Piezoelectric MEMS switch to activate event-driven wireless sensor nodes

We have developed piezoelectric microelectromechanical systems (MEMS) switches and applied them to ultra-low power wireless sensor nodes, to monitor the health condition of chickens. The piezoelectric switches have 'S'-shaped piezoelectric cantilevers with a proof mass. Since the resonant frequency of the piezoelectric switches is around 24 Hz, we have utilized their superharmonic resonance to detect chicken movements as low as 5-15 Hz. When the vibration frequency is 4, 6 and 12 Hz, the piezoelectric switches vibrate at 0.5 m s ^-2 and generate 3-5 mV output voltages with superharmonic resonance. In order to detect such small piezoelectric output voltages, we employ comparator circuits that can be driven at low voltages, which can set the threshold voltage (V_th) from 1 to 31 mV with a 1 mV increment. When we set V_th at 4 mV, the output voltages of the piezoelectric MEMS switches vibrate below 15 Hz with amplitudes above 0.3 m s^-2 and turn on the comparator circuits. Similarly, by setting V_th at 5 mV, the output voltages turn on the comparator circuits with vibrations above 0.4 m s^-2. Furthermore, setting V_th at 10 mV causes vibrations above 0.5 m s^-2 that turn on the comparator circuits. These results suggest that we can select small or fast chicken movements to utilize piezoelectric MEMS switches with comparator circuits.