberry Pi Touch Display 2.
Touch Display 2 is available in 5-inch and 7-inch versions, uses a 720×1280 display, supports five-finger capacitive touch and connects through GPIO power and a DSI ribbon connection. The listed prices researched for the US market were $40 for the 5-inch version and $60 for the 7-inch version; retailer pricing, tax and availability may differ.
Raspberry Pi 5 requires the appropriate 22-way-to-15-way cable for Touch Display 2, while older boards use the older 15-way arrangement. The display’s portrait orientation, cable routing, wall-mounting requirements and power arrangement also affect whether it suits a thermostat retrofit. A mounting accessory such as the SmartiPi Touch 2 can help with physical mounting, but it is not an electrical enclosure, HVAC interface or code-compliance solution.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Security and reliability gaps in the original design
An HTTP endpoint that enables a relay is convenient for a private experiment but dangerous when exposed beyond a trusted local network. The original architecture should not be placed directly on the public Internet or protected only by an obscure URL.
A current implementation should provide authentication and authorization, validate every input, protect browser actions against CSRF, segment the control device from untrusted networks, avoid port forwarding and record actuator changes. Remote access should use a properly secured mechanism rather than exposing the Flask port.
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- COMPATIBILITY: Specifically designed to work seamlessly with Raspberry Pi boards for easy integration
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File-based JSON settings also need protection against simultaneous writes, partial writes and power loss. A supervised service, atomic state updates, backups and a tested rollback procedure are more appropriate for a permanently installed controller.
What a responsible 2026 build must address
- HVAC compatibility: Confirm that the equipment accepts the proposed contact closures and identify all stages, fan functions, reversing valves and safety interlocks.
- Electrical separation: Use an appropriate isolated interface and enclosure. Do not treat a hobby relay board as installation-grade hardware without verifying its construction and ratings.
- Safe defaults: Define relay states for startup, shutdown, lost sensor, lost network, corrupted storage and service failure.
- Compressor timing: Add minimum-off and minimum-run timers where required by the equipment.
- Sensor validation: Reject missing, stale or implausible readings instead of running heat or cooling indefinitely.
- Local operation: The thermostat should remain usable without Internet access and, ideally, without network boot infrastructure.
- Thermal management: Keep sensors away from Pi heat and ensure the wall enclosure does not trap excessive heat.
- Recovery: Provide a fast way to restore a conventional or certified thermostat if the Pi fails.
- Testing: Test each relay, sensor, reboot state and fault condition on a bench before connecting the HVAC system.
RRDTool and modern data choices
RRDTool was a sensible choice for the original project. It stores recent measurements at high resolution and older data at progressively reduced resolution, making it useful for long-running graphs without unlimited storage growth.
The project records more than room temperature: humidity, CPU temperature, outdoor temperature, heater state, air-conditioner state and HVAC duty cycle. Keeping both measurements and actuator state is important because it lets the owner examine runtime, overshoot and possible insulation problems.
RRDTool is not mandatory today. A small installation might use SQLite or an append-only log; a larger home-automation system might use its own recorder or a time-series database. The choice matters less than retaining reliable timestamps, sensor validity and equipment state.
Should you reproduce, modernize or avoid the design?
| Choice | Best fit | Main concern |
|---|---|---|
| Reproduce the original | Learning GPIO, I²C, Flask, PHP, RRDTool and kiosk interfaces | Old software assumptions and weak production safeguards |
| Modernize it | Builders who want a local custom controller | Requires careful service, security, timing and fault design |
| Use a home-automation platform | Users wanting schedules, dashboards and integrations | The HVAC interface still must be suitable and supported |
| Use a commercial thermostat | Reliable everyday HVAC operation | Less customization and possible vendor dependence |
A custom controller is a poor fit when the equipment uses proprietary communicating controls, high voltage, millivolt signaling or an unidentified interface. It is also a poor fit when there is no safe fallback thermostat, no service access or no way to keep the equipment safe if the Pi stops working.
Practical construction checklist
Safe to prototype on a bench
- Read an MCP9808 and verify its address.
- Build a touchscreen dashboard with simulated relay outputs.
- Log temperatures and simulated equipment states.
- Test hysteresis, minimum-off timing and reboot behavior.
- Run the interface locally without exposing it to the Internet.
Requires HVAC and electrical review
- Identifying the existing thermostat terminals and equipment type.
- Selecting the relay or dedicated HVAC interface.
- Choosing an enclosure, cable routing and wall-box arrangement.
- Verifying control voltage, staging and manufacturer requirements.
Must be tested before connection
- Sensor disconnection and invalid-temperature handling.
- Relay states during startup, shutdown and power restoration.
- Network loss and storage failure.
- Compressor lockout and minimum-cycle timing.
- Manual recovery using the backup thermostat or approved control.
Final verdict
“Hack My House: Raspberry Pi As A Touchscreen Thermostat” is a strong educational project because it connects sensing, GPIO, relays, web software, data logging and building automation in one system. Its most valuable lessons are the I²C wiring failure, the importance of sensor placement and the use of hysteresis to reduce relay chatter.
It should not be treated as a current copy-and-paste thermostat replacement. The original hardware and software are historical, the HTTP control interface is not a security model, and hysteresis does not replace HVAC-specific safety controls. For a 2026 build, reuse the architecture and ideas, but modernize the software, isolate the HVAC interface, add explicit failure handling and have the installation reviewed by someone qualified to work with the equipment and building wiring.
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