Build guide: prototype 0 (Solo Starter)

This guide walks you through building the first Open Bed Climate prototype: the Solo Starter kit, one zone, three TECs, Tier 0 (off-the-shelf modules only, no custom PCB, no mains wiring).

⚠️ Nobody has built this yet. Open Bed Climate is in the design phase. Every number in this guide comes from the design documents and the thermal model, not from a working unit. Where the docs leave something open, the guide says so. You are the person who finds out what really happens. Please write it down and submit a build report.

Safety first

⚠️ Read SAFETY.md before you buy anything. This build combines water, ~250 W of DC power and, in stage B, a sleeping person.

• You never wire mains. Both supplies are certified plug-in bricks with an IEC inlet. Everything you wire is ≤ 24 V DC.
• Hardware cutoffs are not optional. Bimetal cutoffs on every cold plate and heatsink drop the zone relay even if all software fails. You will test them.
• Never leave the build running unattended until it has passed the full commissioning checklist (step 7). Stage A exists so that you find problems on a bench, not in a bed.
• Have the electrical part reviewed by a qualified person before you sleep on it.
• The safety limits (43 °C max setpoint, 45 °C water trip, 75 °C hot-side trip, flow interlock, 10 s heartbeat timeout) are compiled constants in the firmware. This guide never asks you to change them, and neither should you.

Who this is for

A careful hobbyist who can:

• solder through-hole parts and wires to a module (Pico 2, ADS1115 breakout, relay terminals),
• use a multimeter for voltage, resistance and continuity,
• read a simple schematic and follow a pin table,
• install software from a web page or run a few terminal commands.

You do not need to write code, design a PCB or touch mains wiring.

Two stages

Stage	What	Thermal load	Goal
A: bench	Steps 1–7	a tube coil in a ~10 L insulated water bath instead of a mat	prove that every safety function works, and measure the numbers the model only estimates
B: bed	Step 8	the water mat on the bed	first night with conservative limits

Do not start stage B until every checkbox in stage A is ticked.

Steps and time

Times are hands-on estimates for a first build. Waiting time (the 24 h leak test, long measurement runs) is listed separately because you don’t need to work during it, but you must stay nearby whenever TECs are powered.

#	Step	Hands-on	Waiting
1	Tools and safety	1 h	—
2	Control board: Pico 2, ADS1115, NTC dividers, RS-485, relay drivers, fail-safe inputs	5–7 h	—
3	Power path: 24 V brick → DPS5020 → fuse → zone relay → polarity relay → TECs	3–4 h	—
4	Thermal stack: cold plates, TECs, heatsinks, cutoffs, NTCs	3–4 h	paste cure per datasheet
5	Water loop: reservoir, pump, flow sensor, bench bath, leak test	3 h	24 h leak test
6	Software: flash the Pico 2, install the hub, adopt in Home Assistant	1–2 h	—
7	Bench commissioning and measurements	4–6 h	1–2 days of attended measurement runs
8	Bed stage	3–4 h	24 h mat leak test, first night
	Total	≈ 23–31 h	≈ 4–5 days elapsed

Parts

The shopping list is hardware/shopping/solo-starter.md. This guide refers to parts by their role id from hardware/parts.toml (for example tec_drive, thermal_cutoff), because the role says what a part must do and the examples change. Small parts without a role (resistors, transistors, fittings, wire) are named generically and marked (no role yet).

Roles used in this build:

Module	Role ids
Power	tec_psu (24 V, 280 W), aux_psu (12 V, 80 W), tec_drive (DPS5020), polarity_relay (zone relay + DPDT polarity relay + 20 A fuse)
Control	control_mcu (Pico 2), hub_controller (ESP32-S3), temp_sensors, thermal_cutoff, leak_sensor
Thermal	tec_module ×3, cold_plate ×3, heatsink, pump, flow_sensor, reservoir
Water mat (stage B)	water_mat, quick_disconnect
Sensing (optional, later)	bcg_sensing (Solo Starter default: load cells). Not covered in this guide: it doesn’t affect the thermal or safety path.

Reference documents

The guide follows these documents. If the guide and a document disagree, the document wins, and please open an issue.

• hardware/WIRING.md: pin map, bring-up order (written for Duo Standard; this guide adapts it to one zone)
• firmware/opod-pico2/src/board.rs: pin map source of truth, kit constants
• docs/design/01-thermal-fluidics.md, 02-electrical-safety.md, 04-firmware-protocol.md
• hub/esphome/README.md: what the hub shows in Home Assistant

Known gaps (read before you start)

These are open in the design and affect a Solo build. Each step repeats the relevant one.

1. Use the Solo Starter firmware. The control firmware is built per kit. The solo-starter build sets 3 TECs × 5.0 A and makes zone 1 inert (outputs held low, no Modbus traffic, no faults). Another kit’s build would set the DPS to 18 A, which is 6 A per TEC on your 3 TECs. The hub image still has both sides: hide the “Right” entities in Home Assistant (step 6).
2. One heatsink sensor in software. The firmware reads one water NTC and one hot-side NTC per zone; extra sensors per cold plate are planned (temp_sensors role). The bimetal cutoffs on every plate and heatsink are your per-TEC protection.
3. Unverified parts. The DPS5020 register map, current scaling and output ripple are unverified on real hardware. The reservoir level sensor has no tested example yet. Thermal-cutoff examples are listings without UL. You will check these on the bench.