Motorola Flash Innovation 2003 – Design Contest
The advantage of using acoustic waves to measure the moisture content of soil is that acoustic waves measure the moisture content in a volume of soil that radiates outward from the acoustic source to the detector rather than measure the moisture content at only one point. Each detector is solar-powered and uses wireless transceivers to send the measured data to a server. The heart of each source and detector unit (SDU) is the 68HC908QT4. The SDU’s acoustic source consists of a piezoelectric speaker excited by a 5-kHz square wave generated by the 68HC908QT4. The SDU’s detector unit consists of an electret microphone whose output is AC coupled to a high-gain, band-pass filter. The result is then peak detected before being sampled by the microcontroller’s ADC. The ’908QT4 is connected to a Manchester CODEC and RF transceiver for communications with a desktop computer.