COLDGUARD · CG-WIFI-A01
Retrofit temperature, door and power monitoring for freezers, chilled cabinets and small cold rooms. Two probes, local alarms, battery-backed recording and Wi-Fi replay connect on-site events with remote oversight.
Place an air probe in the storage zone and a second probe in a thermally buffered reference. Track cabinet temperature, prolonged door opening and supply loss during sales and restocking. Keep the display probe away from the evaporator outlet and heated door frame.
Retrofitting prep refrigerators, upright freezers and ingredient storage cabinets gives kitchen staff local audible alerts and operators a history by device. Chilled ingredients and frozen stock require different high/low limits, recovery hysteresis and opening delays.
Use T1/T2 for two representative positions, such as a warm return-air zone and a shelf away from the cooling unit. A two-probe device provides local observations; larger rooms need additional devices and a temperature-mapping plan for spatial coverage.
Assign a device identity to each asset, compare alarm history and separate store connectivity from cabinet failures. A service team can use repeated probe, door, clock or storage faults to plan maintenance and document corrective action.
Air temperature reacts rapidly when a door opens or defrost starts. A buffered reference responds more slowly. Compare both histories with planned operating windows, and choose delay settings long enough for the normal cycle while preserving alarms for persistent excursions.
Sense the low-voltage output of the monitored adapter before the battery-backed power path. The battery supports the monitor after supply loss. Immediate remote delivery also requires the Wi-Fi router and upstream connection to remain powered; otherwise events are recorded locally for later upload.
Two powered DS18B20 probes use separate data lines on GPIO4 and GPIO5, each with a pull-up to 3.3 V. The digital interface carries a unique ROM identity and CRC-protected readings. Independent channels simplify probe identification and prevent one shared-bus fault from masking the other channel.
T1 can follow cabinet air; T2 can follow a suitable buffered reference or a second cabinet location. A reference container needs compatible food-area materials, controlled fill and secure placement. Its reading is a proxy for thermal response, not a direct measurement of the food core.
The DS18B20 chip specifies −55 to +125°C measurement range and ±0.5°C accuracy from −10 to +85°C. Colder freezer operation needs calibration and an error budget for the complete probe assembly. Apply separate T1/T2 offsets from a traceable comparison at the intended operating points.
Temperature limits use continuous-duration gates and 1°C recovery hysteresis to reduce repeated switching around a threshold. Invalid probe readings create probe-fault alarms rather than ordinary temperature excursions. Alarm raise and recovery edges are captured immediately, alongside periodic telemetry.
The active buzzer is driven through a transistor; a green LED communicates local operation. The push button temporarily mutes audible output without clearing the underlying alarm or stopping data capture. Mute behavior belongs to the local device, so connectivity is not needed for on-site handling.
The firmware example uses frozen-storage limits. A chilled cabinet requires a different compiled alarm profile; high/low limits and timing are defined in the alarm-core source. An operator should not assume that the example frozen profile suits a chilled product or a regulated storage specification.
| Event | Raise condition | Delay / recovery |
|---|---|---|
| High temperature · each probe | > −12°C | 180 s; clear at ≤ −13°C |
| Low temperature · each probe | < −25°C | 180 s; clear at ≥ −24°C |
| Probe fault | Invalid / missing reading | 15 s; valid reading restores |
| Door open | Contact indicates open | 60 s; closing clears |
| Power loss | Adapter sense absent | 3 s; supply restoration clears |
| Low battery | < 3.4 V | 30 s; clear at ≥ 3.5 V |
| Storage / clock fault | Health flag false | Immediate health reporting |
ESP32-S3-DevKitC-1-N8R8 provides the controller, 8 MB flash, 8 MB PSRAM and 2.4 GHz Wi-Fi. DS3231 maintains a local time base; a 3.3 V microSD interface stores pending records. Digital inputs cover door position and adapter supply; ADC measures battery voltage with a divider and calibration coefficient.
Local sensing and alarm evaluation are separated from SD file ownership and HTTP upload. Only the storage task accesses SD. Bounded queues and explicit storage/clock faults make a stalled network or write failure visible without relying on an unbounded retry in the sensing loop.
The monitor observes temperature, door and power events; it does not switch the refrigeration compressor. Bring only SELV low-voltage wiring into the enclosure. Door input uses a passive contact with debounce; route signal cables separately from compressor and defrost power wiring.
| Function | GPIO | Interface |
|---|---|---|
| T1 / T2 | 4 / 5 | 1-Wire · 3.3 V |
| Door contact / supply sense | 6 / 7 | Digital input · low-voltage only |
| Buzzer / status LED / mute | 8 / 9 / 10 | Transistor drive / LED / push button |
| Battery divider | 1 | ADC |
| DS3231 | 17 / 18 | I²C · SDA / SCL |
| microSD | 11 / 12 / 13 / 14 | SPI · CS / MOSI / CLK / MISO |
A 5 V adapter feeds the BQ24074 power-path charger. Its OUT rail feeds the MiniBoost stage, nominally 5.2 V, then a resettable fuse and disconnect switch before the controller 5 V input. A protected 1S 2,000 mAh lithium cell connects to the battery input. Nominal charge current is set to 0.5 A.
The optional charge-control circuit uses two Semitec 103AT-2 NTCs on the battery surface, analog comparators, hysteresis and a power-on supervisor. Its permit chain controls charger CE and is separate from the firmware. Nominal thermal switching points are about 10.2°C and 34.8°C; component tolerance, NTC attachment and thermal gradients belong in the final trip-point calculation.
Battery voltage is reported in volts; it is not a state-of-charge percentage. Estimate runtime from usable cell energy and measured average load: runtime ≈ usable Wh × conversion efficiency / average W. Include Wi-Fi retries, buzzer duty, SD writes, temperature and the low-voltage cutoff; capacity alone does not define a guaranteed backup duration.
| Module | Selected device | Role |
|---|---|---|
| MCU | Espressif ESP32-S3-DevKitC-1-N8R8 | Control, sensing and Wi-Fi |
| Probes | 2 × Adafruit 381 / DS18B20 | Powered waterproof probe assembly |
| Door contact | Adafruit 375 | Passive magnetic contact |
| RTC | Adafruit 3013 / DS3231 | Timestamp & backup coin cell |
| microSD | SparkFun BOB-00544 | 3.3 V SPI storage |
| Charger | Adafruit 4755 / BQ24074 | Power path and charging |
| Boost | Adafruit 4654 / MiniBoost | Nominal 5.2 V output |
| Cell | Adafruit 2011 · 1S 2000 mAh | Protected cell; verify connector polarity |
| Thermal sensing | 2 × Semitec 103AT-2 | Separate charge permit inputs |
Periodic telemetry is captured every 10 seconds; alarm edges are captured between periodic samples. Each record carries device ID, boot counter and sequence number. SD records use CRC and temporary-file rename; grouped storage holds up to 16,384 pending records. At a purely periodic 10-second rate this is about 45.5 hours; alarm edges also consume queue entries.
Upload the oldest pending record first. The server treats the device/boot/sequence tuple as an idempotency key and returns an acknowledgement before the device retires a record. A lost acknowledgement therefore leads to a safe retry. The dashboard separates capture time, server receive time and connectivity freshness.
DS3231 provides timestamps when a valid clock is available, with NTP synchronization after connectivity returns. When a timestamp is unavailable, retain boot/sequence/uptime and mark clock health explicitly. Storage faults and lost-record counters are telemetry fields, so gaps remain visible.
Use HTTPS ingest with an explicit trusted root CA and a per-device token. Empty CA configuration disables HTTPS upload; plain HTTP is restricted to an explicitly enabled lab mode. Provision each device with its own Wi-Fi credentials and identity, and keep real tokens outside downloadable examples.
| Group | Fields | Meaning |
|---|---|---|
| Identity | device_id / boot / seq | Device identity and replay ordering |
| Time | captured_at / uptime_ms | Capture time and local elapsed time |
| Measurements | t1 / t2 / battery_v | Two temperatures and battery volts |
| State | door_open / mains | Door and adapter availability |
| Health | storage_ok / clock_ok / lost_log_records | Storage, time and visible record loss |
| Events | alarms / changes | Active alarms, raise and recovery edges |
A Python/SQLite service stores telemetry and alarm edges, with device registration, latest readings, offline state and per-device history. Operators can inspect both temperatures, door/mains state, battery volts and storage/clock health. Use the history to distinguish a transient opening from a persistent excursion.
Alarm notification is decoupled from ingest through an outbox and worker. Configure an approved email or HTTPS webhook channel, credentials and recipients at deployment. Retries, delivery errors and duplicates are handled independently from receiving device telemetry; remote delivery is checked through provider receipts.
The application listens on loopback by default. Put an authenticated TLS reverse proxy in front of operator access, retain device-token protection on ingest and separate demo data from operational records. Back up SQLite consistently, document restore steps and review device freshness and notification failures during maintenance.
The 270 × 220 × 78 mm main body combines a warm-white lid, graphite base, removable carrier, battery tray and wall bracket. A 3 mm wall/floor baseline, corner fasteners and alignment lip define the housing. The front groups a green status LED, circular mute button and 15-hole buzzer outlet.
The 234 × 184 mm carrier separates controller, charge-control circuit, charger, boost, RTC and SD modules. The battery tray reserves 68 × 44 × 16 mm for the protected cell and sensor leads. Position the controller antenna toward the enclosure edge and leave the reserved RF region free of metal fasteners and wiring.
Four bottom Ø12.5 mm openings serve T1, T2, door contact and 5 V supply through matching cable glands. Mount externally in a dry accessible position, away from washdown, condensation and hot compressor surfaces. Buzzer perforations are open; select another sealing arrangement when the installation requires a rated wet environment.
The rear bracket adds 9 mm depth; including the front button, the outer assembly reaches about 88.5 mm before cable bend space. Use the bracket slots and a suitable mounting surface rather than drilling blindly into a refrigeration cabinet. For ESP32-S3 RF design, keep at least the recommended 15 mm clearance around the module antenna and evaluate the installed signal path near metal cabinets.
Identify the storage range, usual opening duration, defrost cycle and Wi-Fi coverage. Place both probes consistently, mount the door magnet within its specified switching distance and label each cable. Keep cable exits from compromising the door seal; use existing service routes or approved feedthroughs.
Follow the endpoint list for signal, power and common ground. Check battery connector polarity before connecting. During USB flashing disconnect the external 5 V and battery power path. Provision 2.4 GHz Wi-Fi, root CA, device ID/token and calibration offsets, then choose the application alarm profile.
Check both probes against a reference, opening/closing state, audible mute, supply loss and restoration. Interrupt Wi-Fi to observe local recording, restore it and compare sequence continuity in the server. Use the supplied UART recording tools to save timestamps, identity and status for installation handover.
Choose the complete resource bundle or individual system, firmware, software, charge-control and enclosure files. The engineering documents and assembly notes are primarily in Chinese; component names, signal definitions and source files use shared technical identifiers.
SHA-256 3eaf873fc33b9f622494ae92c10ba89cd4a8832e96537ab17822fbbd4c58393cHardware, firmware & monitoring softwareZIP · 1012.6 KB ↓SHA-256 f7019906b22bd6cee20f0bba56fefc76f0ffef95bea90d7bd69a7aabfb94165bCharge-control electronics & KiCad sourcesZIP · 276.1 KB ↓SHA-256 7f3789cef00165a6747bb84aabe2ae89766e4fc6311a3dd7a29b37e4f11b7df4Complete enclosure design & editable CADZIP · 1953.6 KB ↓SHA-256 05ec044768e137f9d154750c40f57908f8848456610f55965ff5d3a09fd0ad08Serial commissioning & recording toolsZIP · 23.8 KB ↓SHA-256 b2c51e84efebd6606d29cff9f2768e42958f3e1a251a92032c96a7646413ee3bEnclosure dimensions & assembly drawingsPDF · 192.4 KB ↓SHA-256 196809efd7b6cbc252917f5e88cf1a7f8218ad00c08a925c4b5c036148923661Enclosure assembly · STEPZIP · 264.6 KB ↓SHA-256 f01e6e88f9589cf777ff471492b71e3873f70fc2ffd1f3d0b1513de33f8898e3System purchasing BOMCSV · 8.5 KB ↓SHA-256 e48f61ea8678b35ec90512a6a08c19ac7e92eb46f316119b73beac51f4b4d7d0Wire endpoint listCSV · 1.8 KB ↓SHA-256 8b536ca5f0bb46c98920408adbee5be8c5a27210e24a0d146e9bfb79a72c7032Wiring & electrical interface guideTXT · 3.3 KB ↓SHA-256 0d36838bf7bc9e6b6be50eb35bd60bf5bdf900bcaa701ff560a4192586b97efeCharge-control schematicPDF · 107.0 KB ↓SHA-256 15429cf4e1c623f663c3e24ea008e9360bca35e7abbc4abc19ad7af65c442fc0Charge-control PCB layoutPDF · 128.6 KB ↓SHA-256 3c051fc3aec0228d2ec2809e0357e48a4b613ee2ea01e263b7111b1b0e35bd09File manifest & SHA-256 checksumsJSON · 3.1 KB ↓SHA-256 2c9050ea6b23ded9c31a343cc66eff093233a6b85c6f9592e9ae98481a304c8bThis architecture monitors the cabinet and records events. Compressor control requires a separate controller and a specified switching/protection circuit.
The sensor interface serves both ranges; the alarm profile, probe assembly, placement and calibration must match each storage specification. The displayed example is a frozen-storage profile.
The monitor battery supports local operation. Remote delivery also needs a powered router and upstream network. With no network, records stay in the SD queue and replay after reconnection.
Register independent device identities and tokens, deploy enough measurement points and group assets operationally by store. Size server retention, backup frequency and notification routing for the device count.
Share the cabinet type, storage temperature range, probe positions, door behavior, defrost cycle, Wi-Fi conditions, backup needs and fleet size. These inputs define the electronics, harness, enclosure, firmware profile and monitoring service.
Discuss a cold-storage project ↗