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It is a floating ESP32 thermometer that sends pool-temperature readings over Wi-Fi to Adafruit IO, while a separate Adafruit MagTag displays the latest reading indoors or poolside. The project solves a simple family problem—checking the water without making a trip—but it is a convenience monitor, not a swimming-safety system.
How the pool monitor works
Blake documented the Pool Temperature Monitor on Hackaday.io in 2021 after his children repeatedly asked whether their grandparents’ pool was warm enough to visit. The system has three parts:
Water temperature → DS18B20 → ESP32 → Wi-Fi → Adafruit IO/MQTT → MagTag display- Floating sensor node: An ESP32 reads a waterproof DS18B20 temperature sensor inside a PVC enclosure.
- Cloud transport: The ESP32 connects to Wi-Fi and publishes the reading to Adafruit IO.
- Remote display: An Adafruit MagTag retrieves the feed over MQTT and shows the temperature with weather information.
That arrangement avoids running a cable across the pool deck and means family members can see the result without opening a phone app.
What is inside the floating unit?
The enclosure uses approximately 1.5-inch PVC pipe with a bottom cap and threaded top cap. Inside are an ESP32 module and breakout board, a waterproof DS18B20 probe, an 18650 lithium-ion cell, and a TP4056-based charging and protection board. A 3D-printed internal structure holds the electronics and flotation material, while foam helps the unit float and steel BBs act as ballast so it remains upright.
#1 Best Overall
- Waterproof Design IP65-rated main unit resists water splashes, making it suitable for wet environments such as pools, bathtubs, or kitchens without compromising performance.
- Wide Temperature Range Equipped with a 1.5M 304 stainless steel probe for accurate readings from -40°C to 115°C, ideal for measuring in refrigerators, ventilation ducts, fermentation bottles, and more.
- Real-Time LCD Display Clear, high-definition LCD screen shows current temperature instantly, so you don’t need to check your phone every time.
- App Monitoring & Data Export View live temperature data and historical charts via the app, or export temperature logs for analysis and record-keeping.
- Flexible Installation & Zigbee Connectivity Works as a Zigbee sub-device (requires Zigbee hub), with magnetic or hanging mount options for easy placement in different locations.
The floating design can measure water away from the pool wall and avoids plumbing modifications. It does not, however, guarantee an average pool temperature. Sunlight, depth, circulation, stratification, and contact with the wall or liner can all affect the reading. For more consistent results, a recreation should keep the sensor at a repeatable depth and make the float easy to retrieve.
The DS18B20 measures temperature—not pool safety
The DS18B20 is a digital 1-Wire temperature sensor. Adafruit’s waterproof version is pre-wired and sealed for wet environments, supports 3.0–5.0V systems, and is specified at approximately ±0.5°C over much of its operating range.
Those figures describe the component, not the finished floating product. System accuracy also depends on calibration, sensor placement, thermal gradients, cable sealing, software conversion, and heat absorbed by the enclosure. Displaying several decimal places does not make the overall installation equally precise.
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Rank #2
- High Accuracy: This indoor-outdoor thermometer hygrometer offers ±0.54℉/±3%RH accuracy. Whether in the jungle, or on your patio, its 2s sampling rate monitors small humidity/temperature changes, compact design at 2.39 ounces, making it your reliable companion for mountaineering, hiking, camping, and gardening.
- Portable Design: IP65-rated Waterproof thermometer helps you withstand rain and moisture during camping, hiking, and rock climbing. The dustproof and sun-resistant casing can withstand tougher outdoor environments. Pocket-sized design for easy storage in hard-shell jackets and backpacks. (Please note that this product cannot be immersed in water.)
- Durable and Reliable: This wireless thermometer features a robust nylon lanyard with an adjustable buckle to attach to your backpack, stroller, pet carrier, golf bag, or carabiner clip for convenient outdoor use. It can also be effortlessly hung indoors, in your room, greenhouses, patios, or on doors and windows. In Bluetooth mode, the battery can last for a year, avoiding frequent replacement.
- Smart Alerts: This outdoor waterproof thermometer supports both app monitoring and standalone display. You can connect via Bluetooth for preset temp./humid. thresholds and receive instant alerts via the app when values exceed the range. Or view the data directly on the screen. Supports connection to GoveeLife product gateways, enabling wireless remote monitoring.
- Record at Anytime: This thermo-hygrometer supports 20-day curve readings and 2-year free data storage and export, enabling you to gain insights into seasonal patterns. It helps you optimize gear selection for camping, picnics, and skiing, or to track indoor temperature and humidity changes—keeping you prepared at all times.
Most importantly, temperature alone does not establish that a pool is safe. This device does not test pH, free chlorine, combined chlorine, cyanuric acid, water clarity, electrical faults, entrapment hazards, supervision, physical access, or lightning risk.
Why deep sleep matters
The ESP32 sleeps between measurements to preserve battery power. When its timer wakes it, the controller must read the probe, connect to Wi-Fi, publish the value, and return to sleep. Wi-Fi is usually the expensive part of that cycle.
The project initially targeted readings about once per hour, then moved to approximately 30-minute sensor updates after software and board changes. The display was intended to refresh every two hours. Blake estimated nearly two months of battery life under that usage pattern. These are project-specific 2021 observations, not guaranteed results for every ESP32 board, battery, firmware version, or Wi-Fi environment.
Rank #3
- DS18B20 Temperature Sensor Cable with Probe(Pack of 5)
- Wiring: Red(VCC), Yellow(Data), Black(GND)
- Wide temperature range of -55 ℃ ~ +125 ℃
- Power supply: 3.0V ~ 5.5V
- Tips: Using a 4.7K resistor between the Data and VCC will make the testing of the probe easilier.
The MagTag is the display, not the pool sensor
The MagTag provides a low-power, glanceable endpoint for the data. The current Adafruit MagTag is the 2025 edition: a 2.9-inch grayscale e-paper Wi-Fi display using an SSD1680 driver. It includes an ESP32-S2, USB-C, battery charging, buttons, NeoPixels, a speaker, an accelerometer, and a light sensor. Adafruit listed the single-unit price as $34.95 on August 18, 2026, with limited stock shown at that time.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsThere is an important compatibility issue for anyone recreating the project. Adafruit changed the display in July 2025 from the discontinued ILI0373 to the SSD1680. Current Arduino and CircuitPython releases support the newer display, but 2021 code may require adaptation. A builder should confirm the exact MagTag revision and software libraries before wiring or ordering parts.
The honest engineering story: waterproofing failed first
PVC pipe is a sensible starting material, but it is not automatically a waterproof electronics enclosure. Later project updates reported condensation under the screw cap, liquid inside the enclosure, and slight rusting of the steel ballast. The builder suspected that a child pulling the float underwater may have contributed and planned to replace old thread sealant with fresh sealant.
Rank #4
- IP65 Dustproof/Waterproof, Suitable for Indoors & Outdoors: Outdoor Thermometer is IP65 dustproof and waterproof and can work safely even on rainy days. They're easy to use, lightweight, and portable. Perfect for kitchens, bathrooms, greenhouses, refrigerators, guitar cases, humidors, attics, basements, cigar humidor and crawlspaces.
- More Sensitive to Temperature & Humidity: With its built-in industrial Swiss Sensirion sensor you can expect more accuracy up to ± 1.8% RH, ± 0.36 ° F, with a faster and more accurate 4sec sampling interval to always monitor the temperature and humidity in your living environment.
- Supports Data Exporting: 394ft connecting distance (no obstacles) allows you to quickly and clearly view the temperature and humidity yearly, monthly, daily, or by the hour. Enjoy up to 68 days On-board Data Storage and up to 2 years In-App Data Storage(Unlimited cloud storage if SwitchBot Hub is added). Data can be exported as. csv file for more in-depth analysis of your environmental. Supports multiple methods of data calibration.
- Compact and Durable: indoor outdoor thermometer wireless features a simple and compact design, with a lanyard attached to allow you to hang it to just about anything. With its low power consumption design, you can use it for up to two years, and do not need to replace the removable batteries frequently.
- Smarter Push Notifications (SwitchBot Hub required): Our hygrometer now supports Dewpoint, and VPD readings with absolute humidity as well as classic temperature and humidity readings that can be used as Scene conditions. When used with a SwitchBot Hub, should any preset value be exceeded, our app can push a notification alert to allow you to react in real-time to changes in freezer, wine cellar, instrument case, attic, basement, crawlspace, reptile tank, terrarium, and more.
That failure is the most useful lesson in the project. A modern version should:
- Seal the DS18B20 cable entry deliberately rather than relying on a tight opening.
- Use fresh, compatible sealant and decide whether permanent adhesive sealing is preferable to easy service access.
- Assemble the enclosure dry and consider an appropriate internal moisture-control strategy.
- Test the empty enclosure by immersion before installing electronics or a battery.
- Allow for battery replacement without repeatedly damaging the seal.
- Account for chlorine, ultraviolet exposure, temperature cycling, and aging of 3D-printed parts.
- Retrieve the unit before swimming, pool maintenance, or any activity that could pull it underwater or damage it.
Do not recharge a wet lithium-ion battery or power wet electronics. Remove the device from service and inspect it after any suspected water ingress.
Connectivity problems were part of the prototype
The project experienced intermittent Wi-Fi connection and Adafruit IO publishing failures, including a failure after the home internet went offline overnight. Changing the ESP32 board and updating the code improved operation, but also exposed a battery-voltage reporting problem: the replacement board measured battery voltage differently.
Best Value
- IP65 Waterproof & Multi-Scene Use Boasts an IP65 waterproof rating, making it perfect for water-splash environments like greenhouses, bathrooms, and warehouses. Ideal for monitoring humidity and temperature in areas prone to moisture or daily indoor spaces needing stable detection.
- Zigbee Smart & Real-Time Data Display As a Zigbee-enabled device, it detects ambient temperature and humidity, showing real-time readings on the LCD screen. Check current conditions instantly via the app for convenient environmental info access.
- Chart View & Export for Analysis Supports viewing and exporting temperature and humidity charts on the app. Helps track environmental changes over time, enabling better management of spaces like greenhouses and warehouses for optimized conditions.
- Special Waterproof & Breathable Membrane Equipped with a unique waterproof and breathable membrane, it ensures stable operation and accurate measurements even in humid settings. Moisture won’t interfere with performance, guaranteeing reliable monitoring.
- Flexible Installation: Magnetic & Hanging Install it your way—magnetically attach it or hang it up. Suits various setups, like attaching to greenhouse frames, hanging in bathrooms, or placing in warehouses, adapting to different space layouts.
A dependable recreation should treat freshness and failure states as part of the product design. The sensor firmware should use connection and publish timeouts, bounded retries with backoff, and a watchdog or scheduled recovery path. Each upload should include:
- Temperature
- Battery voltage or battery state
- A timestamp
- A sensor and connection status
The MagTag should show the reading’s age and mark it as stale when the cloud feed has not updated. A last-known value without a timestamp can be more misleading than no value at all.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What a modern adaptation should build
- Choose the controller carefully. Select an ESP32 board with reliable deep sleep, 3.3V-compatible logic, a documented battery-measurement path, and an antenna suitable for the enclosure location.
- Wire the probe correctly. Connect power, ground, and 1-Wire data, and add the pull-up resistor required by the selected library and wiring arrangement.
- Protect the battery. Use a protected 18650 cell and verify the TP4056 board’s charging and protection behavior. Do not charge the device while it is floating unless the enclosure and connector are specifically designed for safe service.
- Validate readings. Reject implausible temperatures and report sensor faults instead of publishing a convincing but incorrect number.
- Design recovery behavior. On a temporary Wi-Fi outage, retry for a limited time and sleep rather than draining the battery indefinitely. On a cloud outage, retain the last value but label it stale.
- Test the enclosure first. Immersion-test the empty enclosure, inspect for condensation, and only then install electronics and the battery.
- Make the float serviceable. Keep the sensor depth consistent, secure the ballast, remove sharp or loose parts, and provide a reliable retrieval method.
For the display, use a MagTag or another Wi-Fi display that shows the current temperature, last update time, data age, battery state, and an explicit offline message. If adding a “warm enough” label or notification, use a configurable personal threshold and hysteresis so the status does not flip repeatedly around one temperature.
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| Option | Best for | Main trade-off |
|---|---|---|
| DIY floating monitor | Open hardware, customization, logging, alerts, and learning | Requires programming, cloud setup, battery maintenance, and serious enclosure work |
| Simple floating thermometer | Lowest complexity and fewest failure points | No remote access or historical data |
| Phone-connected sensor | App-based remote readings without soldering | May use proprietary hardware, apps, or recurring services |
| Home Assistant sensor | Local automation and integration | More setup, but potentially less cloud dependence |
| Fixed or in-line sensor | Repeatable placement and circulation-system integration | Requires installation or plumbing work |
| Commercial smart monitor | Finished hardware and vendor support | Usually less open and potentially more expensive |
The DIY route is attractive because it is adaptable, not necessarily because it is the cheapest way to read water temperature. The original documentation lists the principal parts and links to code, but it does not provide a complete wiring diagram, full bill of materials, exact pin assignments, complete Adafruit IO setup, enclosure-sealing procedure, or verified current software path. It should therefore be treated as an adaptation roadmap rather than a ready-to-follow build tutorial.
Does it automatically know when the pool is warm enough?
No. The documented system reports temperature; people decide whether the water feels suitable for the intended activity. A modern version could add configurable “cold,” “comfortable,” or “warm” labels, alerts, and separate preferences for children, adults, exercise, or recreation. Those thresholds are comfort choices, not universal swimming-safety standards.
Bottom line
This is a clever family-focused IoT project: a floating ESP32 thermometer, an online data feed, and a separate e-paper display turn a routine pool check into a glanceable household status. Its concept is practical, but dependable construction depends on solving water ingress, Wi-Fi recovery, stale-data reporting, battery servicing, and current MagTag compatibility. If remote readings and customization matter, it is a strong maker project. If the goal is simply to know the water temperature, a local floating thermometer will be easier and more reliable.
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