Circuit Sauna · rev A.1 · 2026

The enclosure heater that proves it's working.

Field electronics are rated to −40 °F (−40 °C); winters aren't. Circuit Sauna keeps cold-exposed boards inside their operating window on 12/24 V DC — and reports temperature, duty cycle, and faults over WiFi/MQTT, so a dead heater is a 2 am alert instead of a spring-time discovery.

12–24 V DCrated for charging battery banks, to 31 V
20–60 Wplug-in, field-replaceable nichrome element
MQTTtemp · current · duty · element health · faults

The gap

Every incumbent enclosure heater — STEGO, DBK, the panel-shop house brands, including STEGO's low-voltage DC line — is thermostat-only: no telemetry, no fault reporting, no way to know it's dead until the site is. Circuit Sauna measures its own heater current, so a burned-out element, a stuck switch, or a detached probe is a distinct alarm within minutes, not a spring-time discovery. For unattended DC sites, proof of life is the product.

The board

Two-layer, 100×70 mm, all through-hole, hand-serviceable: ESP32-DevKitC socketed on headers (swaps in ten seconds), all field wiring on pluggable terminals along one edge — element swaps at −4 °F (−20 °C) are a connector, not a screwdriver. IRLB8721 logic-level MOSFET switches the element at 1 Hz time-proportioning; remote TMP36 probe with open-lead and detached-probe detection; current-sense shunt on the return leg.

12–24V ─[fuse]─┬───[element]──[85°C stat]──[104°C TCO]── IRLB8721 ─[shunt]─ GND
               ├─[TVS]                        gate ◄─┬─ GPIO26 (firmware)
               │                                     └─[5°C stat + zener] ← boot
               └─[schottky]─[wide-in buck 3.3V]─ ESP32 · GPIO34◄TMP36 GPIO35◄shunt

Four independent safety layers

1. A 185 °F (85 °C) bimetal cutout on the element mount — works with the MCU dead or mid-flash. 2. A one-shot 219 °F (104 °C) thermal fuse that latches open on true runaway: a failed switch means safe-and-dead (and reported), never weeks of oscillating heat. 3. A 41 °F (5 °C) thermostat on the MOSFET's gate: below 41 °F the element heats open-loop even when it's too cold for the ESP32 to boot — the classic smart-heater failure is that the brain freezes before it can turn the heat on — yet the contacts carry microamps and a live firmware always retains authority. 4. Watchdog, sensor-sanity window, 80% duty ceiling, current-based fault discrimination — every fault published over MQTT.

Sizing honesty: 40 W holds roughly +32 °F (+18 °C) over ambient on a small bare plastic cabinet (insulated cabinets need less — run the model). Deep-cold sites get the 60 W SKU. A 24 V element dissipates ~58 W at 29 V charge voltage; the design sizes for that, not the nominal.

Economics

Launch price $129. Loaded COGS $32.81 at qty 100 — 75% gross margin. One avoided truck roll to a dead site pays for the unit.

QtyPartsLaborLoaded COGSGM @ $129
10$33.30$14.58$49.3262%
100$23.10$8.75$32.8175%
1,000$16.39$4.67$21.6983%

Status & roadmap

Design rev A.1 (July 2026), generalized from a unit already deployed and surviving winters, then hardened by a 20-agent adversarial design review (16 confirmed findings, all fixed in A.1). Next: KiCad layout → −40 ° cold-chamber test (F and C agree down there) of the bootstrap sequence → three pilot units in real off-grid sites this coming winter. Mains variants and UL listing follow revenue; hazardous-location ratings are explicitly out of scope for now.