This sensor uses a small and low cost single solar cell to harvest energy (size of the solar cell is 20 mm x 6 mm). The energy can be used as it is harvested or partly stored in a super capacitor to also allow operations when there is not enough illumination. The stored energy is monitored for optimal usage by means of an algorithm, which decides (based on of the available energy) how often to do a measurement and to send the results to smartphones or gateways via Bluetooth Smart.