WeatherShield is here!

I kept mentioning this in the forum from time to time and I’m happy to release the first batch of WeatherShields which is now available in the shop. These are highly accurate I2C temperature/humidity (Si7021) and atmospheric pressure (BMP180) sensors. Credit goes where it’s due – this was inspired by this forum post and its author mr. A, but it’s somewhat different than the one presented there. There is a sample sketch to read the data from this shield, schematics is at the end of this post.

Some of the features:

  • –40°C to +85 °C temperature range (Si7021)
  • ± 3% RH (max) 0–80% RH humidity range (Si7021)
  • Best of all these sensors are very low power!
    • The Si7021 has an active conversion consumption of 150uA and standby of 60nA, and BMP180 ranges between 3-12uA in active mode and 0.1uA in standby.
  • Very Fast sample times, far superior to sensors like DS18B20 which require a long ridiculous sample reading time of up to 1s. By comparison Si7021 requires about 4-10ms sample conversion time depending on reading resolution (8-14bit)
  • The shield can be stacked on/under a Moteino (not a MoteinoMEGA)
  • Small prototyping area where you can add a little circuit, connect it to the Moteino pins through thin hookup wire
  • The BMP180 sensor also gives temperature readings that are pretty good but it is primarily an atmospheric pressure sensor, and Si7021 has a magnitude better accuracy for temperature
  • Onboard P-mosfet driven VIN/battery monitor. This is a VIN-4.7k+10K-GND voltage divider that can be enabled by setting A3 to OUTPUT LOW and reading the VIN voltage on A7, then disabling it to save power by setting A3 to INPUT (HighZ which disconnects any battery drain through this circuit).

These are much different than popular hobby sensors like DS18B20 or DHT11/DHT22 which are in a different price range and much more limited, so they are not meant to be general purpose sensors. These boards come at a price and instead they are precision sensors for serious weather monitoring enthusiasts and offer a set of features which makes them very battery/remote monitoring friendly and along with Moteino they can make a very small battery operated node. There is a battery friendly sketch available.

Comparing readings between 2 units:

This is how they look fresh out the reflow conveyor:

6 thoughts on “WeatherShield is here!

  1. Hi,
    Si7021 has a min. readable temperature of -10 Celsius degrees. No less. Is that true?
    What kind od mosfet did you use to turn on the voltage divider? I’m curious becouse looking for low voltage gate threshold one (2.1-3.3 for mosfet to be full opened).
    For me si7021 is very attractive due to low power cons. when standby and power voltage range( compared to f.ex. Dht22)
    Regards,
    Jakub

  2. Cool board! Those chips are indeed very low power. A few years ago I designed a stand-alone temperature/pressure/altitude graphing tool called Backwoods Logger that used the BMP085, the predecessor to the BMP180 chip used here. It ran continuously for about a year on a single CR2032 watch battery. http://www.bigmessowires.com/backcountry-logger/

    For my purposes, I found the temperature accuracy of the BMP chip to be fine – I think it’s rated for 0.1 degree C accuracy. Weather Shield looks like it would be great for long-term weather monitoring at a fixed location. My project was aiming more for a portable device you’d carry with you into the wilds. 🙂

    • Very nice to hear from you and your feedback with the BMP085. I added one of these to my mailbox notifier and I am quite stoked that my mailbox now tells me temp/humidity/pressure in addition to last open timing, it’s a good place to have a little weather monitor, I will try to write up about it. I also read your recent article about the manufacturing failure, I really understand that kind of problem, and having read similar other stories lead me to make a huge leap of faith and get my own assembly equipment, something I don’t regret. It’s still hard but at least there is an extra degree of control and if I fail I can slap myself instead of someone else 🙂

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