An autonomous self-powered acoustic transmitter using radioactive thin films

We present a radioactively powered acoustic transmitter for application in wireless sensor nodes. The acoustic transmitter integrates a piezoelectric acoustic transducer with a self-reciprocating radioisotope powered piezoelectric micropower generator for long-life (tens of years) autonomous operation. The radiated β-particles from the radioisotope are used to direct charge an air-gap capacitor structure, formed by the collector plate at the tip of a piezoelectric unimorph cantilever beam and the thin-film radioisotope source plate placed directly underneath the collector. The resulting electrostatic forces draw the collector plate towards the source. The collector plate eventually makes contact with the source discharging the air-gap capacitor and releasing the cantilever. The sudden release excites the unimorph mechanical impulse response, causing the cantilever to oscillate. The mechanical vibrations induce charges in the piezoelectric section of the monomorph, which are used to drive the acoustic transmitter. The stored mechanical energy is first converted into electrical energy by the piezoelectric unimorph and then into radiated acoustic energy in the external piezoelectric speaker. Eventually, the vibrations decay and a new cycle begins for the self-reciprocating actuator. At the end of every reciprocation cycle, acoustic transmission at the design frequency of 170 Hz can be sensed using a high sensitivity (-34 dB 1 V/1 Pa) microphone placed as far as 5 cm from the speaker.