There are several RFID sensor platforms available right now. This will be based on the RF430FRL15X series from Texas Instruments. It has the most flexibility of any low cost energy harvesting RFID sensor platform available at this point, because it's a full blown microcontroller. Other RFID sensor ICs make you live with a very small sensor voltage range, and strange transfer functions. TI's micro allows you to scale the output any way you want.