I backed this great Kickstarter project for $235.
The Smart Citizen Kit is an Open-Source platform that comprises 3 technological layers: a hardware device, a website and online API, and a mobile app. Below is an overview of them, and if you’re interested, all the gory details can be found on the Smart Citizen website.
The first layer is a piece of hardware comprised by two printed-circuit boards: an interchangeable daughterboard or shield, and an arduino-compatible data-processing board. We have nicknamed the shield developed for this campaign ‘The Ambient Board.’ As the name suggests, it carries sensors that measure air composition (CO and NO2), temperature, light intensity, sound levels, and humidity. Once it’s set up, the ambient board is able to stream data measured by the sensors over Wi-Fi using the FCC-certified, wireless module on the data-processing board. The device’s low power consumption allows for placing it on balconies and windowsills. Power to the device can be provided by a solar panel and/or battery. All the design files (schematics and PCB layout) for this Open-Source, Arduino-compatible device are available.
So it has a lot of sensors including a small solar panel and its also Open Source! So you are also able to upload the data directly to your own site or database. Also when internet is not available the data will be logged on the onboard storage.