A Resettable, Wireless and Passive Fall-Down Recorder Using a Magnetic Droplet with an LC Circuit

A fall-down recorder can record the happening of an object falling down during object delivery or transportation, even after the object is recovered to its initial state. Here, a resettable, wireless and passive fall-down recorder is proposed by integrating a droplet-based inertial force switch with the inductor-capacitor ( LC ) circuit for wireless signal transmission. A magnetic droplet is used in the switch with a converging microchannel to control the desired threshold acceleration. When the inertial force exceeds the designed threshold acceleration, the magnetic droplet will pass through the converging microchannel to a deep cavity to become a different state. By connecting the switch with the sensing LC circuit, the state of the recorder can be wirelessly identified through the LC resonant frequency. After triggering, this switch can be remotely reset to its initial state by an external magnet. A proper hydrophobic surface treatment is employed to facilitate the movement of the magnetic droplet and reduce the variation on the threshold acceleration. The capabilities, including switching, recording, wireless reading, and resetting of the proposed fall-down recorder, are successfully designed, fabricated, and verified with ten recording-reset tests.