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The problem
Cold storage rooms protect dairy, pharma and food inventories, but most are watched manually — a thermometer is glanced at a few times daily. A compressor that dies quietly overnight, a door left ajar during loading, or an unreported power cut can warm a whole chamber for hours, and spoiled stock is found only at the next check. Commercial cold-chain loggers exist, but they are expensive closed boxes that teach nothing. Student builds usually stop at one breadboard temperature sensor and never complete the real loop: multi-zone sensing, per-zone thresholds, door and power awareness, cloud logging, alerts that reach someone, and a summary of the day's exposure. This project completes that loop. An ESP32 node watches two zones with waterproof DS18B20 probes, reads humidity from a DHT22, tracks the door with a reed switch and senses mains power — evaluating every reading on-device, buzzing locally, logging to the cloud, and pushing alerts when anything crosses your thresholds. The dashboard shows live gauges, 24-hour temperature curves with threshold bands, and a daily cold-chain risk score from time spent out of range.
How it works
- A waterproof DS18B20 probe is placed in each of the two storage zones; the DHT22 sits in the main chamber; a reed switch mounts on the door and a sense tap watches mains power.
- The ESP32 samples all sensors every 10 seconds (design target) and compares each reading against the configured per-zone thresholds.
- Any breach — temperature, humidity, door open beyond the grace period, or power loss — immediately sounds the buzzer, updates the OLED, and fires a push notification.
- Every 60 seconds the node POSTs a JSON sample over Wi-Fi to the cloud backend; samples buffer in RAM during Wi-Fi outages and backfill on reconnection.
- The dashboard renders live gauges, the 24-hour temperature curves with threshold bands, system status and the alert event log.
- Firmware accumulates weighted breach-minutes per zone into the daily cold-chain risk score shown on the dashboard.
- Once, at installation, the owner runs the documented calibration procedure against an ice-point reference and records the correction offsets used in displayed readings.
Tech stack:
- ESP32 (Wi-Fi sensor node)
- DS18B20 waterproof probes · 1-Wire bus
- DHT22 temperature-humidity module
- Reed switch door sensor
- Arduino firmware (C++)
- Wi-Fi telemetry · JSON over HTTP
- Firebase-class cloud logging
- HTML/CSS/JS dashboard demo
| Parameter | Value |
|---|---|
| Controller | ESP32-WROOM-32 module (Datasheet value) |
| Temperature probes | 2 × DS18B20 waterproof, −55 to +125 °C, ±0.5 °C in −10 to +85 °C (Datasheet value) |
| Probe resolution | 12-bit, approximately 0.0625 °C steps (Datasheet value) |
| Humidity | DHT22, 0–100 %RH at ±2–5 %RH, −40 to +80 °C at ±0.5 °C (Datasheet value) |
| Door sensor | Magnetic reed switch, open/closed state (Datasheet value) |
| Display + alert | 0.96-inch OLED + 5 V buzzer (Datasheet value) |
| Default zone thresholds | Chill +2 to +8 °C; freezer −24 to −18 °C (Configurable design target) |
| Door-open grace period | 5 min before alert (Configurable, design target) |
| Sample interval | 10 s local; cloud push every 60 s (Design target) |
| Power backup | 3.7 V Li-ion module, approximately 4–6 h node runtime (Design target) |
| Alert channels | Buzzer + OLED + Wi-Fi push webhook (Implemented) |
Project features
- [Dual-zone temperature sensing] Two waterproof DS18B20 probes on one 1-Wire bus — one per storage zone — with per-zone high/low thresholds you configure; datasheet ±0.5 °C accuracy in −10 to +85 °C.
- [Humidity, door and power sensing] A DHT22 watches chamber humidity, a magnetic reed switch reports door open/closed with a configurable grace period, and a mains-sense line with a Li-ion backup keeps the node logging and alerting through power cuts.
- [Live cloud dashboard] The working single-file dashboard demo: per-zone live gauges, 24-hour temperature curves with threshold bands, door/power/Wi-Fi status cards, a cold-chain risk score, and a timestamped alert log.
- [Three alert channels] Threshold breaches, door-left-open and power events fire a local buzzer, an on-device OLED message, and a Wi-Fi push notification via an HTTP webhook (e.g. Telegram or Blynk) — SMS through a SIM800L module is an optional customization, not in the base build.
- [Cold-chain risk scoring] Firmware accumulates weighted breach-minutes per zone — time outside thresholds multiplied by a severity weight — into a daily Low/Moderate/High risk score, presented honestly as a simplified exposure proxy, not a certified food-safety metric.
- [Cloud data logging with buffering] One sample per 60 seconds posted as JSON over Wi-Fi to a Firebase-class backend; when Wi-Fi drops, the ESP32 buffers readings in RAM and backfills them on reconnection.
- [Buyer-run calibration procedure] An ice-point and reference comparison procedure for the probes is documented step by step, so the viva discussion on accuracy is concrete instead of quoted from a datasheet.
What is included
- Assembled prototype: ESP32 board, 2 × waterproof DS18B20 probes, DHT22 module, reed-switch door sensor, buzzer, OLED, Li-ion backup module, wiring
- Firmware source code with threshold logic, risk scoring, buffering and Wi-Fi telemetry
- Working single-file dashboard demo (the exact app shown in the gallery screenshot)
- Wiring diagram and sensor placement guide
- Buyer-run calibration procedure document
- Project report PDF (background, DS18B20/DHT22 datasheet analysis, methodology, results)
- PPT presentation for final review
- Viva Q&A preparation document (1-Wire, thresholds, risk scoring, accuracy limits)
Limitations & prerequisites
- Academic prototype, not a food-safety certified cold-chain system — the risk score is a simplified exposure proxy, never a certified spoilage metric.
- DS18B20 ±0.5 °C applies only from −10 to +85 °C; at freezer temperatures near −20 °C the datasheet allows ±2 °C — the report states the achieved error honestly after calibration instead of quoting the best figure.
- The datasheet accuracies are component values, not measured results; the report documents the calibration offsets found during the buyer-run procedure.
- Cloud logging and push alerts need Wi-Fi; without it the node keeps sensing and buzzing locally but no remote alert is delivered.
- The DHT22 samples every 2 seconds and drifts with condensation — humidity is indicative, not lab-grade.
- SMS alerts are NOT in the base build; they need the optional SIM800L customization.
- Probes read the air where they sit — placement away from door draughts and evaporator blast is part of the installation procedure.
Frequently Asked Questions
Which controller is used and why?
The ESP32. It has built-in Wi-Fi for cloud logging and push alerts, 1-Wire support for the DS18B20 bus, and enough GPIO for the reed switch, buzzer and OLED — one board does sensing, evaluation and telemetry where an Arduino would need an add-on Wi-Fi module.
How does the cold-chain risk score work?
Firmware tracks minutes each zone spends outside its thresholds and multiplies them by a severity weight (freezer breaches weight more), summing a daily score in Low/Moderate/High bands. It is a simplified exposure proxy designed to make neglect visible — not a certified food-safety spoilage metric, and the page says so.
What happens during a power cut?
The mains-sense line detects the loss instantly, the node sends a "power outage" push alert while Wi-Fi is still up, and the Li-ion backup keeps sensing and logging for approximately 4–6 hours — a design target, tested by the buyer during calibration.
Does it work without Wi-Fi?
The sensing, thresholds, buzzer and OLED keep working fully offline — only cloud logging and push notifications need Wi-Fi. Samples buffer in RAM and backfill to the cloud automatically when the link returns.
Is the freezer probe really accurate enough?
The DS18B20's ±0.5 °C figure holds only from −10 to +85 °C; at −20 °C the datasheet permits ±2 °C. The documented ice-point calibration procedure tightens the actual offset, and the report states the calibrated error honestly rather than quoting the datasheet as a result.
Is this project suitable for a final-year project?
Yes — for Electronics, Electrical and IoT programs. It combines real multi-sensor embedded design (1-Wire, humidity, door and power sensing), threshold and scoring logic, cloud telemetry with offline buffering, and a working dashboard, with honest calibration and accuracy limits that make strong viva material.
Components & software requirements
- ESP32 (Wi-Fi sensor node)
- DS18B20 waterproof probes · 1-Wire bus
- DHT22 temperature-humidity module
- Reed switch door sensor
- Arduino firmware (C++)
- Wi-Fi telemetry · JSON over HTTP
- Firebase-class cloud logging
- HTML/CSS/JS dashboard demo
Delivery information
Built-to-order project. Delivery timeline is shared after order confirmation based on current queue.
Support terms
Complete documentation, setup guide, and viva preparation included. Support for setup and explanation provided.
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