Corrosion-resistant soil moisture sensor, suitable for dacha automation. Humidity sensors - how they work and how they work Moisture sensors

A soil moisture sensor will help you get rid of monotonous repetitive work, and a soil moisture sensor will help you avoid excess water - it’s not that difficult to assemble such a device with your own hands. The laws of physics come to the aid of the gardener: moisture in the soil becomes a conductor of electrical impulses, and the more there is, the lower the resistance. As humidity drops, resistance increases and this helps track the optimal watering time.

The design of the soil moisture sensor consists of two conductors that are connected to a weak energy source; a resistor must be present in the circuit. As the amount of moisture in the space between the electrodes increases, the resistance decreases and the current increases.

The moisture dries - the resistance increases, the current decreases.

Since the electrodes will be in a humid environment, it is recommended to turn them on via a key to reduce the destructive effects of corrosion. During normal times, the system is turned off and only starts up to check the humidity by pressing a button.

Soil moisture sensors of this type can be installed in greenhouses - they provide control over automatic watering, so the system can function without human intervention at all. In this case, the system will always be in working condition, but the condition of the electrodes will have to be monitored so that they do not become unusable due to corrosion. Similar devices can be installed on garden beds and lawns in the open air - they will allow you to instantly obtain the necessary information.

In this case, the system turns out to be much more accurate than simple tactile sensation. If a person considers the soil to be completely dry, the sensor will show up to 100 units of soil moisture (when assessed in the decimal system), immediately after watering this value increases to 600-700 units.

After this, the sensor will allow you to monitor changes in moisture content in the soil.

If the sensor is intended to be used outdoors, it is advisable to carefully seal its upper part to prevent information distortion. To do this, it can be coated with waterproof epoxy resin.

The sensor design is assembled as follows:

  • The main part is two electrodes, the diameter of which is 3-4 mm, they are attached to a base made of textolite or other material protected from corrosion.
  • At one end of the electrodes you need to cut a thread, on the other side they are made pointed for more convenient immersion into the ground.
  • Holes are drilled in the PCB plate into which the electrodes are screwed; they need to be secured with nuts and washers.
  • Outgoing wires need to be placed under the washers, after which the electrodes are insulated. The length of the electrodes that will be immersed in the ground is about 4-10 cm, depending on the container or open bed used.
  • To operate the sensor, a current source of 35 mA is required; the system requires a voltage of 5V. Depending on the amount of moisture in the soil, the range of the returned signal will be 0-4.2 V. Resistance losses will demonstrate the amount of water in the soil.
  • The soil moisture sensor is connected via 3 wires to the microprocessor; for this purpose, you can purchase, for example, Arduino. The controller will allow you to connect the system to a buzzer to sound a signal when soil moisture decreases excessively, or to an LED, the brightness of the lighting will change with changes in the operation of the sensor.

Such a homemade device can become part of an automatic watering system in a Smart Home system, for example, using the MegD-328 Ethernet controller. The web interface shows the humidity level in a 10-bit system: the range from 0 to 300 indicates that the ground is completely dry, 300-700 - there is enough moisture in the soil, more than 700 - the ground is wet and no watering is required.

The design, consisting of a controller, relay and battery, is removed into any suitable housing, for which any plastic box can be adapted.

At home, using such a humidity sensor will be very simple and at the same time reliable.

The application of a soil moisture sensor can be very diverse. They are most often used in automatic watering systems and manual watering of plants:

  1. They can be installed in flower pots if the plants are sensitive to the water level in the soil. When it comes to succulents, such as cacti, it is necessary to select long electrodes that will respond to changes in humidity levels directly at the roots. They can also be used for other fragile plants. Connecting to an LED will allow you to accurately determine when it is time to carry out.
  2. They are indispensable for organizing watering of plants. Using a similar principle, air humidity sensors are also assembled, which are needed to put the plant spraying system into operation. All this will allow you to automatically ensure watering of plants and a normal level of atmospheric humidity.
  3. At the dacha, the use of sensors will allow you not to remember the time of watering each bed; electrical engineering itself will tell you about the amount of water in the soil. This will prevent overwatering if it has recently rained.
  4. The use of sensors is very convenient in some other cases. For example, they will allow you to control soil moisture in the basement and under the house near the foundation. In an apartment, it can be installed under the sink: if the pipe starts to drip, the automation will immediately report this, and flooding of neighbors and subsequent repairs can be avoided.
  5. A simple sensor device will allow you to fully equip all problem areas of your home and garden with a warning system in just a few days. If the electrodes are long enough, they can be used to control the water level, for example, in an artificial small reservoir.

Making your own sensor will help you equip your home with an automatic control system at minimal cost.

Factory-made components can be easily purchased via the Internet or in a specialized store; most of the devices can be assembled from materials that can always be found in the home of an electrical engineering enthusiast.

More information can be found in the video.

This simple homemade device is used for water or other liquid, in various rooms or containers. For example, these sensors are very often used to detect possible flooding of the basement or cellar with melt water or in the kitchen under the sink, etc.


The role of the humidity sensor is performed by a piece of foil fiberglass with grooves cut into it, and as soon as water gets into them, the machine will disconnect the load from the network. Or if we use the rear contacts, the automatic relay will turn on the pump or the device we need.

We manufacture the sensor itself in exactly the same way as in the previous diagram. If liquid gets on the contacts of the F1 sensor, the sound alarm will begin to emit a constant sound signal, and the HL1 LED will also light up.

Using the SA1 toggle switch, you can change the order of the HL1 indication to a continuous LED glow in standby mode.

This humidity sensor circuit can be used as a rain detector, overflow of a liquid container, water leakage, etc. The circuit can be powered from any five-volt DC power source.

The source of the sound signal is a sound emitter with a built-in sound generator. The humidity sensor is made from a strip of foil PCB with a thin track in the foil. If the sensor is dry, the sound signal does not signal. If the sensor gets wet, we will immediately hear an intermittent alarm signal.

The design is powered by a Krona battery and will last for two years, because during standby mode, the circuit consumes almost zero current. Another bonus of the circuit is the fact that almost any number of sensors can be connected parallel to the input and thus cover the entire controlled area at a time. The detector circuit is built on two 2N2222 type transistors connected in a Darlington manner."

List of radio components

R1, R3 - 470K
SW1 - button
R2 - 15k
SW2 - switch
R4 - 22K
B1 - crown type battery
C1 - capacitor with a capacity of 0.022 µF
T1, T2 - input terminals
PB1 - (RS273-059) piezo buzzer
Q1, Q2 - 2N2222 type transistors

When the first transistor opens, it immediately unlocks the second, which turns on the piezo buzzer. In the absence of liquid, both transistors are securely off and very low current is consumed from the battery. When the buzzer is turned on, the current consumption increases to 5 mA. Sound emitters of type RS273-059 have a built-in generator. If a more powerful alarm is needed, connect several buzzers in parallel or use two batteries.

We manufacture printed circuit boards with dimensions of 3*5 cm.

The test toggle switch connects a 470 kOhm resistance to the input, simulating the action of a liquid, thereby checking the functionality of the circuit. Transistors can be replaced with domestic ones, such as KT315 or KT3102.

An automatic humidity sensor is designed to turn on forced ventilation of a room at high air humidity; it can be installed in the kitchen, bathroom, cellar, basement, garage. Its purpose is to turn on the fans for forced ventilation of the room when the humidity in it approaches 95... 100%.

The device is highly economical, reliable, and its simplicity of design makes it easy to modify its components to suit specific operating conditions. The diagram of the humidity sensor is shown in the figure below.

The scheme works as follows. When the air humidity in the room is normal, the resistance of the dew sensor - gas resistor B1 does not exceed 3 kOhm, transistor VT2 is open, the powerful high-voltage field-effect transistor VT1 is closed, the primary winding of transformer T1 is de-energized. The load connected to the XP1 connector will also be de-energized.

As soon as the air humidity approaches the point of dew, for example, a boil left unattended, the bathroom is filled with hot water, the cellar is flooded with melted water, groundwater, the thermostat of the water heater has failed, the resistance of the gas resistor B1, the sharp alternating current is removed from the secondary winding T1 and supplied to the bridge diode rectifier VD2. Rectified voltage ripples are smoothed out by a high-capacity oxide capacitor C2. The parametric DC voltage stabilizer is built on a composite transistor VT3 with a high base current transfer coefficient of the KT829B type, a zener diode VD5 and a ballast resistor R6.

Capacitors SZ, C4 reduce output voltage ripple. Fans with an operating voltage of 12...15V, for example, “computer” fans, can be connected to the output of the voltage stabilizer. Fans with a total power of up to 100 W, designed for a supply voltage of 220 VAC, can be connected to the XP1 socket. A bridge rectifier VD1 is installed in the open supply circuit of the step-down transformer T1 and the high-voltage load. A pulsating DC voltage is supplied to the drain of the field-effect transistor. The cascade on transistors VT1, VT2 is powered by a stabilized voltage of +11 V, set by the zener diode VD7. The voltage is supplied to this zener diode through the chain R2, R3, VD4, HL2. This circuit design allows the field-effect transistor to be opened completely, which significantly reduces the power dissipated on it.

Transistors VT1, VT2 are included as a Schmitt trigger, which prevents the field-effect transistor from being in an intermediate state, which prevents it from overheating. The sensitivity of the humidity sensor is set by trimming resistor R8, and, if necessary, by selecting the resistance of resistor R7. Varistors RU1 and RU2 protect device elements from damage by network voltage surges. The green LED HL2 indicates the presence of supply voltage, and the red LED HL1 signals high humidity and the device is switched to forced ventilation mode.

You can connect up to 8 low-voltage fans with a current consumption of up to 0.25 A each to the device and, or several fans with a supply voltage of 220 V. If using this device it is necessary to control a more powerful load with a supply voltage of 220 V, then to the output voltage stabilizer, you can connect electromagnetic relays, for example, type G2R-14-130, the contacts of which are designed for switching alternating current up to 10 A at a voltage of 250 V. In parallel with resistor R8, you can install a thermistor with negative TKS, resistance 3.3...4, 7 kOhm at 25°C, placed, for example, above a gas or electric stove, which will allow you to turn on the ventilation also when the air temperature rises above 45...50 °C, when the stove burners are operating at full power.

In place of transformer T1, you can install any step-down transformer with an overall power of at least 40 W, the secondary winding of which is designed for a current value not less than the current of the low-voltage load. Without rewinding the secondary winding “Yunost”, “Sapphire”. Unified transformers TPP40 or TN46-127/220-50 are also suitable. When making a transformer yourself, you can use an W-shaped magnetic core with a cross-section of 8.6 cm2. The primary winding contains 1330 turns of wire with a diameter of 0.27 mm.

Secondary winding 110 turns of winding wire with a diameter of 0.9 mm. Instead of the KT829B transistor, any of the KT829, KT827, BDW93C, 2SD1889, 2SD1414 series will do. This transistor is installed on a heat sink, the size of which will depend on the load current and the magnitude of the collector-emitter voltage drop VT3. It is advisable to choose a heat sink with which the temperature of the VT3 transistor body would not exceed 60°C.

If the voltage on the plates of capacitor C2 with a load connected to the output of the stabilizer is more than 20 V, then to reduce the power dissipated by VT3, you can unwind several turns from the secondary winding of the transformer. The field effect transistor IRF830 can be replaced with KP707V2, IRF422, IRF430, BUZ90A, BUZ216. When installing this transistor, it must be protected from breakdown by static electricity. Instead of SS9014, you can use any of the KT315, KT342, KT3102, KT645, 2SC1815 series. When replacing bipolar transistors, take into account the differences in pinouts.

KBU diode bridges can be replaced with similar ones KVR08, BR36, RS405, KBL06. Instead of 1N4006, you can use 1N4004 - 1N4007, KD243G, KD247V, KD105V. Zener diodes: 1N5352 - KS508B, KS515A, KS215Zh; 1N4737A - KS175A, KS175Zh, 2S483B; 1 N4741A - D814G, D814G1, 2S211ZH, KS221V.

LEDs can be of any general use, for example, AL307, KIPD40, L-63 series. Oxide capacitors are imported analogs of K50-35, K50-68. Varistors - any low or medium power for a classified operating voltage of 430 V, 470 V, for example, FNR-14K431, FNR-10K471. The gas resistor GZR-2B, sensitive to air humidity, was taken from an old domestic video recorder “Electronics VM-12”. A similar gas resistor can be found in other faulty domestic and imported VCRs or in old cassette video cameras. This gas resistor is usually bolted to the metal chassis of the tape drive. Its purpose is to block the operation of the device when the tape mechanism fogs up, which prevents the magnetic tape from wrapping and damaging. The device can be mounted on a printed circuit board measuring 105x60 mm. It is preferable to place the gas resistor in a separate box made of insulating material with holes, installed in a cooler place. It is also recommended to screw it to a small metal plate, perhaps through a thin mica insulating spacer. To protect the mounted board from moisture, the mounting and printed conductors are coated with several layers of FL-98, ML-92 varnish or tsaponlac.

There is no need to paint over the gas resistor. To check the device’s functionality, you can simply exhale air from your lungs onto the gas resistor or bring a container of boiling water closer. After a few seconds, the HL1 LED will flash and the fans connected as loads will begin to fight the increased humidity. In standby mode, the device consumes current from the network about 3 mA, which is very little. Since the device consumes less than 1 W of power in standby mode, it can be operated around the clock without worrying about power consumption. Since the device is partially galvanically connected to the 220 V AC mains voltage, appropriate precautions should be taken when setting up and operating the device.

As a result of numerous experiments, this soil sensor circuit appeared on one single chip. Any of the microcircuits will do: K176LE5, K561LE5 or CD4001A.

The air humidity sensor, the diagram and drawings of which are attached, makes it possible to fully automate the process of monitoring and managing the relative humidity of the air in any room. This humidity sensor circuit makes it possible to measure relative humidity in the range from 0–100%. With very high accuracy and stability of parameters

Light and sound alarm for water boiling. - Radio, 2004, No. 12, pp. 42, 43.
. - Circuitry, 2004, No. 4, pp. 30-31.
Constant" in the cellar. - CAM, 2005, No. 5, pp. 30, 31.


If you forget for a long time about the dishes with water placed on a hot stove, several liters of water evaporated and the dishes are damaged will not make you happy. To prevent this from happening, you can assemble a simple device that, when placed, for example, in the kitchen, will notify you with sound signals about high humidity in the room.

The circuit diagram of the high air humidity alarm is shown in Fig. 1. At the same time, it can also signal about a puddle that has formed on the floor, which will reduce troubles in the event of damage to plumbing or heating equipment or the sink overflowing when the tap is left open for a long time and the drain hole is clogged.

Rice. 1. Diagram of a high air humidity alarm

Gas resistor B1 is used as a sensitive element in the alarm device. These were used in video cassette recorders and video cameras to block the operation of the tape mechanism when the air humidity inside the device was high. Logic elements DD1.1 and DD1.2 form a pulse generator that follows approximately 15 times per minute. This frequency is set by resistors R13, R15, R16 and capacitor C9. Thanks to the VD7 diode, the pulses are significantly (about 10 times) shorter than the pauses between them.

When the gas resistor is dry, its resistance does not exceed 1...3 kOhm and the voltage at the connection point of resistors R4, R5, R7 is not enough to open transistor VT1. Transistor VT2 is also closed. The logical voltage level at the lower (according to the circuit) input of element DD1.1 is low, which prohibits the operation of the pulse generator on elements DD1.1 and DD1.2, and the output of element DD1.2 is set to a low level, which in turn prohibits the operation of the pulse generator audio frequency on elements DD1.3 and DD1.4.

If the humidity of the air surrounding the gas resistor increases (to check, it is enough to exhale two or three times onto the gas resistor from a distance of 5...10 cm), then the resistance of the gas resistor will increase to 10...20 MOhm. With the increased voltage at the base, transistor VT1 will open, and transistor VT2 will open along with it. A high logical voltage level will be set at the lower (according to the diagram) input of element DD1.1. Both pulse generators will work. The piezo sound emitter HA1 will emit sound signals every 4 seconds lasting about 0.5 seconds.

Feedback through resistor R7 accelerates the opening and closing of transistors VT1, VT2 and creates a slight hysteresis in their switching characteristics. This ensures clear, rattling-free operation of the alarm when the humidity slowly approaches the threshold. The response threshold is set using trimming resistor R3.

The device will also give a signal if transistor VT1 remains closed and transistor VT2 opens as a result of contacts E1 and E2 being closed by spilled water. Resistors R6 and R8 not only limit the base current of transistor VT2, but also reduce the risk of electric shock to a person who touches the contacts. Mains voltage can reach them as a result of water penetration inside the alarm device or failure of the insulation between the windings of transformer T1.

To prevent the alarm from annoying you with sound signals while the reasons for its operation are eliminated, by pressing the SB1 button you can block the operation of the generators for approximately 18 minutes. For this amount of time, capacitor C8, discharged by pressing the button, will be charged through resistor R17. Resistor R22 limits the discharge current of the capacitor, protecting the button contacts from burning. It should be noted that the restoration of the low resistance of gas resistor B1 after exposure to high humidity is very slow. Therefore, to get rid of annoying signals, you may need to press the SB1 button several times.

The piezo sound emitter HA1 is connected to the outputs of elements DD1.3, DD1.4 through emitter followers on transistors VT5, VT6 and VT7, VT8. This increases the load capacity of the generator and makes it possible to connect several sound emitters to it in parallel, placing them, for example, in different rooms.

The HL1 LED signals that the alarm device is connected to the network, and the HL2 LED turns on when sound signals are given, as well as when the generators are blocked by a low voltage level on capacitor C8. Capacitors C1 and C2 prevent false alarms caused by interference.

The 220 V mains voltage is supplied to the primary winding of the step-down transformer T1 through protective resistors R1 and R2. Varistor RU1 protects the transformer from network voltage surges. A voltage of about 17 V from the secondary winding of the transformer rectifies the diode bridge VD2-VD5. All components of the stabilizer are powered by a voltage of +9.2 V, obtained from a voltage rectified using transistors VT3 and VT4 using a stabilizer. Its value depends on the stabilization voltage of the zener diode VD6.

Since the design uses a low-power step-down transformer from a Xerox copy machine as T1, designed for a load current of about 10 mA, the current through the zener diode is chosen to be very small - less than 1 mA. The small power of the transformer also determined the choice of the nature of the sound signal - a short tone pulse and a long pause.

You can also use a more powerful transformer, for example TPK-2-12V, designed for a load current of up to 0.21 A. For self-manufacturing of a transformer, an W-shaped magnetic core with a cross-sectional area of ​​the central rod of 2 cm 2 is suitable. The primary winding should consist of 5900 turns of winding wire with a diameter of 0.06 mm. The secondary winding, containing 500 turns, is wound with wire with a diameter of about 0.2 mm. The magnetic circuit plates are assembled across the roof. The finished transformer can be coated with epoxy compound.

Most of the device parts are placed on a circuit board measuring 75x45 mm, shown in Fig. 2. Resistors R6, R8 and resistors R1, R2 with varistor RU1 are mounted on small separate boards.

Rice. 2. Placement of device parts on a circuit board measuring 75x45 mm

A ready-made board from the network adapter was also used, on which diodes VD2-VD5 and capacitor C3 were installed. After manufacturing, all these boards are coated on the installation side with a moisture-proof varnish, for example XB-784. Together with the T1 transformer, they are placed in a plastic case with dimensions of 160x110x32 mm from the RR-701R security alarm receiver.

Gas resistor B1, removed from the Funai VCR, is mounted on a massive metal plate and together with it is placed in a plastic case measuring 46x42x15 mm (Fig. 3) with holes for air access. Its sensitivity is significantly higher than that of the domestic gas resistor GZR-2B, used in the design described in the article “Light and sound indicator for water boiling away” (Radio, 2004, No. 12, pp. 42, 43). Nevertheless, GZR-2B and other similar gas resistors can also work in the described alarm device.

Rice. 3. Gas resistor B1 on a metal plate

The device can use fixed resistors of any type (MLT, S1-4, S1-14, S2-23). It is desirable that resistors R1 and R2 are non-flammable. Trimmer resistor R3 is miniature in a housing that protects it from external influences. It is highly undesirable to use open trimming resistors (for example, SP3-38) due to their low reliability. Varistor RU1 - HEL14D471K or another disk type with a classification voltage of 470 V.

Oxide capacitors - K50-68, K53-19, K53-30 and their imported analogues. Capacitor C8 should have low leakage current. The copy used by the author has a leakage current of less than 10 nA at a voltage of 18 V. The remaining capacitors are ceramic K10-17, K10-50, KM-5 or their analogues. Capacitor C4 must be designed for a voltage of at least 35 V.

Instead of diodes 1 N4002, any of the 1N4001-1 N4007, UF4001 -UF4007, as well as the KD208, KD209, KD243 series are suitable. Diodes 1N4148 can be replaced with 1SS244, 1N914, KD510A, KD521A, KD521B, KD522A, KD522B. Zener diode BZV55C-10 is replaced by TZMC-10, KS210Ts, KS210Ts1, 2S210K1, 2S210K, 2S210Ts, transistors 2SC1685 and 2SC2058 - with 2SC1815, 2SC1845, SS9014, as well as the KT3102, KT6111 series , and transistor 2SA1015 - on SS9012, SS9015, 2SA733 or series KT3107, KT6112. Replacing transistors 2SC2331 - 2SC2383, SS8050, BD136, BD138, KT646A, KT683A. Instead of transistors 2SA1273 and 2SA1270, SS8550, 2SB564, BD231, KT639A, KT644A, KT684A are suitable. Please note that replacement transistors may differ in case type and pin layout.

The K561LA7 chip will be replaced by the domestic KR1561LA7, N564LA7, 564LA7 (the last two in different packages) or the imported CD4011A.

Choke L1 is a small-sized industrial-made inductance with an inductance of at least 100 μH and a winding resistance of 3...30 Ohms. Button SB1 - PKN-125.

Sound emitter HA1 is a piezoelectric ringing device for a telephone set. Its own capacitance is 0.03 µF. Other piezo emitters are also suitable, even larger ones, designed for a voltage of at least 20 V. Several such emitters can be connected in parallel. Instead of a piezo emitter, an electromagnetic telephone capsule or a dynamic head with a winding resistance of at least 32 Ohms, for example PQAS57P3ZA-DZ, can be connected to the output of the device through a non-polar coupling capacitor.

A water leakage sensor can be made, for example, from a plate of fiberglass foil on one side. The foil is divided along a broken line by a gap into two insulated parts, one of which serves as electrode E1, and the second as electrode E2. The longer the gap, the higher the probability that the first drops of water falling on the plate will fall on it and short-circuit the electrodes.

Several of these sensors, connected in parallel, can be placed in the most dangerous places from the point of view of water leakage, for example, under heating radiators, washing machines, and water pipe joints. The box with the gas resistor is placed in the place of the room most susceptible to fogging at high humidity, but not on the window.

Trimmer resistor R3 sets the alarm threshold. If the “dry resistance” of gas resistor B1 takes too long to recover after a decrease in humidity, resistors R4 and R5 with three times less resistance can be installed in the alarm. You can increase the sensitivity of the leaked water sensor by increasing the resistance of resistor R9 to 100 kOhm. By selecting the resistance of resistor R20, you can set the desired tone of the sound signals. To make it easier to check the functionality and set up the alarm, capacitor C8 can be temporarily disabled.


Publication date: 13.09.2015

Readers' opinions
  • Ivan / 04/05/2016 - 09:28
    Is there a block diagram, a description of the microcircuits and a printed circuit board?

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 activities of 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.

Instruments that measure humidity are called hygrometers. They can also be called humidity sensors. In everyday life, humidity is an important parameter. It is important for farmland and machinery.

Human health depends on the percentage of humidity. Weather-dependent people are very sensitive to this parameter. The health of patients with asthma and hypertension also depends on it. When the air is dry, healthy people feel drowsiness, skin irritation, and itching. Excessively dry air provokes respiratory diseases.

In plants and factories, humidity affects the safety of raw materials and manufactured products, and machines. In agricultural land, humidity affects the soil and its fertility. To obtain information about humidity, hygrometers (humidity sensors) are used.

Classification of humidity sensors

Some devices are made calibrated for a certain humidity, but for precise adjustment you need to know the exact value of this parameter in the air.

Humidity is measured using the following parameters:
  • Air and gases are determined by humidity in g*m 3 at an absolute value, or at a relative value in RN.
  • Solid objects, liquids, measured in % of sample weight .
  • Liquids that do not mix, the humidity is measured parts of water(ppm).
Capacitive humidity sensors

These sensitive elements can be thought of as elementary capacitors with two plates with air between them. This is the simplest design. Air does not conduct electricity when dry. When it changes, the capacitance of the capacitor also changes.

A more complex design is a capacitive sensor with a dielectric, which changes significantly with humidity. This method improves the quality of the sensor compared to the air type.

The second type is best used for measurements on solid objects. The object is placed between the plates of a capacitor, which is connected to the oscillation circuit, to the generator. The frequency of the oscillation circuit is measured, and the capacitance of the sample is calculated based on the result.

This method of measurement contains negative aspects. When the moisture content of the material is less than 0.5 percent, the accuracy will be low, the material must be free of high permeability substances. The most important is also the geometric shape of the object, which should not change in the experiment for measuring humidity.

The third type of sensor is a thin film hygrometer, which includes a substrate with two comb-shaped electrodes. They are coverings. To compensate the temperature, 1 thermocouple is included in 1 sensor.

Resistive humidity sensors

Resistive sensors consist of 2 electrodes. They are applied to a substrate. A layer of conductive material is applied to the electrodes. But this material changes its resistance value significantly depending on humidity.

Aluminum oxide became a material suitable for sensitivity. It absorbs moisture from the outside, its resistance changes significantly. As a result, the total resistance of the sensor network is highly dependent on humidity. The passing current value will indicate the humidity value. The advantage of such sensors is their low cost.

Thermistor version of the sensor

A thermistor hygrometer includes two thermistors of the same type. These are nonlinear components. Their resistance is directly proportional to temperature. One of the thermistors is located in a sealed chamber with dry air. The 2nd thermistor is located in a chamber with holes. Humid air flows through them. This humidity needs to be determined. The thermistors are connected in a bridge circuit. The potential difference is applied to one diagonal, readings are taken from the other.

At zero voltage at the output of the thermistors, their temperature is the same, so the humidity of both thermistors is also equal. At zero voltage the humidity is different. Therefore, humidity is calculated from the measured voltage.

The question arises: why does the thermistor temperature change when humidity changes? You can answer this way. As humidity increases, water evaporates from the surface of the thermistor and the temperature of the thermistor decreases. The higher the humidity indicator, the more rapidly these processes proceed, and the thermistor cools down faster.

Optical humidity sensors

Its action for determining humidity is based on the dew point. When this dew point state is reached, the liquid and gas reach thermodynamic equilibrium.

If the glass is placed in a gas environment with a temperature above the dew point, and the temperature of the glass is further reduced, condensation will form on the glass. This is the process of water turning into a liquid state. The temperature of this transition is called the dew point. The temperature of this point depends on the pressure and humidity of the environment. As a result, if we can determine temperature and pressure, we can easily calculate humidity. This method is the main one.

A simple sensor circuit includes an LED that emits light onto the surface of a mirror, reflecting it and changing its direction. In our case, it is possible to change the temperature of the mirror by heating or cooling with a particularly precise temperature control device. A thermoelectric pump can be used. A temperature sensor is mounted on the mirror.

Before starting measurements, the temperature of the mirror is set so that its value is greater than the dew point. Then the mirror is cooled. Water drops will form on the mirror, as a result of which the light beam coming from the LED will be refracted and scattered, which will lead to a decrease in the current in the photodetector.

Using information from the photodetector, the regulator will maintain the temperature on the mirror, and the temperature sensor will determine the temperature. Knowing the pressure and temperature, determine the humidity.

The optical sensor has maximum accuracy compared to other analogues. Among the disadvantages are the increased cost and considerable energy consumption, as well as maintenance, which consists of keeping the surface of the mirror clean.

Electronic hygrometer

Its operating principle is to change the electrolyte with which the insulating material is coated. There are devices with automatic heating that maintain the dew point temperature.

The dew point temperature is measured over a lithium chloride solution. This solution is very sensitive to the smallest changes in humidity. For maximum convenience, a thermometer is attached to the hygrometer. This hygrometer has increased accuracy and a small error. It can measure humidity at any ambient temperature.

Conventional electronic hygrometers with two electrodes are most famous. Two electrodes are stuck into the soil. The degree of current conductivity determines humidity. Before purchasing a sensor, you need to decide what it will be used for, measurement range, accuracy, etc. The most accurate device is an optical sensor. Depending on the conditions, you need to pay attention to the protection class and measurement temperature range.

DIY humidity sensors
Many craftsmen want to make a hygrometer for a fan with their own hands. For this work they will need modern digital devices:
  • Touch sensors and temperatures (DНТ 11, DНТ 22).
  • Data processing device based on Arduino.

– a device consisting of a set of microprocessors assembled on inexpensive microcontrollers. It has open, understandable schemes. Anyone can find out on the Internet what components are included in the scheme and what the price will be. Connecting a fan to such a device is not difficult. An interesting fact is the interaction of such a device with a computer. There are many drivers and special programs with which you can work and perform various operations.

Considering the current cost, I would like to make my own exhaust fan complete with a humidity sensor. But such devices are recommended to be manufactured for more complex tasks. You can, for example, connect many different equipment into one network. Many companies install humidity sensors on their equipment. As a result, there is no serious point in doing this and doing what has already been done a long time ago.

If you make a humidifier for your home and try to connect it to a fan, then it's a completely different matter. For such purposes, it is necessary to develop several schemes.

You can also find a similar humidity sensor for a fan. These are available on Honeywell equipment. Their action is based on the way the capacitor operates. Concepts such as “special polymer insulation” or “platinum electrodes” can be daunting. These devices are not cheap. First you need to study this issue and decide whether it is necessary or not. Assembling the circuit for measuring the analog value and calibrating the sensor will be quite difficult work.

The Regeltechnik company produces combined sensors for measuring humidity and temperature, both for the external environment and for inside buildings and premises.

Duct humidity sensors

There are channel type hydrostats. Their application is still not very clear. In factory conditions this can somehow be explained. A power plant has control over many parameters. There, high humidity in the ventilation duct can be detected by the automatic control system as a malfunction of the equipment.

For household purposes, a duct fan with a humidity sensor is not useful anywhere, since it is not intended for monitoring environmental values. If a duct fan is operated in many rooms at once, and moisture forms in the duct, then this is a command signal to increase the speed of the fan motor. This occurs at close to idle speed. In this case, a humidity sensor with a fan will become a powerful energy saving system. This system will only operate at full capacity when necessary.

You can also control the operation of the recuperator and similar equipment. Its meaning is that during normal operation, electricity is saved.

It is recommended to create humidity within the range of 40-60 percent. Sometimes in such cases the task of moisturizing appears. A fan with a humidification device can reach nominal parameters automatically, since it has a built-in hygrostat, in other words, a steam generator. These devices are in demand in summer in dry climatic conditions. Fans can fight the vagaries of nature using a digitally controlled system. There is no such thing as bad weather, but the microclimate can always be optimized.