Several applications require hermetically sealed environments, where physical parameter measurements such as temperature, humidity, or pressure are measured and, for several reasons, a battery-less operation is required. In such applications, wireless data and power transfer are necessary. This application report shows how to implement an easy-to-use low-power wireless humidity and temperature sensor comprising an SHT21 from Sensation, an MSP430F2274 microcontroller, and a TMS37157 PaLFI (passive low-frequency interface). The complete power for the wireless sensor and the MSP430F2274 is provided by the RFID base station (ADR2) reader included in the eZ430-TMS37157 demo kit.

The application is divided into four steps:

  • Charge phase: Generate an RF field of 134.2 kHz from the ADR2 reader to the wireless sensor
  • module to charge the power capacitor.
  • Downlink phase: Send command or instructive to the wireless sensor to start measurement.
  • Measurement and recharge phase: Trigger measurement of temperature, recharge the power
  • capacitor on the senor device and trigger humidity measurement.
  • Uplink phase: Send measurement results via RF interface (134.2 kHz) back to the ADR2 reader.