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ZigBee eWeLink temperature sensor - interior

bulek01 1290 4

TL;DR

  • A ZigBee eWeLink SNZB-02 temperature and humidity sensor was opened after its temperature reading drifted badly.
  • The board contains a TLSR8258F512ET32 microcontroller, a CH8305 temperature/humidity IC, an LED driver transistor, and a pairing button.
  • The TLSR8258F512ET32 combines 64KB RAM, 512KB flash, and Zigbee 3.0 / 802.15.4 radio support in one package.
  • The real fault was a discharged battery: Zigbee2Mqtt still showed 3%, but the sensor was only getting 2.2V.
  • Replacing the batteries fixed the measurements, and a boost converter might extend battery life, though it could reduce radio range.
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  • Zigbee temperature and humidity sensors with manual and mounting accessories


    I present the inside of a temperature and humidity sensor on Zigbee.
    The idea to present the inside came to me after it stopped working properly.
    An offset appeared on my temperature reading. When it was 20 degrees it was showing 3 degrees.
    I ordered a new one of the same kind and it turned out....
    I chose this sensor for my application because it has the best range of all the ones I tested.
    At my place, 2.4GHz barely gets out of the room and this one managed to get through to the other side of the house.

    My sensor is read by Zigbee2Mqtt, the documentation and model page for this sensor can be found here:
    https://www.zigbee2mqtt.io/devices/SNZB-02.html
    Zigbee SNZB-02 eWeLink sensor status with 3% battery level


    It is called eWeLink on the packaging.
    Close-up of Zigbee eWeLink temperature and humidity sensor packaging

    When the sensor stopped working, I saw what was inside to check for any damage. It had been mounted on the north window outside for several months.

    Zigbee sensor battery module with visible cell and white enclosures in background

    Interior of Zigbee sensor with exposed purple PCB and electronic components
    Close-up of Zigbee sensor PCB with TLSR8258 microcontroller
    Close-up of Zigbee sensor PCB with visible SMD components
    Close-up of a PCB with CH8305 sensor and SMD components
    Inside everything looks correct, nothing is flooded.

    As you can see there are few components inside:
    -mikrokontroler TLSR8258F512ET32
    -temperature and humidity sensor ch8305
    - lED and its control transistor in SOT-23
    - a button to pair with the network and start instantaneous measurement(didn't catch the photo)

    TLSR8258F512ET32 64KB 512KB
    Qualified for Bluetooth 5.3, AOA/Tx, Bluetooth Mesh, 802.15.4, Zigbee 3.0, RF4CE, HomeKit, Dual-Mode, 2.4G Proprietary
    Block diagram of RISC-based MCU with integrated 2.4 GHz radio and peripherals
    As you can see from the brief description the microcontroller has 512kB flash 64kB RAM. So it is quite large. In addition, it is equipped with a radio chip with quite a lot of capabilities. It has everything integrated into one housing with inverters and memory and radio. It works with voltages from 1.8V to 3.6V.

    Temperature sensor ch8305
    Technical specifications of CHT8305SC sensor with pinout diagram
    This is a small 3x3mm IC in a DFN housing. It has an I2C interface. The average current consumption is 3uA. It operates over a supply voltage range of 2.5V to 5.5V.

    After buying a new sensor, I re-purposed the battery from the old one, connected it to the mains and found that this sensor was also showing the temperature reading incorrectly.
    The only common element left was the battery. I fitted new ones and found that everything worked. Zigbee2Mqtt was showing that there was still 3% battery, so I didn't pay attention to that right away. The other sensors I have work to the end of the battery until they stop responding completely.
    Measuring the battery voltage with a multimeter and it comes out 2.2V, which I thought was enough, there should still be some energy pulling from them, especially as the Zigbee connectivity was working fine.
    A quick measurement on the board and it turned out that the sensor is powered directly from the batteries, and is running from 2.5V to 5.5V. Which is the direct cause of the problem.

    Summary
    The sensor consists of two temperature sensor ICs and a Chinese microcontroller with radio circuitry.
    The reason for not working turned out to be a discharged battery, even though there was still 3% left.

    I wonder if there are inverters that could be added here to get the maximum out of these batteries. Such a battery can be discharged to 1V, hence there would be 2V which would need to be bumped up to 3.3V. Such an inverter would also need to draw little current at no load in order not to discharge the battery additionally. I wonder how this would also affect the range, my experience with inverters is that they are very difficult to reconcile with the radio system, the range decreases significantly.

    Cool? Ranking DIY
    About Author
    bulek01
    Level 17  
    Offline 
    bulek01 wrote 336 posts with rating 293, helped 12 times. Been with us since 2006 year.
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  • #3 21819197
    @GUTEK@
    Level 31  
    Posts: 1560
    Help: 163
    Rate: 367
    What time does this thermometer report the temperature?
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  • #4 21819462
    bulek01
    Level 17  
    Posts: 336
    Help: 12
    Rate: 293
    Thanks for the info on the inverter, it looks cool but unfortunately it's not cheap. I might be tempted to give it a try.
    As for reporting, the sensor sends data when the temperature changes. I have no idea what the threshold is. Looking at my data the spread between measurements is so 20minutes to a minute or so.
  • #5 21819485
    krzbor
    Level 29  
    Posts: 1731
    Help: 40
    Rate: 1044
    Basically, it's a software error that should show 0% battery. It should show that from 1.2V on the cell. I have a general question though - how do you rate the durability (failure-free) of modern digital modules made with SMD technology? I mean contemporary ones, because at the beginning of lead-free technology there were problems. I am writing in the context of IOT circuits - which, once installed, we are not going to move for a long time. At my house, such a system is the alarm. It has worked for 26 years - both the board and the keypad. I hope it will reach half a century. Of course, I'm leaving out circuits that operate at high loads (such as LED power supplies) and LED bulbs that operate at high temperatures. I am referring to Zigbee sensors and lighting / roller shutter controllers. Perhaps someone distributes such circuits and has some insight into failure rates? The heart of the IOT chip at my place is an RPi with passive cooling. Has anyone "died" with this computer?
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FAQ

TL;DR: If your Zigbee eWeLink/SNZB‑02 shows wildly wrong temps (e.g., 20°C actual vs 3°C shown), replace the batteries; “An offset appeared on my temperature reading.” [Elektroda, bulek01, post #21818785]

Why it matters: This FAQ helps DIYers quickly diagnose misreads, power issues, and reporting behavior for Zigbee temperature/humidity sensors.

Quick Facts

How often does the Zigbee eWeLink/SNZB‑02 report temperature?

It reports on change, not strictly on a timer. In real logs, updates appeared anywhere from about 1 minute to about 20 minutes. As the user put it, “the sensor sends data when the temperature changes.” This behavior helps save battery during stable conditions. [Elektroda, bulek01, post #21819462]

Why is my SNZB‑02 showing the wrong temperature after months of use?

Low supply voltage is the prime suspect. The CH8305 sensor needs at least 2.5 V. A measured 2.2 V caused severe misreads, even while the platform still showed 3% battery. New cells immediately restored correct readings. Replace batteries first before deeper troubleshooting. [Elektroda, bulek01, post #21818785]

What chips are inside this Zigbee temperature/humidity sensor?

It uses a TLSR8258F512ET32 SoC for Zigbee radio and control, plus a CH8305 temperature/humidity IC. The board also includes an LED with a small SOT‑23 transistor and a push button for pairing/instant read. The enclosure interior showed no moisture damage in the teardown. [Elektroda, bulek01, post #21818785]

Is this device eWeLink or Sonoff SNZB‑02—what’s the exact model?

The packaging is branded eWeLink, while Zigbee2MQTT recognizes it as the SNZB‑02 family. The teardown and usage notes reference the Zigbee2MQTT SNZB‑02 page and show eWeLink branding on the box. Functionally, it behaves like a standard SNZB‑02 sensor. [Elektroda, bulek01, post #21818785]

Can I add a boost converter (e.g., MCP1624) to squeeze more life from the batteries?

Yes, a forum member pointed to using an MCP1624-based boost inverter, as discussed in a related LED candle project. A booster could raise two nearly-spent cells back to 3.3 V operation. Consider cost, layout space, and quiescent current before modding. [Elektroda, Sentymentalny, post #21819085]

Will a boost converter hurt Zigbee range or stability?

It can. The teardown author noted that inverters are hard to reconcile with radios and can significantly reduce range. RF noise and ripple from the converter may degrade sensitivity. If you add a booster, use good filtering and short leads. [Elektroda, bulek01, post #21818785]

Zigbee2MQTT showed 3% battery, but readings were wrong—bug or hardware issue?

One participant called it a software-side issue and argued it should read 0% from about 1.2 V per cell. In practice, the board still ran its radio while the sensor misread. Treat low percentages as a prompt to replace cells early. [Elektroda, krzbor, post #21819485]

What voltage does the CH8305 sensor require?

The CH8305 operates from approximately 2.5 V to 5.5 V. Below that, readings can be offset or invalid. Its typical average current is around 3 µA, which suits long-life battery designs when the supply is within range. [Elektroda, bulek01, post #21818785]

Can I mount this sensor outdoors or on a window exterior?

One user mounted it for months on a north-facing exterior window. The inside looked clean with no flooding. However, the post does not claim a weatherproof rating. If used outdoors, shield it from rain and direct sun for reliability. [Elektroda, bulek01, post #21818785]

Quick fix: how do I test if low voltage is my problem?

  1. Install fresh cells in the sensor.
  2. Confirm correct temperature in your Zigbee2MQTT dashboard after pairing.
  3. Measure the removed cells; if near 2.2 V total, low voltage likely caused the misreads. Replace with new, quality batteries. [Elektroda, bulek01, post #21818785]

What is Zigbee2MQTT in simple terms?

Zigbee2MQTT is a bridge that lets Zigbee devices talk to your home automation via MQTT. It supports many products, including SNZB‑02, and exposes readings to platforms like Home Assistant. It helps unify vendors under one hub. “Zigbee2MQTT — Introduction”

What triggers a manual measurement on this sensor?

There is a push button that pairs the device and also triggers an instantaneous measurement. Use it after battery replacement or when testing placement. Watch the LED to confirm action and check for a fresh payload in your logs. [Elektroda, bulek01, post #21818785]

How is range in tough 2.4 GHz environments?

The author chose this unit because it out‑ranged others tested. At their home, typical 2.4 GHz struggled to leave one room, but this sensor reached the far side of the house. Placement and interference still matter. [Elektroda, bulek01, post #21818785]
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