Step 1: Tools and safety

Goal: a safe workspace, the right tools, and a clear picture of what can go wrong before you touch a part.

Time: about 1 h.

Tools

Tool	Used for	Notes
Soldering iron (temperature-controlled) + solder + flux	control board, wire joints
Multimeter with continuity beep and a 10 A range	every checkpoint	a second meter helps: one on volts, one on amps
Wire stripper, side cutters, crimp tool for blade/ring/ferrule terminals	power path	crimp, don’t just solder, anything that carries TEC current
Small screwdrivers, hex keys	relay terminals, heatsink clamps
Hot-air gun (or a hair dryer that gets hot enough)	cutoff test (> 50 °C on a cold plate)
Contact or probe thermometer (thermocouple)	cutoff test, NTC calibration, bath reference	independent of the build’s own sensors
Kitchen scale	measuring the bath water mass
Optional: waterproof DS18B20 probe on the hub	logs the bench bath temperature in Home Assistant	or read your reference thermometer by hand
Measuring jug (1 L) + stopwatch	flow-rate check
Plug-in mains power meter	PSU input power	the only thing you plug between the wall and the bricks
Paper towels	leak test	lots
Zip ties, insulating tape, heat-shrink
Optional: debug probe (a second Pico running debugprobe)	firmware log over SWD	without it you rely on the hub telemetry
Optional: signal generator or a spare microcontroller	simulated flow pulses in step 7	a spare Pico/Arduino with a 10-line blink sketch works

Workspace

1. A table you can get wet: tile floor, or the whole bench setup inside a large plastic tray (a storage-box lid or shower tray) that holds the whole loop volume with margin (the loop holds ~1–2 L; a tray of ≥ 5 L is plenty).
2. Electronics above the water level and away from the plumbing. Plan the layout now: the wet side (reservoir, pump, cold plates, bath) and the dry side (PSUs, DPS, relays, control board) with the drip tray under the wet side.
3. One mains power strip with a switch within reach. This is your emergency stop: it removes power from both bricks at once.
4. A smoke detector in the room, and a CO₂ or powder fire extinguisher for electrical fires nearby.

What can go wrong

Hazard	How it happens	What protects you	You verify it in
Burn (stage B)	water too hot for prolonged skin contact	43 °C setpoint cap, 45 °C trip, 50 °C bimetal on each cold plate	step 7
Fire / overheating	TEC hot side not cooled (fan stopped, heatsink loose), overcurrent	75 °C firmware trip, 80 °C bimetal per heatsink, 20 A fuse, PSU overcurrent protection	step 7
Flooding	loose fitting, cracked tube, coupling that doesn’t seal	leak sensor → TECs and pump off; drip tray; 24 h leak test	step 5
Pump stall / air lock	dry pump, kinked tube	flow interlock (no TEC current below 0.15 L/min, NO_FLOW after 20 s), level sensor	step 7
Runaway driver	DPS misprogrammed or failed: up to ~21 V on TECs rated 14.4 V	zone relay (opens on any cutoff), fuse, PSU OCP	step 3, step 7
Hub crash	Wi-Fi/ESP32 hangs	control board goes safe 10 s after the last heartbeat	step 7
Control board crash	firmware bug, brown-out	1 s hardware watchdog; external 4.7 kΩ pull-downs (≤ 8.2 kΩ, erratum E9) keep every output off	step 2, step 7
Condensation	water below room dew point	insulation, drip tray, dew-point guard on the hub	step 4, step 8

Rules for the whole build

• ⚠️ Unplug both bricks before you change any wiring. The DPS keeps its input capacitors charged for a few seconds; wait, then measure 0 V before you touch the TEC path.
• ⚠️ Never power a TEC without its heatsink and without water flowing on its cold plate. A bare TEC destroys itself in seconds and can reach temperatures that burn skin.
• ⚠️ Never bypass a thermal cutoff with a jumper “just for a test”. If one is in the way, the test is wrong.
• Keep a build log: date, what you did, what you measured. Photos of every wiring stage. The log is the raw material for your build report, and it is how other people learn from your build.

✅ Checkpoint

• I have read SAFETY.md and know which limits I must not change.
• The wet side stands in a tray that holds ≥ 5 L, and the dry side sits above it.
• A switched power strip is my emergency stop.
• Multimeter works (continuity beep tested, 10 A range fuse intact).