| Pesticide residues in our food is a serious problem threatening people's daily life. A multi-channel detection system for pesticide residues based on C8051F MCU is designed in this paper. Unlike the traditional method of which only one sample solution can be detected at the same time, this instrument detects 96 solutions every time. Optical, mechanical and electronic integration enormously enhances the efficiency and the accuracy.This system works on base of the theory that absorption law in spectroanalysis. Absorbency analyzing theory is also called Ultraviolet-visible Spectrophotometry, UV-Vis which is based on speciality and extent of absorbency for substance molecule, a method to quantitative analysis.In another hand, under a certain condition pesticide of organophosphorus carbamate and cholinesterase have a special biochemical reaction: after the hydrolysis of cholinesterase, products of the hydrolysis react with a certain chromogenic reagent, the color changes after the reaction. But the reaction is affected by the pesticide of organophosphorus carbamate, they can restrain the reaction and the color will not change. The inhibitory rate is correlated with the concentration of pesticide positively. The system can judge the degree of the pesticide of organophosphorus carbamate in the samples by detecting the inhibitory rate.The working process is: the monochromatic light generating from the LED goes through the solution which is put into an 8*12 holes test-tube moved by a step-motor and shoots to silicon photocells. The photocells convert the light signal to current signal. The current signal is transported to a amplifier circuit and is changed into voltage signal which is subsequently received by A/D converter of a MCU. The MCU is in charge of controlling the step-motor, display, communication and data processing.The system uses C8051020F device as its MCU. The C8051F020 family utilizes Silicon Labs' proprietary CIP-51 microcontroller core. The CIP-51 employs a pipelined architecture that greatly increases its instruction throughput over the standard 8051 architecture. In a standard 8051, all instructions except for MUL and DIV take 12 or 24 system clock cycles to execute with a maximum system clock of 12-to-24 MHz. By contrast, the CIP-51 core executes 70% of its instructions in one or two system clock cycles, with only four instructions taking more than four system clock cycles.The most unique enhancement is the Digital Crossbar. The on-chip counter/timers, serial buses, HW interrupts, ADC Start of Conversion input, comparator outputs, and other digital signals in the controller can be configured to appear on the Port I/O pins specified in the Crossbar control registers.The C8051F020 has an on-chip 12-bit SAR ADC (ADCO) with a 9-channel input multiplexer and programmable gain amplifier. With a maximum throughput of 100ksps, the ADC offers true 12-bit accuracy. The internal voltage reference circuit consists of a 1.2 V, 15 ppm/℃(typical) bandgap voltage reference generator and a gain-of-two output buffer amplifier. So using the internal voltage requires that the input current must be lower than 2.4V.Eight light-emitting diodes are used as light source of which light shoots to the solutions. A voltage stabilizer LM317 supplies a stable voltage to the diodes. During the course of measuring, diodes should be lighted one by one so as not to cause interference to adjacent ones. As a result a decoder 74LS138 is used. Since the output current of 74LS138 is not enough to light the diodes, two power transistors 9013 are connected to a diode.The acquisition circuit is composed of eight silicon photocells and an analog multiplexer CD4051. The current signal emitted by the photocell is so weak that the MCU can not detect exactly. So the output of CD4051 is connected to the input of an amplifier circuit. The amplifier circuit contains a low-pass filter and an integrated operational amplifier OP07. The out voltage from the amplifier circuit and the signal ground are connected to the differential input ports of a instrumentation amplifier AD623 which is configured for unity gain (G = 1) in order to eliminate the common-mode interference.The system equips a four-phase inductor 47BYGH107 type step-motor. Its driving circuit has four triodes TIP122 and four diodes IN4003. It makes multi-detecting possible and the number in one time detecting can be 96.The system employs a LCM240128 made by XINHUALONG Corp. to display the result. The module contains LCD display, driving circuit, controlling module T6963C made by TOSHIBA,PCB main board ,RAM, and backup light.A 4*5 matrix keyboard is used and its nine controlling lines are all linked to the I/O ports of the MCU.The system has the capability of storing 1000 results by using a EEPROM AT24C512 even when it is not powered. The SMBus0 I/O interface between AT24C512 and MCU is a two-wire, bi-directional serial bus. It is compatible withthe I~2C serial bus.It is convenient for users to confirm and record the time by using a real time clock chip SD2303. In this system users could get or modify the time at any time. The interface between SD2303 to MCU is also a two-wire serial bus.A serial printer made by WEIDA is used. It makes that the result can not only be stored in the EEPROM, but also be printed out for people to check out.The system employs a portable switch power. Other internal voltages can be got by using some DC-DC converters.The system is programmed with C language under the development environment of Keil C uVision 2. Modularized programs consist of: multi-chinese character menu, displaying and modifying time, measuring and storing the result, querying and printing the history data and connecting to PC.To validate the dependability and stability of the whole system, a common kind of fast measuring reagent for pesticide residues is used to detect the samples. The result is satisfied, the whole system is succeeded.The whole system performed well in the experiment. We should developed it and make more advantage. An improved system can distinguish the kind of a pesticide and also quantitative analysis can be realized. |
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