Its this one: https://www.amazon.com/gp/product/B00H7HFINS/ref=oh_aui_search_asin_title?ie=UTF8&psc=1 https://www.amazon.com/gp/product/B00H7HFINS/ref=oh_aui_se

Its this one: https://www.amazon.com/gp/product/B00H7HFINS/ref=oh_aui_search_asin_title?ie=UTF8&psc=1 https://www.amazon.com/gp/product/B00H7HFINS/ref=oh_aui_sear... I put a four of them around my house, each connected to a Raspberry Pi. CO2 levels were way higher than I expected.