Implementation plan of chlorophyll content tester based on AVR design

1.1 Introduction

Photosynthesis is a biochemical process in which plants and algae use chlorophyll to convert carbon dioxide and water into organic matter and release oxygen under the irradiation of visible light. As an important link in energy conversion in plant growth, it directly reflects the growth condition of plants. Chlorophyll is an important type of pigment in photosynthesis. In photosynthesis, chlorophyll absorbs the energy of light and uses the energy to synthesize carbohydrates. There are several different types of chlorophyll. Chlorophyll a and b are the main types, which are found in higher plants and green algae. Chlorophyll content is an important indicator to measure plant photosynthesis and growth conditions. Its detection technology is of vital importance in agricultural and forestry production and research.

1.2 Project Background/Motivation for Topic Selection

The relative content of chlorophyll reflects the actual nitro demand of plants, so as to understand the degree of soil nitro deficiency, guide the reasonable application of nitrogen fertilizer, increase the utilization rate of nitrogen fertilizer, and prevent the application of excessive nitrogen fertilizer from polluting the environment, especially water sources. At present, the detection of chlorophyll content mainly relies on photoelectric chlorophyll detectors. Chlorophyll detectors are intelligent agricultural and forestry instruments with high-sensitivity photoelectric sensors as the main detection units. They have the advantages of real-time detection, easy to carry, and no harm to the measured objects. They are widely used in production practice. However, the main producers of such instruments are Japan, the United States, etc. They are not only expensive, but also have no Chinese operating interface, and have single functions, which is not conducive to promotion among the vast number of agricultural and forestry workers in my country. Therefore, it is of great significance to develop a cheap, simple and practical measuring instrument suitable for my country's actual situation.

The chlorophyll photoelectric detector designed in this paper can fill the gap in related instruments in China. It has the advantages of accurate measurement, easy operation, rich functions and easy portability. It provides convenience for the majority of agricultural and forestry workers and also makes its own contribution to the scientific and technological services for the "three rural issues".

2. Demand Analysis

2.1 Functional requirements

The main function of this system is to measure the chlorophyll content of plant leaves and store the measurement results. The detector also integrates a real-time clock module and a digital thermometer to display time and temperature. It can also record and print relevant data according to the situation to simplify the recording method.

Figure 1 System architecture

2.2 Performance requirements

Photoelectric sensors need high ADC accuracy to collect chlorophyll content in leaves and have a strong ability to eliminate noise interference. Taking into account the impact of the environment, temperature sensors are needed to measure the ambient temperature and real-time clocks are needed to provide time information for experimental reference by the detector. Due to the external use environment, the design needs to be portable to facilitate agricultural and forestry workers to sample data anytime and anywhere. Under the conditions of meeting the requirements, cost control provides conditions for large-scale promotion.

3. Solution Design

3.1 System Function Implementation Principle

This design uses known light sources with wavelengths in the red light region (peak wavelength 650nm) and the infrared region (peak wavelength 940nm) to illuminate the leaves. Chlorophyll has a high absorption of light intensity in the red light region and is not affected by carotene, and has a very low absorption of light intensity in the infrared region. TI's OPT101 single-chip photodiode is used to sample the transmitted light data, and the sampled data is processed by A/D through a single-chip microcomputer to analyze the transmission values ​​of the two wavelengths of light. Through the corresponding calculation method, the measurement results of the relative value of the chlorophyll content in the leaves can be obtained. This design uses advanced photoelectric sensors, combines knowledge of biology, chemistry, etc., involves multiple scientific fields, and covers a wide range of knowledge. The system hardware structure block diagram is shown in Figure 2.

Figure 2 System hardware structure diagram [page]

3.2 Hardware platform selection and resource allocation

This design uses ATmega16L in the AVR microcontroller series as the control core of the measuring instrument, combined with the OPT101 single-chip photodiode chip to realize the basic function of chlorophyll content parameter measurement. This measuring instrument will also use the temperature chip DS18B20 and the clock chip DS1302 to measure temperature and time for the reference of the measurer, and with the assistance of LCD and keyboard, the design is easy to operate and more humane. The detector designed this time also uses a micro thermal printer, which can print data instantly, making the design more practical. Utilize the rich peripherals of ATmega16 and use the on-chip E2PROM to store instant data to prevent data loss due to accidental power failure. Conveniently realize the measurement and recording of chlorophyll content at a low price.

3.3 System Software Architecture

The designed detector includes a single-chip minimum system, LCD display, buttons, printer, etc. This design uses ATmega16L in a 40-pin DIP package as the main control chip to build a simple minimum system: including power rectification circuit, temperature measurement circuit, real-time clock circuit, expandable RS232/RS485 communication circuit, LCD display interface circuit, photoelectric sensor interface circuit, external high-precision nearly 14-bit AD converter circuit, working status indication circuit, and buzzer alarm circuit.

The LCD screen is a Jinpeng Electronics C series Chinese display module, with white characters on a blue background. The single-chip microcomputer uses the LCD communication subroutine to communicate with the LCD screen in serial. The communication rate can be flexibly set. The communication rate is based on the LCD screen being able to display clearly and without garbled characters. The LCD screen can display the control method and measurement results in real time, give operation prompts, and facilitate finding stable measurement values ​​for recording or storage, as well as instrument operation.

The buttons are independent light-touch buttons. The button scanning subroutine inside the microcontroller determines whether the button is pressed or released to control the measurement, data storage, data browsing and other operations. Through precise control of the process, a solution with the least buttons is designed without increasing the complexity of the operation.

The detector contains multiple functions, and all of these functions need to be integrated without conflict. It is necessary to consider both the overall performance of the rate and the working conditions of each module from many aspects. It requires multiple adjustments and debugging. After hundreds of software and hardware debugging, the final work reaches the best state and the design is completed.

3.4 System Software Process

The software flow of the system is shown in Figure 3. After the system starts, the microcontroller port is initialized first, then the welcome message is displayed, the key command is read, and the data in the E2PROM is read or the sensor is controlled to perform a measurement according to the command. If the "read E2PROM" command is read, the subroutine related to reading E2PROM is called, and the E2PROM is read and displayed on the LCD screen. If the "measurement" command is read, the subroutine related to the sensor is called, a measurement is performed, the result is displayed on the display, and it is asked whether to store it. At this time, if "yes" is selected, the subroutine related to writing to E2PROM is called to store the data; if "no" is selected, the data of this measurement is discarded, and the main interface is returned to wait for key commands. When displaying and storing data, two data values, chlorophyll content data and time, are included.

3.4 Expected Results of the System

Since the temperature, time, LCD display and other modules are relatively mature modules that we often use, we combined these two modules into one for testing. After more than a dozen data readings, we compared them with the data measured by special instruments, and then improved them, and finally controlled the error within the allowable error range of such measurements. The test of the sensor module is mainly to measure different samples, and then compare the values ​​of their measured data after processing to obtain the relative content of chlorophyll. When testing E2PROM, we store a set of data, cut off the power for a period of time, and then selectively read out part or all of them. After repeated tests, we determined that E2PROM is working properly.

Therefore, the design is expected to be able to measure the relative content of chlorophyll in leaves, and store the time, temperature and other information of the measurement results into E2PROM for later use or direct printing with a micro thermal printer. In addition, the battery life is long, which fully meets the needs of field work.