The Si7021 is a low-cost, easy-to-use, highly accurate, digital humidity and temperature sensor. This sensor is ideal for environmental sensing and data logging and perfect for build a weather stations or humidor control system. All you need are two lines for I2C communication, and you’ll have relative humidity readings and very accurate temperature readings as a bonus!
There are only four pins that need to be hooked up in order to start using this sensor in a project. One for VCC, one for GND, and two data lines for I2C communication. This breakout board has built-in 4.7KΩ pullup resistors for I2C communications. If you’re hooking up multiple I2C devices on the same bus, you may want to disable these resistors.
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Very easy setup, controlled by a Raspberry Pi 2 with CE’s Python script for this module: https://github.com/ControlEverythingCommunity/SI7021/blob/master/Python/SI7021.py
Highly recommended for temp and humidity in one package.
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This hooked up to the weather station I built easily. Just pulled in the libraries, added a few calls and cal factors and my station now has humidity and dew point stats. Weather Underground quality checkers gave my homemade station the gold seal once I added this sensor. Easy to use and reports accurately.
I spent 20 minutes (figuratively) pulling out my hair trying to get this thing connected to my Arduino. Then I realized: I had not physically connected all four jumper wires!
In all seriousness, this sensor works very well and I have had no real problems with it. It’s connected to my ESP8266 Thing Dev Board for logging data to ThingSpeak.
Got it working quickly, integrated it into a wireless outdoor reporting station. Sits next to a DHT-11 that quit measuring humidity in a matter of weeks after deployment (the second DHT-11 to fail quickly – a DHT-22 (less precise) lasted a year or so in the same place). I’m hoping that the PTFE cover over the Si7021 helps it survive a long time in our tough environment - it has to survive pollen and 100% humidity and sprinkler mist here in Florida.
The only negative was mounting. I soldered header pins with the short side on the sensor side, then trimmed them further, drilled a small hole in the plastic case for the sensor to protrude into (recessed just below the outer surface), and mounted it with the sensor into the hole and just put a big piece of tape over the board to hold it in position. The case is mounted with the sensor facing down to protect it from dust and droplets. I might hot-glue some sort of screen over it to keep bugs out, but it’s ok for now. We’ll see how long it lasts.
I have several of these boards. They just work, first time every time. I2C communication is simple and reliable.
Worked almost out of the box using an Si7021 Arduino library compiled for the Z-Wave.me Z-Uno. There was a compile error (Z-uno compiler bug, I think) that required a workaround. Much easier that I expected.
It took some doing to verify the accuracy of the two sensors I purchased. Turns out it’s not easy to accurately measure RH. Eventually I was able to use a table salt mixture in a small plastic container and both sensors read within 2 percentage points of the 75.5% theoretical value. I also built a desktop psychrometer that calculates RH and DP from wet bulb temp, dry bulb temp and atmospheric pressure. It took some work to get the wet bulb to cool properly but again both sensors were within tolerance at 25% RH. My next project is building a cooled mirror DP sensor using a Peltier device to get one more independent calibration check but I’m pretty sure it will confirm what I’ve seen so far.