Water is an important part of natural soil. It not only affects the physical properties of the soil, but also restricts the dissolution and transfer of nutrients in the soil and the activity of microorganisms. It is also an important factor in composing the soil fertility, and it is the basis of all plants. The basic conditions for survival. Therefore, the determination of soil moisture content has important implications for the implementation of precision agriculture, water-saving irrigation, and improvement of agricultural production efficiency. Figure 1 Soil Water Suction Sensor There is a water inlet on the gas gathering pipe. When it is used, open the rubber plug, fill it with water, and then plug it. When the gas in the pipe is discharged, bury the clay head in the soil. The clay head is a sensing part with many tiny pores. After the clay head is infiltrated with water, a water film is formed in the pores. When the water-filled and sealed soil moisture sensor is inserted into the unsaturated soil, the water film is in contact with the soil. The water is connected to generate negative pressure in the water inside the instrument. This negative pressure acts on the solid-state piezoresistive sensor. The sensor converts the negative pressure into electrical signals so that an electrical signal of soil suction at any point within the range of 0-100 kPa can be obtained. And, the two have a good linear relationship. Figure 2 principle block diagram According to the above design principles, the designed soil moisture detection conversion circuit is shown in Figure 3. Figure 3 soil moisture detection conversion circuit First of all, using the drying weighing method to prepare the water content The soil moisture content was measured by this circuit device and the dry weighing method, and the results were compared as shown in Table 1. Table 1 Comparison of measurement data
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At present, the methods for measuring soil moisture can be summed up in two major categories. One is the variable position sampling measurement (such as drying weighing method), and the other is the in situ sampling measurement (such as neutron method, C-ray method, Time domain reflectometry method, frequency domain emitter method, sensor method, etc.). Different methods have their own advantages and disadvantages. Drying weighing method has a simple principle and high precision, but its operation is cumbersome and it cannot be continuously monitored in the field in real time.
Although the neutron method and the C-ray method can be quickly and accurately monitored outdoors, there are radioactive substances that endanger human health. The time domain reflectometry method and frequency domain transmitter method are expensive, and the sensor method is safe and reliable. It can measure soil moisture quickly and directly, has no soil sampling, does not destroy the soil structure, and can continuously monitor and output electrical signals. It is very suitable for Monitoring the dynamic distribution and change of soil moisture in the field facilitates long-term observation and accumulation of field water potential data. Therefore, it is widely used to monitor soil moisture in the field.
This paper designs a sensor method, which is mainly applied to the application of facility agriculture. The objects of its use are mainly agricultural planters and managers. Therefore, the design strives to be simple and easy to use, fast and direct reading, and low in price.
1 Soil Moisture Analyzer Test Switching Circuit Working Principle 1.1 Basic Principle of Soil Water Suction Sensor At present, there are many kinds of moisture sensors. This design uses soil water suction sensor. Because it has the advantages of simple structure, convenient use and high sensitivity, it can be fixed point. Positioning the continuous measurement of soil moisture movements, in time to reflect the vertical and horizontal changes in soil moisture.
The soil water suction sensor is based on the principle that the soil water suction is related to the soil moisture content. In general, the smaller the soil moisture, the greater the suction; the other hand, the smaller the suction. Its structure is shown in Figure 1. 
A solid-state piezoresistive sensor is a kind of force-sensing device made by utilizing the piezoresistive effect of single crystal silicon. Single-crystal silicon is used as a matrix, and four equivalent strain resistances are diffused by an integrated process technology to form a Wheatstone bridge. . When the pressure is not applied, the bridge is in equilibrium; when the pressure is applied, the resistance of one pair of bridge arms becomes larger and the other pair becomes smaller, so as to achieve the purpose of measuring the pressure of gas and liquid. For example, a CYG01 piezoresistive sensor generates a potential difference of 5mV when the suction force is 0 under a certain supply voltage, and a potential difference of 85mV when the suction force is 100kPa. Sensor power supply DC, in order to solve the problem of temperature drift, there are generally constant voltage source and constant current source, in the close-range measurement, with a constant voltage source 9V power supply, can be used to supply multiple sensors; if the measurement distance is far, in the accuracy When the requirement is high, the constant current source 6~10mA can be used for power supply, and each constant current source only supplies one sensor.
1.2 Soil Moisture Detection Conversion Circuit Principle The principle block diagram of the soil moisture detection conversion circuit is shown in Figure 2. 
Soil moisture detection conversion circuit consists of soil water suction sensor, signal conditioning circuit, display device and power supply. The basic principle of the design is that the soil water suction sensor sends the size-conversion electrical signal of the soil moisture content to the signal conditioning circuit, and the conditioning circuit processes the signal and sends it to the display device for display.
1.3 Soil Moisture Detection and Conversion Circuit Design 
When the circuit is working, the soil moisture sensor converts the soil moisture content H (%) into soil water suction and acts on the piezoresistive sensor.
The piezoresistive sensor converts soil water suction into a differential voltage signal output to a double-ended input single-ended output differential amplifier. Here, the differential amplifier is used because it has a good effect of suppressing zero drift, and at the same time, the double-ended input signal can be converted into a single-ended output signal. Since the output and input of the differential amplifier are inverting, the output signal is sent to the inverting amplifier again, and the output is again inverted while amplifying the signal, thereby converting it into a voltage signal in phase with the output of the piezoresistive sensor. . In order to adjust the amplification of the inverting amplifier as needed, a potentiometer is connected in series with the feedback resistor, and at the same time, the signal is zeroed and corrected. Finally, the output signal of the conditioning circuit is sent to the digital voltmeter to display the final output voltage value. A soil moisture meter is prepared based on the calibration of the test data, and the soil moisture content can be read directly.
2 Test of Soil Moisture Tester 2.1 Test Calibration of Soil Moisture Detection and Conversion Circuit 
100%, of which m1 is the water quality and m2 is the soil dry soil quality in g) is 8%, 12%, 15%, 18%, 22%, 25%, 28%, 30%, 34%, 36%, 38%, 43% of soil sample groups, using this device to measure the output voltage value, list and make the curve between soil moisture content and output voltage, according to which the circuit device calibration, and made Soil moisture header.
2.2 Comparison of results 
3 test results The measurement results show that: The resolution of this circuit device is 0.1%, and the difference between the measurement value and the drying weighing method is within 5%, indicating that the resolution and measurement accuracy of the device is relatively high . In particular, using this device to measure soil water content is fast (measurement takes only a few seconds at a time), numerical display, very intuitive, small size, easy to carry, can measure moisture content at any time, it is suitable for the field, greenhouse, lawn Use on other occasions.
Basic characteristics
1. Invention patent: a roll of film is automatically cut and translated into two packaging films on the machine, and two lines of articles are automatically packed. One machine has two capacity, small area, and saves people and energy.
2. Touch screen control, simple operation, intuitive display, modular design of electrical appliances, digital control.
3. There are three driving modes: double frequency conversion, double servo and three axis servo. No mechanical transmission, stable and reliable operation, low noise and high tracking accuracy.
4. Material position adjustment: automatic or jog button adjustment.
5. Material conveying width adjustment: one handle can adjust two rows of conveying width at the same time.
7. Two sets of adjustable bags can adjust the height and width of the bag.
8. There is a traction guide wheel in front of the longitudinal heat sealing wheel, and the traction guide angle can be adjusted during operation
Transformer circuit working principle and design of soil moisture detecting instrument