The RnMonitor project aims to develop a novel and low-cost Cyber-Physical System (CPS) which integrates computation, networking, and physical processes by taking advantage of embedded computers and wireless networks to monitor and control Indoor Air Quality (IAQ). The RnMonitor CPS will be designed for online monitoring and active mitigation of radon gas concentration based on Cloud-based services for storage, processing and reasoning. Sensing is a crucial element in a CPS and our aim is to design and develop a low-cost radon probe with wireless connectivity that will be used as a sensing element in the RnMonitor CPS, see Figure below.