Moisture alarm. High humidity alarm Moisture content sensors

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, technology.

Human health depends on the percentage of humidity. weather dependent people 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, itching. Excessively dry air provokes respiratory diseases.

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

Classification of humidity sensors

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

Humidity is measured by the parameters:

• Air and gases are determined by humidity in g * m 3 at an absolute value, or at a relative value in RN.
• Solids, liquids, measured in % of sample weight .
• Liquids that are not miscible, moisture is measured parts of water(ppm).

Capacitive humidity sensors

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

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

The second type is best used for measurements on solid objects. The object is placed between the plates of the 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 from 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 poor, the material must be free from substances with high permeability. The most important is also the geometric shape of the object, which should not change in the experiment to measure the humidity.

The third type of sensor is a thin-film hygrometer, which includes a substrate with two comb-shaped electrodes. They are covers. For temperature compensation, 2 thermocouples are included in 1 sensor.

Resistive humidity sensors

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

Aluminum oxide has become a suitable sensitivity material. It absorbs moisture from the outside, its resistance changes significantly. As a result, the impedance of the sensor network has a large dependence on humidity. The value of the passing current will show about the humidity value. The advantage of such sensors is their low cost.

Thermistor version of the sensor

The thermistor hygrometer includes two thermistors of the same type. These are non-linear 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 in the chamber with holes. Through them comes wet air. This humidity needs to be determined. Thermistors are connected in a bridge circuit. The potential difference is applied to one diagonal, the 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 when the humidity changes, the temperature of the thermistor changes. You can answer like this. When the humidity rises, water evaporates from the surface of the thermistor and the temperature of the thermistor decreases. The higher the humidity index, the more rapidly these processes proceed, the thermistor cools down faster.

Optical humidity sensors

At the heart of its action of determining humidity is the dew point. When this dew point condition is reached, the liquid and gas acquire the equilibrium of thermodynamics.

If the glass is placed in a gaseous environment with a temperature above the dew point, then the temperature of the glass is lowered further, then condensation will appear on the glass. This is the process by which water changes to 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 medium. As a result, if we can determine the temperature and pressure, then we can easily calculate the humidity. This method is the main one.

A simple sensor circuit includes an LED that emits light onto the surface of a mirror that reflects and changes its direction. In our case, it is possible to change the temperature of the mirror by heating or cooling with a temperature control device of particular accuracy. You can use a thermoelectric pump. 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 droplets 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 current in the photodetector.

Possessing 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 the highest accuracy compared to other analogues. Among the disadvantages are the increased cost and considerable energy consumption, as well as maintenance, which consists in maintaining the surface of the mirror in a clean state.

Electronic hygrometer

Its principle of operation is to change the electrolyte with which the insulating material is coated. Self-heating devices are available to maintain the dew point temperature.

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

Conventional electronic hygrometers with two electrodes are widely known. Two electrodes are stuck into the soil. Humidity is determined by the degree of current conductivity. 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, measurement temperature range.

DIY humidity sensors

Many craftsmen want to make a hygrometer for a fan with their own hands. For such work, they will need modern digital devices:

• Sensors and temperatures (DHT 11, DHT 22).
• Arduino-based data processing device.

- a device consisting of a set of microprocessors assembled on inexpensive microcontrollers. It has open clear schemes. Anyone can find out on the Internet what components are included in the scheme, 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 an exhaust fan with my own hands, complete with a humidity sensor. But such devices are recommended to be made for more difficult tasks. It is possible, for example, to connect many different equipment. Many firms mount humidity sensors on manufactured equipment. As a result, it makes no serious sense to do this, and do 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 this is a completely different matter. For such purposes, it is necessary to develop several schemes.

You can find a similar humidity sensor for the fan. These are available on Honeywell equipment. Their action is based on the way the capacitor works. Such concepts as “special polymer insulation” or “platinum electrodes” can scare away. These devices are not cheap. First you need to study this issue and decide whether it is necessary or not. Enough hard work there will be both an assembly of an analog value measurement circuit and a calibration of the sensor.

Regeltechnik manufactures combined type sensors for measuring humidity and temperature, both for the external environment and for indoor buildings and premises.

Duct humidity sensors

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

For a household, a duct fan with a humidity sensor will not be useful anywhere, since it is not designed to control environmental values. If the 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 when the mode is close to idle. In this case, the humidity sensor with fan will become powerful system saving electricity. The operation of this system at full capacity will be carried out only when necessary.

It is also possible to control the operation of the recuperator and similar equipment. Its meaning lies in the fact that in normal mode, electricity is saved.

Humidity is recommended to create within 40-60 percent. Sometimes the task of moisturizing appears in such cases. A fan with a humidification device can reach the nominal parameters automatically, since it has a built-in hygrostat, in other words a steam generator. These devices are in demand in summer period in dry climatic conditions. Fans can, with the help of a digitally controlled system, fight the vagaries of nature. bad weather does not happen, but the microclimate can always be optimized.

It's simple homemade device used for water or other liquid, in various rooms or containers. For example, these sensors are very often used to fix the possible flooding of the basement or cellar. melt waters 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 in it, and as soon as water gets into them, the machine will disconnect the load from the network. Or if you use the rear contacts, the automatic relay will turn on the pump or or the device we need.

We make the sensor itself in the same way as in the previous scheme. If liquid gets on the contacts of the sensor F1 sound signal The device will emit a constant sound signal, and the HL1 LED will also light up.

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

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

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

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

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

When the first transistor turns on, it immediately turns on the second one, which turns on the piezo buzzer. In the absence of liquid, both transistors are securely locked and very low current is drawn from the battery. When the buzzer turns on, the current drawn increases to 5 mA. RS273-059 sound emitters have a built-in generator. If you need a more powerful alarm, connect several buzzers in parallel or take two batteries.

We make a printed circuit board with dimensions of 3 * 5 cm.

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

The automatic humidity sensor is designed to turn on the forced ventilation of the room at high 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 the simplicity of the design makes it easy to modify its components for specific operating conditions. The diagram of the humidity sensor is shown in the figure below.

The scheme works as follows. When the humidity in the room is normal, the resistance of the dew sensor - gas resistor B1 does not exceed 3 kOhm, the transistor VT2 is open, the powerful high-voltage field effect transistor VT1 is closed, the primary winding of the 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 dew point, for example, boiled left unattended, the bathroom fills up hot water, the cellar is flooded with melted, groundwater, the temperature controller of the water heater has failed, the resistance of the gas resistor B1 is abruptly removed from the secondary winding T1 and fed to the bridge diode rectifier VD2. The ripple of the rectified voltage is smoothed out by a high-capacity oxide capacitor C2. Parametric Voltage Stabilizer direct current built on a composite transistor VT3 with a high current transfer coefficient of the base type KT829B, a zener diode VD5 and a ballast resistor R6.

Capacitors C3, C4 reduce the 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 can be connected to socket XP1 total power up to 100 W, rated for 220 V AC. A bridge rectifier VD1 is installed in the power supply circuit of the step-down transformer T1 and the high-voltage load. A pulsating DC voltage is applied to the drain of the field-effect transistor. The cascade on transistors VT1, VT2 is powered by a stabilized voltage of +11 V, given by the zener diode VD7. The voltage to this zener diode is supplied through the chain R2, R3, VD4, HL2. Such a circuit solution allows you to open the field-effect transistor completely, which significantly reduces the power dissipated on it.

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

Up to 8 low-voltage fans with current consumption up to 0.25 A each and or several fans with 220 V supply voltage can be connected to the device. 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 a negative TCR, 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 ventilation also when the air temperature rises above 45 ... 50 °C, when the stove burners operate at full power.

In place of the T1 transformer, 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 of at least the low-voltage load current. Without rewinding the secondary winding "Youth", "Sapphire". Also suitable are unified transformers TPP40 or TN46-127 / 220-50. With self-manufacturing of the transformer, you can use a W-shaped magnetic circuit 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 is suitable. 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 desirable to choose such a heat sink with which the temperature of the case of the transistor VT3 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, several turns can be unwound from the secondary winding of the transformer. Field-effect transistor IRF830 can be replaced by KP707V2, IRF422, IRF430, BUZ90A, BUZ216. When mounting 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.

Diode bridges KBU can be replaced with similar 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 application, for example, AL307, KIPD40, L-63 series. Oxide capacitors - imported analogues of K50-35, K50-68. Varistors - any low or medium power for a classification 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 video recorders 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 drive mechanism is fogged up, which prevents winding and damage to the magnetic tape. The device can be mounted on a 105x60 mm printed circuit board. 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, it is possible through a thin mica insulating gasket. To protect the mounted board from moisture, the mounting and printed conductors are covered with several layers of varnish FL-98, ML-92 or zaponlak.

The gas resistor does not need to be painted over with anything. To test the device for operability, you can simply exhale air from the 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 deal with high humidity. In standby mode, the device consumes a current from the network of 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 fear of power consumption. Since the device is partially galvanically coupled to the mains voltage of 220 V AC, appropriate precautions must be taken when setting up and operating the device.

As a result of numerous experiments, this soil sensor circuit appeared on a single microcircuit. Any of the microcircuits is suitable: K176LE5, K561LE5 or CD4001A.

The air humidity sensor, the scheme and drawings of which are attached, makes it possible to fully automate the process of monitoring and controlling the relative humidity 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 parameter stability

Light and sound signaling device of boiling water. - 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.

Water can become a source of great trouble if you do not know in time about its appearance where it is not expected and where it is undesirable, especially in large quantities.

To help a person in such cases and avoid many troubles can be an individual moisture alarm, which can be made very compact. The diagram of the signaling device is shown in Fig.1.

A tone generator is assembled on germanium transistors VT1, VT2, resistors R1, R2, capacitor C1 and a loudspeaker head, which, with good parts, starts to sound as soon as the supply voltage is applied to it. Power is supplied to the generator through a key stage on silicon transistors VT3, VT4, resistors R3 ... R5 and a moisture sensor. Many liquids are electrically conductive and therefore resistive. electric current. So, tap water has an ohmic resistance of several kilos.

Therefore, the ingress of moisture on the sensor is equivalent to the appearance between the base of the transistor VT3 and the "minus" of the power circuit of some resistance, which makes the electrical potential of the base of the transistor VT3 negative with respect to the emitter of this transistor. Such an inclusion for the transistor VT3 is opening, and a current begins to flow through it, which in turn leads to the opening of the transistor VT4. Both transistors, having opened, enter the saturation mode, the electronic key closes, and power is supplied to the tone generator through it. The loudspeaker head (0.5 GDSh-2) begins to sound, the tonality and volume of the sound of which can wake up even a soundly sleeping person. In the standby (standby) mode, the signaling device consumes a current much less than 1 μA. In the alarm mode (when water gets on the sensor), the device consumes no more than 80 mA. Since the device is very economical in standby mode, in the most critical cases, installing a power switch in it is even undesirable.

To make sure that the signaling device is turned on and operable, it is enough to close the plates of its sensor with wet fingers or something metallic. In case of serviceability, he will immediately give a "voice".

The scope of the moisture detector is not limited to security functions. He can follow

filling with liquid of any containers, or it can be used as an electronic "nanny". In the latter case, the sensor(s) are placed under the diapers. As soon as the diapers get wet, the "nanny" will immediately signal this. To bring the electronic "nanny" to the standby position, it is enough to wipe the sensor with a napkin or rag.

MP11A (MP35 ... MP38) can be used as VT1, and MP39 (MP16 ... MP42B) as VT2, i.e. any low-frequency low-power germanium transistors of appropriate conductivity. KT203 is used as VT3, KT814 is used as VT4. Radiator for VT4 is not needed. As a speaker head, any power of 0.25 ... 2 W with a nominal electrical resistance of 8 ohms is suitable. Mounting of the device can be either hinged or using a printed circuit board, the dimensions and configuration of which depend on the dimensions of the parts used and the device case.

As a device sensor, you can use a plate of one-sided foil material, on which contact strips are etched (Fig. 2). You can cut out sensor strips from copper foil and stick them on rubber, leather, etc. Strips should be tinned with solder. Some of the authors advise not to do this, because appearance from this it becomes artisanal. But if you tin well-cleaned and rosin-rubbed printed conductors with a well-heated, stripped and tinned tip of a powerful soldering iron, using small amounts of solder (this is a kind of "know-how"), then the quality of the coating is excellent. At the same time, defects in printed conductors due to microcracks are eliminated, and the service life is increased. printed circuit boards, especially those that, due to the application, cannot be coated with a protective varnish.

The smaller the distance between the strips of the sensor, the higher the likelihood that the alarm will work even if a few drops of rain hit the sensor. The length of the conductors connecting the sensor to the device can be from several tens of centimeters to several hundred meters.

S.N. Kovalenko, Zaporozhye