I found this cool little bluetooth sensor that is completely open source: https://ruuvi.com
It even comes with a standard battery:
You can find the source code (and apk) for the Android app here: https://github.com/ruuvi/com.ruuvi.station
It's rare to find great sensors that are also open source, so I decided to share it here.
It also comes with optional registration, and other features on their site, but it's all completely optional. You can use them locally and even build the app yourself.
It's such a refreshing thing to see these days. Most sensors now force you to sign up to some service or pay to use them. Not this one.
It reminded me of the Polar Heart Rate monitors like the H10 as it can also be used completely locally, without having to use a service. Both of these devices are made in Finland. Looks like they are doing something right over there...