soil moisture sensor working principle and calibration

Posted by Binda on February 18th, 2021

The soil moisture sensor is composed of a humidity detection circuit and sound alarm circuit. RP1 is the preset point for the lower limit of humidity, and RP2 is the preset point for the upper limit of humidity. When the humidity in the soil is between the preset upper and lower humidity limits since the soil resistance between probe "a" and "h" is within the specified range, the potential at point "c" is lower than the sliding end potential of the RPI (comparator non-inverting input), So the comparator "I" output a high level, red does not emit light; the wiper potential of RP2 is lower than the potential at point "c", then the comparator II outputs a high level, and green does not emit light.

If the soil humidity reaches or exceeds the upper limit humidity set by RP2, the resistance between the ab probes becomes smaller, and the probe partial pressure decreases, and the potential of the non-inverting input terminal (point c) of the comparator II is lower than the potential of the inverting terminal of the comparator. (RP2 sliding terminal potential), the comparator II inverts and outputs a low level, green glows, and SS8550II turns on the buzzer to sound, indicating that the soil moisture is too high. If the dry soil reaches or exceeds the lower limit humidity set by RP1, the resistance between probes "a" and "b" will increase, and the probe partial pressure will increase higher than the voltage at the non-inverting terminal of comparator I (RP1 sliding terminal potential), and comparator I will output a low voltage. Level, red light, and SS8550I turn on the buzzer to sound, indicating that the soil moisture is too low. The buzzer design is convenient for the user to set the alarm point. When the setting is completed, the power button SP can be disconnected, and the lower computer will report the error. The sensor probes "a" and "b" are made of two stainless steel rods with a length of 160 mm and a diameter of 5 mm, which are 55 mm apart.

Simply speaking, the working principle of the soil moisture sensor is to determine the level of soil moisture by measuring the resistance between two electrodes. Soil is used as a conductive medium between the electrodes. Low water content results in poor conductivity and high resistance value; high water content results in strong conductivity and low resistance value. Through this reverse correspondence, the approximate detection of soil moisture content can be achieved. Many makers use soil moisture sensors to build soil moisture sensors.

The calibration process of the soil moisture sensor probe is:

1. Adjust the sliding contact of RP1 to the uppermost end, and adjust the sliding contact of RP2 to the lower end to make both red and green go out.

2. Carry out soil moisture debugging, take two soil samples suitable for greenhouse seedlings, one for the upper limit humidity (that is, the maximum soil humidity suitable for greenhouse seedlings), and one for the lower limit of humidity (that is, the lowest soil humidity suitable for greenhouse seedlings). Insert the probe into the upper limit soil and adjust RP2 so that green just lights up and the buzzer just sounds; then insert the probe into the lower limit soil and adjust RP1 so that red just lights up and the buzzer just sounds.

3. Connect the alarm output terminals ALARM1 and ALARM2 of the soil moisture detector to the P1 port of the lower computer. When the lower computer is powered on and reset, the P1 port is high. When the soil humidity is too high or too low, the soil humidity detector ALARM1 or ALARM2 The terminal outputs a low level, and the lower computer scans the level status of the P1 port regularly, assigns the corresponding status word, and enters the sub-processing program.

For the sensor made by this method, its measured resistance cannot be measured with an ordinary multimeter, and it can only be measured with a Wheatstone bridge. Besides, the alarm part can be removed in the actual circuit, just observe the light on and off. The lead wire of the probe needs to use a shielded wire.

Using the hardware control circuit of the soil moisture sensor, the soil moisture sensor buried in the root of the crop monitors the moisture of the root-soil. The sensor transmits the "humidity too high" and "humidity too low" signals to the main controller through the encoder through the detection circuit. The main controller determines the control state.

If the humidity is too high, the irrigation will be stopped, and if the humidity is too low, the solenoid valve connected to the water source is controlled by photoelectric isolation and a relay. When the air humidity is high, the air humidity sensor is reflected in the main controller via A/D conversion to temporarily delay irrigation. The current system status can be displayed by the display circuit, and the system can also manually set the irrigation time by the keyboard. The system also has a fault alarm function. The main controller communicates with the host computer through the communication interface and can monitor the system operation status in real-time or analyze historical data.

The above is an introduction to the working principle and use method of the soil moisture sensor. In actual use, due to the influence of dust, oil, and harmful gases, it will age and reduce the accuracy after a long time. The accuracy level of the humidity sensor should be combined with its long-term stability. Generally speaking, long-term stability and service life are the top issues that affect the quality of humidity sensors. There are few products whose annual drift is controlled at 1% RH, and they are generally around ±2% or even higher.