Energy Harvesting in Automotive Key fob Application

Evolution in Radio Frequency (RF) semiconductor technology has led to highly power efficient devices. A typical automobile key fob for remote lock-unlock operations operates on 3V lithium coin cell battery having 200 mAh capacity and can last up to 75,000 key press events or two to three years. The typical transmission currents are less than 10 mA while sleep currents are less than 0.1 uA. As the lithium coin cell batteries are not rechargeable, they need to be replaced and safely disposed. Improper disposal of lithium batteries impose risk to the environment as lithium is highly poisonous and reactive. This paper proposes to replace the coin cell battery with a RF energy harvesting circuit involving voltage multiplier circuit consisting of zero bias schottky detector diodes and a hybrid energy storage capacitor. Authors have conducted experiments as well as simulation to evaluate the feasibility of the RF energy harvester replacing conventional coin cell battery. RF energy harvesting is less efficient due to losses during transmission and reception. This can be overcome by the limited number of operations of a key fob throughout the day and ease of placing it near commonly available RF sources in the vicinity such as mobile phones, wireless routers, microwave ovens, Bluetooth devices etc. The authors have modified the standard printed circuit board based loop antenna present in typical automotive remote key fob so that it can be used for RF energy harvesting. The proposed scheme can be extended to other low power battery operated RF devices after due consideration of usage profile.