Wireless Sensor Node: monitoring low-rate phenomena using a low-power RF-enabled microcontroller

A low-power wireless enabled sensor module built from commercial off-the-shelf components (COTS) is presented. This sensor/transmitter module is the basic component of a low cost telemetry system that can monitor low time-constant phenomena, such as temperature and humidity in our implementation, in which energy preservation is a target specification. The heart of the module is an RFPIC microcontroller (MCU) with a RF-UHF transmitter from Microchip. The module is normally kept in sleep mode to decrease energy consumption; in this condition, it uses 1.5 muW (I_sleep<0.5 muA @ 3 V). The module is woken up by a low frequency (125 kHz) magnetic field pulse that is sensed by a tuned LC tank in the module that drives a low power comparator (MAX919) whose output wakes the MCU up from sleep. When it is woken up, the module takes the intended measurements, uses a current of I_ON =450 muA for tens of ms, and then it transmits the results to a central receiver using a common-use RF ISM band (434 MHz) and then goes back to sleep. Each burst of transmission lasts =30 ms and uses a current of I_TX=5 mA in this condition, which depends on the output power configuration.