BedCooler

BedCooler

A dual-zone heating and cooling controller for a bed: a 6-layer KiCad board with an STM32G474 real-time controller and a Raspberry Pi Compute Module 5 host.

3D render from the board file. The Compute Module 5 itself has no 3D model; its two connectors sit along the right edge.

What it does

BedCooler regulates body temperature during sleep. Each side of the bed has its own pair of Peltier (thermoelectric) modules that can heat or cool, so two people can each choose their own temperature. Pumps, valves and fans move the heat; thermistors, a reservoir float switch and a water sensor report back. A touchscreen sets targets and schedules, and the microcontroller keeps each side on target.

I built it and use it every night. It pairs with my Slim Sleep Tracker wearable.

Block diagram as SVG. The full presentation package (overview, block diagram, firmware architecture, board renders and schematic pages) is in docs/bedcooler_package.pdf.

Key specifications

Values come from the design files (schematic, board and firmware), not from measurements.

Repository layout

hardware/    KiCad 9 project (schematic, routed board), footprint and symbol libraries,
             3D models (packages3d/), production files (Gerbers, IPC netlist, BOM and placement)
firmware/    bedcoolerstm2: STM32CubeIDE project for the STM32G474
docs/        presentation package PDF, diagrams and renders, an older schematic print

Hardware

Open hardware/BedCooler.kicad_pro in KiCad 9. Schematic sheets:

The board was laid out for the JLCPCB 6-layer stackup JLC06162H-3313, with impedance-controlled traces for the high-speed pairs. The Gerbers are in hardware/production/test_bedcooler.zip, and the fabrication-toolkit archive in hardware/production/backups/ also holds the BOM and placement files. The Gerbers are dated 2025-08-13, before the last board edits, so regenerate them from the board file before ordering.

More views: one-page overview, top render, signal map, functional layout.

The 3D model for the Compute Module 5 is not included; the board references packages3d/471511051.stp, which can be added from the Raspberry Pi CM5 IO board KiCad files.

Firmware

firmware/bedcoolerstm2 is an STM32CubeIDE project (HAL, CubeMX .ioc included).

• The STM32 talks to the Compute Module 5 over USART1 at 115200 baud, 8-N-1, newline-terminated text commands.
• Every 1 s it runs the control loop for each zone. In AUTO mode a zone switches fully to heat or cool with 0.5 °C hysteresis around the target on its thermistor. Manual heat or cool is time-proportional over a 10 s period.
• A safety cutoff turns a zone off above 38.0 °C (safety on|off).
• A fault from an H-bridge driver turns that zone off and sends an alert.
• Fan speed is set by 25 kHz PWM; RPM is read from the tach signal.

Build and flash

1. Install STM32CubeIDE.
2. File > Import > General > Existing Projects into Workspace, and select firmware/bedcoolerstm2.
3. Project > Build All. The Debug build uses about 82 KB of flash.
4. Connect an ST-Link to the SWD header J16 (SWDIO, SWCLK, NRST) and choose Run As > STM32 C/C++ Application.

Open items to verify

• Zone 1 enable: the firmware drives PA5, but the netlist connects the zone 1 driver's EN pin (U4) to PA9.
• Input FETs Q18 and Q19: the source is on the input pad and the drain on +12V; check the behaviour with a reversed supply.
• Fault and fan-tach interrupts: the callbacks exist, but the interrupts are not enabled in this source.
• USB DFU enumerates, but its flash erase and write functions are stubs; use SWD to program.
• In the board file the two Peltier- posts carry nets that reach no other copper; check how the posts are wired on the built board.
• Outer copper weight: the board file stackup says 1 oz; the earlier README listed 2 oz.

Safety

This runs at 12 V and about 350 W, with water circulating near the electronics. Fuse the supply, keep the board away from the water loop, and test the leak and float sensors before leaving it running unattended.

License

CC BY-NC-SA 4.0: Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International. You may share and adapt this work for non-commercial purposes with attribution, under the same license. The ST HAL drivers and USB middleware in firmware/ keep their own ST licenses.