RoomOne: one circuit board of our own

What changed (3 Nov 2026). The rev A bench thermostat is built from bought boards joined by leads: a XIAO ESP32-S3, a SEN66 air module on a cable, an LD2410C radar module, an e-paper board, an ATECC608 breakout, a Pololu regulator and NeoPixel stick, all on a carrier board. RoomOne now gets one circuit board of our own, like CricketOne and CellOne, with every chip placed straight onto it. An assembly house makes and tests it.

• The chips: the ESP32-S3 certified radio module (soldered by its edge pads), an Infineon 60 GHz radar chip with its antennas built in, a Bosch particle sensor with no fan, Sensirion CO2, gas and humidity chips, the e-paper drive, two key switches, eight call-light LEDs, a sounder, three relays, the 24 VAC supply, the security chip and the tamper switch.
• The shape: the thermostat is flat, so this board is one rigid 128 × 98 mm plate that fills the body. Side A faces the room: everything that sees, lights, sounds or is pressed through the front face. Side B faces the wall: the radio, relays, supply and the wires in.
• An air corner. A 1 mm slot cut in an L shape keeps the temperature, humidity, gas and particle chips away from the board’s own heat.
• The rev A bench thermostat stays as it is. It proves the calls and the software, which move to this board unchanged (same ESP32-S3 family).

The board is code (hardware/pcb/roomone-board.mjs). It writes the KiCad board file (hardware/pcb/roomone-board.kicad_pcb), the parts list (roomone-board-parts.csv) and this drawing, and checks every part:

• every part sits on the board with a 0.3 mm edge margin, clear of the air-corner slot, and no two parts overlap on the same side;
• every part’s height fits its side: 6.5 mm on the room side, 16.4 mm on the wall side;
• the key switches, radar chip, sounder, particle sensor, e-paper panel and call-light LEDs each sit on their own window in the thermostat CAD (within 1 mm);
• the air chips are inside the slotted corner, and the temperature chip is at least 30 mm from every warm part (47 mm in this layout);
• the radio module’s antenna end is at the board’s top edge, at least 10 mm from the e-paper panel;
• the wire terminal sits inside the wall plate’s 40 × 30 mm wire opening.

KiCad design rule check: 0 violations, 0 unconnected items (KiCad 10, 3 Nov 2026). Each part is drawn at its real size, as a placeholder outline until its full footprint is added.

Sizes come from the thermostat CAD (cad/roomone-cad.mjs, rev A: 140 × 110 × 32 mm, 135 × 105 inside, front face 25.5 mm in front of the wall plate) and the chip datasheets. Numbers marked est. are estimates.

1. The shape: one flat board, filling the thermostat

Why this shape: a guest-room thermostat is a flat box on the wall. One flat board can reach every window from one side and every wire from the other, so it needs no flex and no second board.

Three changes to the thermostat CAD (made in rev T, 3 Nov 2026; nothing is bought for them):

1. A 6 mm particle window with a clear cover in the front face at x 52, z 1,125, over the particle sensor. The SEN66’s side vents stay for the air corner.
2. A rib on the wall plate (6 × 3 mm, 10.1 mm deep, at x 55, z 1,107) that presses the tamper switch on the board’s back: its face stops 0.3 mm short of the switch body, and the plunger stands above the body (operating position 7.4 ± 0.2 mm over the mounting face, Omron D2F datasheet). Taking the thermostat off the plate opens it.
3. The radar window’s thickness is set for 60 GHz (about 1.5 mm PC-ABS in place of 1.0 mm, est., half a wavelength in the plastic; to be measured).

2. What is on the board

The S1 sensor core (radio module, sensors, power block, security chip and software), in RoomOne’s shape. Only what is different for a guest room is described after the table.

What is different for a guest room:

• Radar on the board. The LD2410C was a module on pins. The BGT60TR13C has its antennas inside its package, so it solders straight on a normal board and sees breathing through the front window. It is still a radar: no camera and no microphone in a sleeping room.
• Air chips on the board. The SEN66 was a module on a cable. Its jobs are split over four chips soldered on the board: particles (BMV080), CO2 (SCD41), VOC and NOx (SGP41), humidity and temperature (SHT45). The smoke-and-vape classifier was trained on SEN66 data, so it is re-checked on these chips (see “Still to check”).
• HVAC relays and 24 VAC stay as in the bench design, now on the same board, with the wire terminal behind the wall plate’s opening.

Sale boards run from the ESP32-S3’s own flash, update over Wi-Fi with two system copies (a bad update rolls back by itself), and never update during a guest’s stay at night.

3. Every connection, checked

Our rule: every pair of parts that join is checked against both makers’ drawings and a search for known problems. No “same as CricketOne” claims.

No adapters, no plug-in modules and no cables inside.

4. Heat

• The whole board draws about 0.6–0.9 W (est., section 5). A plain thermostat draws about 0.3–1 W.
• The problem is not cooling, it is reading. A thermostat must read the room, not itself. The radio module, the supply and the relays are kept on the top and left of the board, the air chips in the bottom-right corner behind the slot, 47 mm from the nearest warm part.
• The body’s bottom vents feed the air corner first; warm air leaves at the top vents, past the radio module.
• Rev A logs its own board temperature beside the room temperature; the bench test compares the SHT45 with a reference thermometer in still air with the radio busy.

5. Power budget (est.)

This is about half the bench thermostat’s 1.3–1.8 W, mostly from the fanless particle sensor and the radar chip in place of the SEN66 fan and the radar module. The first rev A thermostats on a real PTAC replace these estimates.

6. Factory test (every board, every time)

1. Current-limited first power-on (100 mA). A short trips in about 1 s, before anything heats.
2. Every rail within ±3 %; the 24 VAC input at 20 and 30 VAC; a wire on the wrong terminal does no harm (fuse and suppressor).
3. Our small loader flashed over the USB pads; our microSD card in, then boot: the loader starts our program from the card (the card socket added by the design review, 4 Nov 2026).
4. Radar: a moving plate at 1 m and 3 m. Air chips: a clean-air reading in the fixture and one puff of test aerosol (est.). Keys pressed by the fixture’s pins. Each LED colour read by a light sensor; the sounder’s level.
5. Each relay switched with a 24 VAC load; the tamper switch opened and closed.
6. Wi-Fi to the fixture. The unit’s own key goes into the security chip, the ID chip is written and the label is printed. Every result is stored against the serial number.

For each rev A batch: radio pre-scan (FCC Part 15, and the 60 GHz radar rule), 24-hour soak, and UL 60730-2-9 pre-checks.

7. Who makes what

8. Cost per thermostat, electronics (est.)

The CO2 and particle chips are the largest costs (about $20–25 together at volume, est.). The bench parts for the same jobs cost about $146 at qty 1 (bom.csv). From about 1,000 boards the single board costs less than half and needs no hand wiring.

9. Timeline

10. Still to check

1. The BMV080’s flex tail: hot-bar soldered or Bosch’s latched connector, and its window, cover and clear-view rules, from Bosch’s datasheet and integration guide.
2. Whether the BMV080, SGP41 and SCD41 together see vape aerosol and smoke as clearly as the SEN66, and still tell steam and hair spray apart (side-by-side test on the bench thermostat).
3. The 60 GHz radar rule for a wall device in each market (FCC Part 15.255 in the US, the EU short-range device rules), and breathing detection at 5 m through the 1.5 mm rev T window.
4. The radio module’s range with its antenna facing the wall plate inside the body, from a corridor access point.
5. The G5V-1 relay contacts (0.5 A at 125 VAC) against the inputs of the PTACs and fan-coil valves we support.
6. The push-in terminal’s maker, wire range and footprint, and the e-paper panel’s tail pad drawing.
7. Temperature reading error with the radio busy, against a reference thermometer.

Sources

• Espressif ESP32-S3-WROOM-1 datasheet (18 × 25.5 × 3.1)
• Infineon BGT60TR13C, 60 GHz radar, antennas in package, 6.5 × 5.0 × 0.9
• Infineon BGT60TR13C shield application note
• Bosch BMV080 datasheet (4.4 × 3.0 × 20 including the flex)
• BMV080 sensor body 4.4 × 3.0 × 3.0 (SparkFun)
• Sensirion SCD41, 10.1 × 10.1 × 6.5, reflow solderable (Arrow)
• Sensirion SGP41
• Sensirion SHT45
• TI LMR36015, 60 V buck
• Microchip ATECC608B
• Omron G5V-1 datasheet
• Omron D2F datasheet
• Omron B3F datasheet
• The thermostat, its sizes and its calls: RoomOne’s design
• The board platform: our own boards
• The camera short that started our board rule: the first stump, section 9

Design review (4 Nov 2026)

Every board is checked by the same rules (products/_shared/board-rules.mjs), one for each lesson any of our boards has taught us. The full review, with every number and its source, is on the board page. It changed this board:

• Added J4, a hinged, latched microSD socket (side B at x −45, y −30): the maker flashes our small loader into the ESP32-S3, and it starts our program from the card (lesson from CricketOne: software on the card, no setup).
• Temperature chip: 47 mm from the nearest warm part; the board’s own warming is measured on rev A against a reference thermometer and calibrated out.
• In the thermostat CAD (rev T, 3 Nov 2026): the board is placed in the thermostat (cad/roomone-t-cad.mjs) and every part is checked in 3D against the body and the wall plate with a 0.3 mm skin inside every wall, windows included. Nothing touches: the closest part is 2.7 mm from the front face and 2.9 mm from the wall plate (the wire terminal). The body now has the 1.5 mm radar window for 60 GHz and the 6 mm particle window, and the wall plate the rib for the tamper switch (the three changes in section 1). Every bought bench part is out; the board does every job.
• Fast lines on a solid ground plane (to prove, lesson: planes, from the CricketOne test-board layout of 4 Nov 2026): USB (90 Ω); antenna feed (50 Ω, keep-out on every layer); card lines (length-matched, no vias under the socket). Ground on the layer next to the parts (inner layer 1, supply on inner layer 2; the whole bottom of a 2-layer section), one via beside each ground and supply pad and none in pads, no copper or vias in the antenna keep-out on any layer or under the card socket, the impedance lines routed first with a test coupon. The quote asks for it; the routed board’s design rule check proves it.

The request for quote is /build/roomone-quote.