16.04.2014
Determining quantitative indicators of the humidity of gaseous media, liquids, solids and granular bodies is a sought-after task for almost all areas of industry, economic and scientific activity, and various types of production. All methods for determining humidity indicators are divided into direct and indirect. The direct method involves the direct separation of dry matter in the material under study from moisture. The principle of indirect methods is to measure physical quantities that have a functional relationship with the moisture content of a substance or material.
The need to continuously measure, monitor and regulate the moisture content in various substances has contributed to the development and development of compact sensor devices - moisture sensors. They have greatly facilitated the process of round-the-clock detection of the concentration of water molecules in the analyzed material. Modern touch sensors must meet a number of requirements: in addition to high accuracy, sensitivity and speed of operations, these devices must have a wide measuring range, coverage of several orders of magnitude of the analyzed quantity, and stability of readings.
Sensor Applications
Measuring humidity indicators is necessary in such areas of activity as:
- chemical production;
- fuel transportation;
- pharmaceuticals;
- polymerization;
- livestock farming;
- product storage;
- maintenance of refrigerators and freezers;
- timber processing industry;
- work of food shops;
- agricultural industry, etc.
Types of humidity sensors
Sensors for measuring humidity are classified according to various criteria, for example by:
- state of aggregation and structural features of the material to be analyzed;
- conditions and mode of operation - there are sensors for continuous and discrete control and measurement activities;
- the method of taking measurements - the sensors are of flow-through and submersible types;
- method for determining humidity indicators.
The last criterion contributed to the identification of two large groups that are in high demand: sorption and sorption-impedance sensors.
Sorption humidity sensors
To determine and control minor moisture concentrations, sorption-type sensors are used, the measurement principle of which is based on piezosorption and sorption-impedance monitoring methods.
The main functional element of such sensors is a sorption layer, which, upon contact with the research environment, is capable of absorbing water vapor. Often this layer is played by a polymer film or material based on highly porous inorganic oxides.
The higher the dimensional characteristics of the internal cavities of the material, the more efficient the sensor based on it is. Therefore, the optimal analyzing elements are porous and mesoporous materials. It is important to note that an increase in the moisture sensitivity of sensors using such material may also be accompanied by an increase in the error of the measurements taken. In this regard, the development and production of humidity sensors requires special control and adherence to technologies for forming the sensitive element.
Sorption sensors used to monitor the humidity of various environments can have a “sandwich” structure. The sensor is manufactured on substrates made of glass-crystalline material or polycor filler. The electrodes are made of nickel with vanadium coating. The sensitive hydrophilic layer is represented by a special nanostructured film of polymers; its formation occurs using a special technique. A particularly thin gold coating is applied to the layer of the resulting dielectric film (the membranes of this film are capable of selectively transmitting water molecules), which takes on the functionality of the second electrode. The direct arrangement of contacts at the level of the lower electrode ensures reliable design. The time constant matters:
- for a relative humidity sensor – 1-2 s;
- for a microhumidity sensor - from 10 to 180 s, such a wide range is determined by the dependence on the level of the studied moisture concentration.
A special heat treatment technology for the humidity sensor helps reduce the device error to 2%.
Sandwich type humidity sensor:
1. Sensor base;
2. Bottom electrodes;
3. Sorbent film;
4. Top electrode.
The operation of humidity sensors often involves the use of temperature meters. This helps improve the accuracy of environmental studies, ensure correct conversion of units of measurement and obtain the most accurate values of absolute and relative humidity.
A special role is given to relative humidity sensors when monitoring the atmosphere, climate of industrial premises and residential buildings. Also, the operation of hydrometeorological equipment, including probes, is essential without these sensors.
Sensors used to monitor microhumidity parameters are in demand when studying extremely pure active gases and their media (an example is argon or oxygen). Therefore, electronics industries, laboratory buildings, etc. cannot do without such measuring equipment.
Sorption-impedance sensors
Sorption-impedance sensors help determine the moisture concentration in various environments. The advantages of these humidity monitoring devices are:
- high sensitivity rates;
- simple manufacturing technology;
- compactness of the product.
The operation of such a sensor is based on the dependence of the complex resistance of the sorption layer on the volume of moisture absorbed by it. Such humidity sensors can have two design options:
- the above-described “sandwich” structure;
- with planar placement of electrodes, often have a comb shape.
The calibration characteristics of sorption-impedance humidity measuring instruments depend on the sorption material. Initially, hygroscopic ion-forming additives in the form of salts (such as lithium chloride, beryllium fluoride, etc.) acted as a sorption layer. Measuring sensors of this type are characterized by shortcomings - low stability of indicators, less sensitivity and a high probability of errors.
Based on this, modern manufacturers rarely use ion-forming salts as an independent moisture-receptive agent. Hygroscopic salt has acquired an auxiliary role in the production of sensors - it is used as an impregnation material or additive to increase moisture sensitivity. The main application in various fields is impedance meters with polymer sorbents (both organic and inorganic) based on metal oxides. The coating can be thin-film or thick-film.
The process of improving humidity sensors
In both domestic and foreign production of humidity sensors, an effective direction of development is visible - the development of innovative moisture-sensitive compositions. In general, this industry is characterized by the following features:
- the inevitable transition to group planar microelectronic production technology (both thin-film and thick-film are used);
- creation of multitasking devices, for example, integrated temperature and humidity sensors. The operation of such sensors not only helps to increase the accuracy of the measurements taken, but also leads to a simplification of the process of their calibration;
- bringing to a unified design system of humidity sensors, as well as signal processing facilities against the backdrop of the widespread use of microprocessors.
The existence of a wide variety of models of humidity sensors can be explained by the fact that none of them is universal. Each type of sensor has its own specifics, has advantages and disadvantages, which means the choice of device should take into account the characteristics of its application.
Humidity monitoring with EXIS instrumentation
Based on manufactured humidity sensors, Ecological Sensors and Systems JSC develops automated multi-channel systems, as well as stationary and mobile versions of control and measuring instruments. The latter are used for monitoring relative humidity and temperature indicators (devices of the IVTM-7 line), in studies of microhumidity of gases (IVG-1 line).
It is worth noting that in publications for scientific research and technical purposes, the concept of a humidity sensor implies devices that contain a moisture-sensitive element (sensor) and an electrical circuit for receiving and converting the signal from the sensor into the required value. This is why monitoring devices are often called sensors.
The devices being developed are used in solving problems in production conditions, providing conditions for comfortable and safe work for workers in various industrial fields. An example is the use of measuring instruments in electronics, chemical plants, nuclear power plants, etc.
The manufactured devices have all the necessary characteristics for combining devices into a common measuring network. The configuration of such a network may include multi-channel and single-channel devices, network and portable models, and measuring transducers. The operation of innovative measuring systems is characterized by a distributed control scheme, remote control (including via the Internet) and other modern technologies for control and measurement activities.
Home plumbing accidents often do not happen suddenly. First it will start to leak, then drip, and then it may burst. And the neighbors above may also start flooding. And it’s better to find out about this early, and not when the rain from the ceiling wakes you up. For my own peace of mind, I decided to play it safe and make an audible humidity alarm. Now I have such a toy next to every radiator, under every sink and in other water-hazardous places. This vigilant guard will warn of danger with the howling of a police siren. The device can also be used to signal high humidity in the room or the formation of condensation.
Specifications:
Supply voltage - 12 volts.
Current consumption at rest - no.
Current consumption in operating mode is 20 mA.
Details:
D1- K561LA7- 1 pc. Analogue - CD4011A.
T1, T2- KP505- 2 pcs. Any n-channel MOSFET with a gate voltage not exceeding 3 volts.
C1- 0.1 µF. Ceramics.
C2, C3- 22 nf. Ceramics.
R2- 1 room - 1 piece. Resistor 0125W.
R4- 3.3 rooms - 1 pc. Resistor 0125W.
R6- 47 rooms - 1 pc. Resistor 0125W.
R1- 68 rooms - 1 pc. Resistor 0125W.
R3- 100 units - 1 pc. Resistor 0125W.
R5- 220 room - 1 pc. Resistor 0125W.
ZP-18- 1 pc. Any piezoceramic emitter.
S1- Any switch.
Bat 12 V - AA battery from the alarm key fob.
Description of work:
As humidity increases, the resistance of the sensor decreases and transistor T2 opens. Both generators of the D1 chip are turned on. The generator based on elements D1-3 and D1-4 operates at a frequency of approximately 1 hertz, the generator based on elements D1-1 and D1-2 operates at the frequency of your emitter (you need to adjust it for maximum volume, in my case about three kilohertz). Transistor T1 with a frequency of 1 hertz connects and disconnects capacitance C3 connected in parallel to capacitance C2, because of this the tone of the second generator changes and an imitation of the sound of a siren is obtained.
Setting:
When assembled correctly, the device does not require configuration.
To reduce the sensitivity of the device, you need to reduce the resistor R5, to increase the sensitivity, increase it.
With these elements, the alarm is triggered by hand touch.
To increase the volume, you can select the frequency using C2 and C3 to suit your resonator.
Any two conductors located close to each other can be used as a humidity sensor. I cut several adjacent tracks on foil PCB.
There are not many parts and connections, so I decided not to make a printed circuit board.
It's hard to say anything about the price, all the details were at hand. The most expensive element is the battery - 30 rubles.
Documentation
How to choose a humidity sensor
The most important technical parameters to review when selecting a humidity sensor are:
- accuracy
- repeatability
- interchangeability
- long-term stability
- recovery from condensation
- resistance to chemical and physical pollution
- size
- frame
- price
Additional factors to consider may include replacement cost, calibration, design complexity, signal amplifier reliability, and data processing circuitry. To consider all the offers that are available on the modern electronic components market, it is necessary to consider the main types of humidity sensors and the general operating principles of each of them.
Capacitive relative humidity (RH) sensors
Capacitive humidity sensors are widely used in modern industrial equipment, household appliances and telemetry systems for collecting meteorological data.
Such sensors are structurally composed of a substrate on which a thin-film polymer or metal oxide is located between two conductive electrodes. The sensitive surface is covered with a porous metal electrode to protect against contamination and condensation. The substrate is usually made of glass, ceramic or silicon. Incremental changes in the dielectric constant of a capacitive humidity sensor are almost directly proportional to the relative humidity of the surrounding air. When the humidity fluctuates by 1%, the capacitance changes by 0.2-0.5 pF, and at 50% humidity (25°C) the fluctuations can reach from 100 to 500 pF.
Capacitive humidity sensors are characterized by a low temperature coefficient, the ability to operate at high temperatures (up to 200°C), the ability to fully recover from condensation and moderate resistance to chemical fumes. The response time of the sensors ranges from 30 to 60 s for a humidity step of 63%.
Modern technologies for the production of capacitive sensors have integrated many advances in semiconductor electronics to achieve minimal parameter offset and hysteresis during long-term operation. For example, thin-film capacitive sensors can integrate a monolithic signal amplifier chip on the substrate. Often modern signal amplifiers have a CMOS generator to smooth the linear output signal.
Capacitive dew point sensors
Thin-film capacitive sensors are characterized by a discrete signal change at low relative humidity. Their work is characterized by stability and minimal shift throughout the entire period of operation. However, such sensors do not have a linear output when the relative humidity drops below a few percent. This feature of the sensors led to the development of a dew point measurement system that combines a capacitive sensor with a microprocessor circuit that stores calibration data in a non-volatile memory unit. This approach to solving the problem has significantly reduced the cost of hygrometers and dew point transmitters, which are used in air conditioning systems and telemetry systems for collecting meteorological data. The sensors are mounted on a chip that has a voltage output signal depending on the level of relative humidity. Microprocessor control stores the voltage level at level 20 in the temperature range -40...27°C. The reference values are confirmed with a NIST hygrometer using Peltier cooled mirror technology. The voltage level at the dew point and freezing point is stored in the EPROM memory of the sensor. The microprocessor uses this data to calculate a linear relationship algorithm while simultaneously measuring dry bulb temperature and water vapor pressure. Once the water vapor pressure is determined, the dew point temperature is calculated from the thermodynamic relationship stored in the EPROM memory. Correlation with chilled mirror measurement technology is above ±2°C for dew point in the range -40...-7°C and above ±1°C in the range -7...27°C. The long-term stability of the sensor is less than 1.5°C per year. Metrology measuring instruments based on this principle are widely used in various applications due to their attractive price compared to instruments based on chilled mirror technology.
Resistive humidity sensors
Resistive humidity sensors detect changes in the electrical resistance of a hygroscopic medium (for example, a conductive polymer, salt, or treated substrate).
Resistive sensors are bifilar wound. Once coated with a hygroscopic polymer, their resistance is inversely proportional to humidity.
Typically, resistive sensors consist of metal electrodes deposited on a substrate using a photoresistor or wound onto a plastic or glass electrode cylinder. The substrate is coated with a saline or conductive polymer. When it is dissolved or placed in a liquid substance, it coats the sensor evenly. In another case, the substrate can be treated with some chemical reagent, for example, acid. The sensor absorbs water vapor and the ionic groups break down, which increases electrical conductivity. The response time for most resistive sensors is 10 to 30 seconds for a 63% measurement step. The resistance range of a typical resistive element ranges from 1 kΩ to 100 MΩ.
Most resistive sensors use AC excitation voltage without DC bias to prevent polarization of the sensor. The resulting current is converted and rectified into a DC voltage signal for further amplification, linearization, or analog-to-digital conversion.
The nominal frequency is 30 Hz to 10 kHz.
Resistive sensors are not completely resistive due to the capacitive effect in the range of more than 10-100 MΩ. The main advantage of resistive humidity sensors is their excellent interchangeability (typically ±2% RH), which allows the use of a resistor to calibrate the signal amplification circuit at a fixed humidity level. This eliminates the need for humidity calibration standards. The accuracy of each resistive humidity sensor can be measured in a calibration tank or using a dedicated computer system. The operating temperature range of resistive humidity sensors is from -40 to 100°C.
In domestic and commercial use, the service life of such sensors is more than 5 years, however, exposure to chemical vapors and other contaminants (oil, for example) can lead to their premature failure. Another disadvantage of resistive humidity sensors is their tendency to shift values when operating in condensate if a water-soluble coating is used. Resistive sensors have significant temperature dependence when used in environments with large temperature changes (greater than 10°F). At the same time, a thermal compensation circuit can be added to the sensor design to increase its accuracy. Thus, the main advantages of resistive sensors are small size, low cost, interchangeability and long-term stability.
Modern resistive sensors are designed using a ceramic coating to reduce the confluence of environmental conditions when condensation occurs. The sensors consist of a ceramic substrate with metal electrodes deposited using photoresist technology. The surface of the substrate is coated with a conductive polymer (or mixed ceramic composition), and the sensor itself is placed in a protective plastic housing with a dust filter.
The binding material is ceramic powder suspended in a liquid medium. After the surface is coated and dried, the sensors are treated with high temperature. The result is a thick film coating, insoluble in water, which completely protects the sensor from condensation.
Once immersed in water, the typical recovery time to 30% for a ceramic backed sensor is 5-15 minutes, depending on air speed.
The interchangeability of sensors is less than 3% in the measurement range of 15-95% relative humidity. The accuracy is ±2%. When the sensor is used in conjunction with a signal amplification circuit, the output voltage is directly proportional to the relative humidity of the environment.
Thermally conductive absolute humidity sensors
Such sensors measure absolute humidity by determining the difference between the thermal conductivity of dry air and air saturated with water vapor.
Thermally conductive sensors are often used to measure absolute humidity at high temperatures. Their operating principle is very different from resistive and capacitive sensors.
If the air or gas is dry, it has a significant ability to absorb heat. A typical example is desert climates. The desert is very hot during the day, but at night the temperature drops sharply due to the dry atmospheric climate. Conversely, humid climates cannot cool as quickly because heat is stored by water vapor in the atmosphere.
Thermally conductive humidity sensors (or absolute humidity sensors) consist of two matched NTC thermistors connected in a bridge circuit. The output voltage of the bridge is directly proportional to absolute humidity. One thermistor is hermetically sealed in dry nitrogen, and the body of the other is open.
When current passes through the thermistors, the thermal resistance increases the temperature to more than 200°C. The heat dissipated from a sealed thermistor is greater than that of an open thermistor due to the difference in thermal conductivity between water vapor and dry nitrogen. Since dissipated heat creates different operating temperatures, the difference in thermistor resistance is proportional to absolute humidity.
A simple assembly of resistors gives an output voltage in the range of 0 - 130 g/cub.m at 60°C. Calibration is performed by placing the sensor in dry air or nitrogen and adjusting the output signal to zero. Absolute humidity sensors have a long service life, their operating temperature reaches 300°C, and the sensor housing is resistant to chemical vapors.
The device that measures humidity levels is called a hygrometer or simply a humidity sensor. In everyday life, humidity is an important parameter, and often not only for ordinary life itself, but also for various equipment, and for agriculture (soil moisture) and much more.
In particular, our well-being depends a lot on the degree of air humidity. Particularly sensitive to humidity are weather-dependent people, as well as people suffering from hypertension, bronchial asthma, and diseases of the cardiovascular system.
When the air is very dry, even healthy people feel discomfort, drowsiness, itching and irritation of the skin. Often, dry air can provoke diseases of the respiratory system, starting with acute respiratory infections and acute respiratory viral infections, and even ending with pneumonia.
In enterprises, air humidity can influence the safety of products and equipment, and in agriculture, the influence of soil moisture on fertility, etc. is clear. This is where the use of humidity sensors - hygrometers.
Some technical devices are initially calibrated to a strictly required value, and sometimes in order to fine-tune the device, it is important to have the exact value of humidity in the environment.
Humidity can be measured by several of the possible quantities:
To determine the humidity of both air and other gases, measurements are carried out in grams per cubic meter when talking about the absolute value of humidity, or in RH units when talking about relative humidity.
For measuring the humidity of solids or liquids, measurements as a percentage of the mass of the test samples are suitable.
To determine the moisture content of poorly mixed liquids, the units of measurement will be ppm (how many parts of water are in 1,000,000 parts of the weight of the sample).
According to the principle of operation, hygrometers are divided into:
capacitive;
resistive;
thermistor;
optical;
electronic.
Capacitive hygrometers, in their simplest form, are capacitors with air as a dielectric in the gap. It is known that the dielectric constant of air is directly related to humidity, and changes in the humidity of the dielectric lead to changes in the capacitance of the air capacitor.
A more complex version of the capacitive humidity sensor in the air gap contains a dielectric with a dielectric constant that can vary greatly under the influence of humidity. This approach makes the sensor quality better than simply having air between the capacitor plates.
The second option is well suited for making measurements regarding the water content of solids. The object under study is placed between the plates of such a capacitor, for example, the object can be a tablet, and the capacitor itself is connected to an oscillatory circuit and to an electronic generator, while the natural frequency of the resulting circuit is measured, and from the measured frequency the capacitance obtained by adding the test sample is “calculated.”
Of course, this method also has some disadvantages, for example, if the sample humidity is below 0.5%, it will be inaccurate, in addition, the sample being measured must be cleared of particles with high dielectric constant, and the shape of the sample is also important during the measurement process; it should not change during the course of the study.
The third type of capacitive humidity sensor is the capacitive thin film hygrometer. It includes a substrate on which two comb electrodes are applied. In this case, comb electrodes play the role of plates. For the purpose of temperature compensation, two additional temperature sensors are additionally introduced into the sensor.
Such a sensor includes two electrodes that are deposited on a substrate, and on top of the electrodes themselves is applied a layer of material that has a fairly low resistance, which, however, varies greatly depending on humidity.
Aluminum oxide may be a suitable material for the device. This oxide absorbs water well from the external environment, while its resistivity changes noticeably. As a result, the total resistance of the measurement circuit of such a sensor will depend significantly on humidity. Thus, the level of humidity will be indicated by the amount of current flowing. The advantage of sensors of this type is their low price.
A thermistor hygrometer consists of a pair of identical thermistors. By the way, let us recall that this is a nonlinear electronic component, the resistance of which strongly depends on its temperature.
One of the thermistors included in the circuit is placed in a sealed chamber with dry air. And the other is in a chamber with holes through which air with characteristic humidity enters it, the value of which needs to be measured. The thermistors are connected in a bridge circuit, voltage is applied to one of the diagonals of the bridge, and readings are taken from the other diagonal.
In the case when the voltage at the output terminals is zero, the temperatures of both components are equal, therefore the humidity is the same. If a non-zero voltage is obtained at the output, this indicates the presence of a humidity difference in the chambers. Thus, the humidity is determined from the value of the voltage obtained during measurements.
An inexperienced researcher may have a fair question: why does the temperature of the thermistor change when it interacts with moist air? The thing is that as humidity increases, water begins to evaporate from the thermistor body, while the temperature of the body decreases, and the higher the humidity, the more intense the evaporation occurs, and the faster the thermistor cools.
4) Optical (condensation) humidity sensor
This type of sensor is the most accurate. The operation of an optical humidity sensor is based on a phenomenon related to the concept of “dew point”. At the moment the temperature reaches the dew point, the gaseous and liquid phases are in thermodynamic equilibrium.
So, if you take glass and install it in a gaseous environment, where the temperature at the time of research is above the dew point, and then begin the process of cooling this glass, then at a specific temperature value, water condensation will begin to form on the surface of the glass, this water vapor will begin to transform into the liquid phase . This temperature will be the dew point.
So, the dew point temperature is inextricably linked and depends on parameters such as humidity and pressure in the environment. As a result, having the ability to measure pressure and dew point temperature, it will be easy to determine humidity. This principle serves as the basis for the functioning of optical humidity sensors.
The simplest circuit of such a sensor consists of an LED shining on a mirror surface. The mirror reflects the light, changing its direction, and directing it to the photodetector. In this case, the mirror can be heated or cooled using a special high-precision temperature control device. Often such a device is a thermoelectric pump. Of course, a sensor is installed on the mirror to measure temperature.
Before starting measurements, the mirror temperature is set to a value that is obviously higher than the dew point temperature. Next, the mirror is gradually cooled. At the moment when the temperature begins to cross the dew point, drops of water will immediately begin to condense on the surface of the mirror, and the light beam from the diode will break due to them, dissipate, and this will lead to a decrease in the current in the photodetector circuit. Through feedback, the photodetector interacts with the mirror temperature controller.
So, based on the information received in the form of signals from the photodetector, the temperature controller will keep the temperature on the surface of the mirror exactly equal to the dew point, and the temperature sensor will indicate the temperature accordingly. Thus, with known pressure and temperature, the main humidity indicators can be accurately determined.
The optical humidity sensor has the highest accuracy, unattainable by other types of sensors, plus the absence of hysteresis. The disadvantage is the highest price of all, plus high energy consumption. In addition, it is necessary to ensure that the mirror is clean.
The operating principle of an electronic air humidity sensor is based on changing the concentration of electrolyte covering any electrical insulating material. There are devices with automatic heating linked to the dew point.
Often the dew point is measured over a concentrated solution of lithium chloride, which is very sensitive to minimal changes in humidity. For maximum convenience, such a hygrometer is often additionally equipped with a thermometer. This device has high accuracy and low error. It is capable of measuring humidity regardless of the ambient temperature.
Simple electronic hygrometers are also popular in the form of two electrodes, which are simply stuck into the soil, controlling its humidity according to the degree of conductivity depending on this very humidity. Such sensors are popular among fans because you can easily set up automatic watering of a garden bed or flower in a pot, in case you don’t have time to water manually or it’s not convenient.
Before you buy a sensor, consider what you will need to measure, relative or absolute humidity, air or soil, what the expected measurement range is, whether hysteresis is important, and what accuracy is needed. The most accurate sensor is optical. Pay attention to the IP protection class, the operating temperature range, depending on the specific conditions where the sensor will be used, and whether the parameters are suitable for you.
Andrey Povny
I have written a lot of reviews about dacha automation, and since we are talking about a dacha, automatic watering is one of the priority areas of automation. At the same time, you always want to take precipitation into account, so as not to needlessly run pumps and flood the beds. Many copies have been broken on the way to seamlessly obtaining soil moisture data. We review another option that is resistant to external influences.
A pair of sensors arrived in 20 days in individual antistatic bags: 
Characteristics on the seller's website:):
Brand: ZHIPU
Type: Vibration Sensor
Material: Blend
Output: Switching sensor
Unpacking: 
The wire has a length of about 1 meter: 
In addition to the sensor itself, the kit includes a control board: 
The length of the sensor sensors is about 4 cm: 
The tips of the sensor look like graphite - they get dirty black.
We solder the contacts to the scarf and try to connect the sensor: 
The most common soil moisture sensor in Chinese stores is this: 
Many people know that after a short time it is eaten up by the external environment. The effect of corrosion can be slightly reduced by turning on the power immediately before the measurement and turning it off when there are no measurements. But this doesn’t change much, this is what mine looked like after a couple of months of use: 
Someone tries to use thick copper wire or stainless steel rods; an alternative designed specifically for an aggressive external environment serves as the subject of a review.
Let's put the board from the kit aside and let's move on to the sensor itself. The sensor is a resistive type, changing its resistance depending on the humidity of the environment. It is logical that without a humid environment the sensor resistance is enormous: 
Let's lower the sensor into a glass of water and see that its resistance will be about 160 kOhm: 
If you take it out, everything will return to its original state: 
Let's move on to tests on the ground. In dry soil we see the following: 
Add some water: 
More (about a liter): 
Almost completely poured out one and a half liters: 
I added another liter and waited 5 minutes: 
The board has 4 pins:
1 + power
2 earth
3 digital output
4 analog output
After testing, it turned out that the analog output and ground are directly connected to the sensor, so if you plan to use this sensor connected to the analog input, the board does not make much sense. If you don’t want to use a controller, you can use a digital output; the response threshold is adjusted by a potentiometer on the board. Connection diagram recommended by the seller when using a digital output: 
When using digital input: 
Let's put together a small layout: 
I used Arduino Nano here as a power source without downloading the program. The digital output is connected to the LED. It’s funny that the red and green LEDs on the board light up at any position of the potentiometer and the humidity of the sensor environment, the only thing is that when the threshold is triggered, the green light shines a little weaker: 
Having set the threshold, we find that when the specified humidity is reached at the digital output 0, if there is a lack of humidity, the supply voltage is: 
Well, since we have a controller in our hands, we’ll write a program to check the operation of the analog output. We connect the analog output of the sensor to pin A1, and the LED to pin D9 of the Arduino Nano.
const int analogInPin = A1; // sensor const int analogOutPin = 9; // Output to LED int sensorValue = 0; // read value from the sensor int outputValue = 0; // value output on the PWM pin with LED void setup() ( Serial.begin(9600); ) void loop() ( // read the sensor value sensorValue = analogRead(analogInPin); // translate the range of possible sensor values (400-1023 - set experimentally) // in the PWM output range 0-255 outputValue = map(sensorValue, 400, 1023, 0, 255); // turn on the LED to the specified brightness analogWrite(analogOutPin, outputValue); // display our numbers Serial.print ("sensor = "); Serial.print(sensorValue); Serial.print("\t output = "); Serial.println(outputValue); // delay delay(2); )
I commented out the entire code, the brightness of the LED is inversely proportional to the humidity detected by the sensor. If you need to control something, then it is enough to compare the obtained value with a certain experimentally determined threshold and, for example, turn on the relay. The only thing I recommend is to process several values and use the average for comparison with the threshold, as random spikes or drops are possible.
We immerse the sensor and see: 
Controller output: 
If you remove it, the controller output will change: 
Video of this test assembly working:
In general, I liked the sensor; it seems resistant to the external environment; time will tell whether this is true.
This sensor cannot be used as an accurate indicator of humidity (like all similar ones); its main application is determining the threshold and analyzing dynamics.
If there is interest, I will continue to write about my country crafts.
Thanks to everyone who read this review to the end, I hope someone finds this information useful. Full control over soil moisture and goodness to everyone!
