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 MC68HC908QT4. The SDU’s acoustic source consists of a piezoelectric speaker excited by a 5-kHz square wave generated by the MC68HC908QT4. 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 MC68HC908QT4 is connected to a Manchester CODEC and RF transceiver for communications with a desktop computer
The heart of each source and detector unit (SDU) is the 68HC908QT4. The SDU’s acoustic source consists of an piezoelectric speaker excited by a 5 kHz square wave gen- erated by the 68HC908QT4. The SDU’s detector unit consists of an electret microphone whose output is ac coupled to a high gain, bandpass ¯lter. The result is then peak detected before being sampled by the ADC unit of the 68HC908QT4. To complete the SDU, a soft- ware serial communications unit has been written for the 68HC908QT so that it can be connected to a Manchester codec and RF transceiver for communications with a desktop computer. Each SDU is powered by two nicads which are trickle charged by solar cells during the day. The 2:4V from the nicads are boosted by a charged pump circuit to +-V to provide the dual polarity supplies required by the SDUs. The main PCB of each SDU is housed in the ornamental cup of a commerical solar powered garden light. The solar cells, trickle charger and nicads which comes with the garden light are also seconded to the SDU. When the SDU is fully assembled into the garden light unit, it is placed in the yard like a normal garden light and if not examined too closely, is comou°aged well enough to look exactly like one | and the best part, the light still works!
To control all these SDUs, any desktop computer which has a serial port and running a programme with the protocol described in this paper can be used. For this project, the programming language used for the server programme is TCL/TK which has the advan- tages of being cross-platform and the ability for producing a GUI with just a few lines of code. The control method selected is a simple client/server model with the desktop computer running as the server and SDU as clients.
Only the server is allowed to com- municate with each SDU. The packets of data sent between clients and server is modeled after TCP/IP packets. Like TCP/IP, each packet has a type or signature, a source and destination address and a body. Author: Cheng-Yang Tan
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