How to assemble a thermostat at home. Diagrams of thermostats for boilers with their own hands. How to make a thermostat for heating with your own hands Homemade high-temperature thermostat

In this article, we will consider devices that support a certain thermal regime, or signal that the desired temperature has been reached. Such devices have a very wide scope: they can maintain the desired temperature in incubators and aquariums, heated floors, and even be part of a smart home. For you, we have provided instructions on how to make a thermostat with your own hands and at a minimum cost.

A bit of theory

The simplest measuring sensors, including those that respond to temperature, consist of a measuring half-arm of two resistances, a reference and an element that changes its resistance depending on the temperature applied to it. This is more clearly shown in the picture below.

As can be seen from the diagram, the resistor R2 is a measuring element of a self-made thermostat, and R1, R3 and R4 are the reference arm of the device. This is a thermistor. It is a conductive device that changes its resistance with temperature.

The element of the thermostat that reacts to a change in the state of the measuring arm is an integrated amplifier in the comparator mode. This mode jumps the output of the microcircuit from the off state to the working position. Thus, at the output of the comparator, we have only two values ​​\u200b\u200b“on” and “off”. The chip load is a PC fan. When the temperature reaches a certain value in the shoulder R1 and R2, a voltage shift occurs, the input of the microcircuit compares the value on pin 2 and 3, and the comparator switches. The fan cools the required object, its temperature drops, the resistance of the resistor changes and the comparator turns off the fan. Thus, the temperature is maintained at a given level, and the operation of the fan is controlled.

Circuit overview

The difference voltage from the measuring arm is fed to a paired transistor with a high gain, and an electromagnetic relay acts as a comparator. When the voltage on the coil is sufficient to retract the core, it is triggered and connected through its contacts to the actuators. When the set temperature is reached, the signal on the transistors decreases, the voltage on the relay coil synchronously drops, and at some point the contacts are disconnected and the payload is disconnected.

A feature of this type of relay is the presence - this is a difference of several degrees between turning on and turning off a home-made thermostat, due to the presence of an electromechanical relay in the circuit. Thus, the temperature will always fluctuate by several degrees around the desired value. The assembly option provided below is practically devoid of hysteresis.

Schematic diagram of an analog thermostat for an incubator:

This scheme was very popular for repetition in the 2000s, but even now it has not lost its relevance and copes with the function assigned to it. If you have access to old parts, you can assemble a thermostat with your own hands almost for free.

The heart of the homemade product is the integrated amplifier K140UD7 or K140UD8. In this case, it is connected with positive feedback and is a comparator. The temperature-sensitive element R5 is a resistor of the MMT-4 type with a negative TKE, which means that when heated, its resistance decreases.

The remote sensor is connected via a shielded wire. To reduce and false operation of the device, the length of the wire should not exceed 1 meter. The load is controlled through the thyristor VS1 and the maximum allowable power of the connected heater depends on its rating. In this case, 150 watts, an electronic key - a thyristor must be installed on a small radiator to remove heat. The table below shows the ratings of radio elements for assembling a thermostat at home.

The device does not have galvanic isolation from the 220 Volt network, be careful when setting up, there is mains voltage on the regulator elements, which is life-threatening. After assembly, be sure to insulate all contacts and place the device in a non-conductive enclosure. The video below shows how to assemble a transistor thermostat:

Homemade transistor thermostat

Now we will tell you how to make a temperature controller for a warm floor. The working scheme is copied from a serial sample. Useful for those who want to familiarize themselves and repeat, or as a sample for troubleshooting the device.

The center of the circuit is a stabilizer chip, connected in an unusual way, the LM431 begins to pass current at a voltage above 2.5 volts. It is this value that this microcircuit has an internal source of reference voltage. At a lower current value, it does not miss anything. This feature of it began to be used in various schemes of temperature controllers.

As you can see, the classic circuit with a measuring arm remains: R5, R4 are additional resistors, and R9 is a thermistor. When the temperature changes, the voltage shifts at input 1 of the microcircuit, and if it has reached the response threshold, then the voltage goes further along the circuit. In this design, the load for the TL431 chip is the HL2 operation indication LED and the U1 optocoupler, for optical isolation of the power circuit from the control circuits.

As in the previous version, the device does not have a transformer, but is powered by a quenching capacitor circuit C1, R1 and R2, so it is also under life-threatening voltage, and you need to be extremely careful when working with the circuit. To stabilize the voltage and smooth out the ripples of the network bursts, a zener diode VD2 and a capacitor C3 are installed in the circuit. To visually indicate the presence of voltage on the device, the HL1 LED is installed. The power control element is a triac VT136 with a small strapping for control through the U1 optocoupler.

With these ratings, the control range is within 30-50°C. Despite the apparent complexity at first glance, the design is easy to set up and easy to repeat. A visual diagram of a thermostat on a TL431 chip, with an external 12 volt power supply for use in home automation systems is presented below:

This thermostat is able to control a computer fan, power relay, light indicators, sound alarms. To control the temperature of the soldering iron, there is an interesting scheme using the same TL431 integrated circuit.

To measure the temperature of the heating element, a bimetallic thermocouple is used, which can be borrowed from a remote meter in a multimeter or bought at a specialized radio parts store. To increase the voltage from the thermocouple to the triggering level of TL431, an additional amplifier is installed on the LM351. The control is carried out through the optocoupler MOC3021 and triac T1.

When the thermostat is connected to the network, the polarity must be observed, the minus of the regulator must be on the neutral wire, otherwise the phase voltage will appear on the body of the soldering iron, through the thermocouple wires. This is the main drawback of this circuit, because not everyone wants to constantly check the correct connection of the plug to the outlet, and if you neglect this, you can get an electric shock or damage electronic components during soldering. Range adjustment is done by resistor R3. This scheme will ensure the long operation of the soldering iron, eliminate its overheating and increase the quality of soldering due to the stability of the temperature regime.

Another idea for assembling a simple thermostat is discussed in the video:

Temperature controller on a TL431 chip

Simple regulator for soldering iron

The disassembled examples of temperature controllers are quite enough to meet the needs of the home master. The schemes do not contain scarce and expensive spare parts, are easily repeated and practically do not need to be adjusted. Homemade data can easily be adapted to control the temperature of the water in the water heater tank, monitor the heat in the incubator or greenhouse, upgrade the iron or soldering iron. In addition, you can restore an old refrigerator by converting the regulator to work with negative temperature values ​​by swapping the resistances in the measuring arm. We hope our article was interesting, you found it useful and understood how to make a thermostat with your own hands at home! If you still have questions, feel free to ask them in the comments.

Autonomous heating of a private house allows you to choose individual temperature conditions, which is very comfortable and economical for residents. In order not to set a different mode in the room every time the weather changes outside, you can use a thermostat or thermostat for heating, which can be installed both on radiators and on the boiler.

Automatic room temperature control

What is it for

• The most common in the Russian Federation is , on gas boilers. But such, so to speak, luxury is not available in all areas and localities. The reasons for this are the most banal - the lack of a thermal power plant or central boiler houses, as well as gas pipelines nearby.
• Have you ever visited a residential building, a pumping station or a weather station far from densely populated areas in the winter, when the only means of communication is diesel-powered sledges? In such situations, very often they arrange heating with their own hands using electricity.

• For small spaces, for example, one duty room at the pumping station is enough - it will be enough for the most severe winter, but for a larger area, a heating boiler and a radiator system will already be required. To maintain the desired temperature in the boiler, we bring to your attention a home-made control device.

Temperature sensor

• This design does not require thermistors or various TCM sensors., here instead of them a bipolar ordinary transistor is involved. Like all semiconductor devices, its operation is largely dependent on the environment, more precisely, on its temperature. As the temperature rises, the collector current increases, and this negatively affects the operation of the amplifying stage - the operating point shifts up to signal distortion and the transistor simply does not respond to the input signal, that is, it stops working.

• Diodes are also semiconductors., and an increase in temperature has a negative effect on them. At t25⁰C, the “continuity” of a free silicon diode will show 700mV, and that of a permanent one will show about 300mV, but if the temperature rises, then the direct voltage of the device will decrease accordingly. So, when the temperature rises by 1⁰C, the voltage will decrease by 2mV, that is, -2mV / 1⁰C.

• This dependence of semiconductor devices allows them to be used as temperature sensors. On such a negative cascade property with a fixed base current, the entire circuit of the thermostat is based (the diagram in the photo above).
• The temperature sensor is mounted on a transistor VT1 type KT835B, the load of the cascade is the resistor R1, and the DC mode of the transistor is set by the resistors R2 and R3. In order for the voltage at the transistor emitter at room temperature to be 6.8V, a fixed bias is set by resistor R3.

Advice. For this reason, R 3 is marked with an * in the diagram, and special accuracy should not be achieved here, if only there were no large drops. These measurements can be made with respect to a transistor collector connected to a common drive power supply.

• Transistor p-n-p KT835B specially selected, its collector is connected to a metal case plate, which has a hole for attaching the semiconductor to the radiator. It is through this hole that the device is attached to the plate, to which the underwater wire is still attached.
• The assembled sensor is attached to the heating pipe with metal clamps., and the structure does not need to be isolated with any gasket from the heating pipe. The fact is that the collector is connected by one wire to the power source - this greatly simplifies the entire sensor and makes contact better.

Comparator

• comparator, mounted on an operational amplifier OP1 type K140UD608, sets the temperature. Voltage is supplied to the inverted input R5 from the emitter VT1, and through R6, voltage is supplied to the non-inverted input from the engine R7.
• This voltage determines the temperature for disconnecting the load. The upper and lower ranges for setting the threshold for the operation of the comparator are set using R8 and R9. The necessary posteresis of the comparator operation is provided by R4.

Load management

• On VT2 and Rel1 the load control device was made and the indicator of the thermostat operation mode is located here - red when heated, and green - reaching the required temperature. Parallel to the Rel1 winding, a VD1 diode is connected to protect VT2 from voltage caused by self-induction on the Rel1 coil when turned off.

Advice. The figure above shows that the permissible switching current of the relay is 16A, which means that it allows load control up to 3kW. Use the device for power 2-2.5kW to lighten the load.

Power Supply

• An arbitrary instruction allows for a real thermostat, in view of its low power, to use a cheap Chinese adapter as a power supply. You can also assemble a 12V rectifier yourself, with a circuit current consumption of not more than 200mA. For this purpose, a transformer with a power of up to 5W and an output of 15 to 17V will do.
• The diode bridge is made on 1N4007 diodes, and the voltage stabilizer is on an integral type 7812. In view of the low power, it is not necessary to install a stabilizer on the battery.

Adjustment of the thermostat

• To check the sensor, you can use the most ordinary table lamp with a metal shade. As noted above, room temperature allows you to withstand the voltage at the VT1 emitter of about 6.8V, but if you increase it to 90⁰C, then the voltage will drop to 5.99V. For measurements, you can use a conventional Chinese multimeter with a thermocouple type DT838.
• The comparator works as follows: if the voltage of the temperature sensor at the inverting input is higher than the voltage at the non-inverting one, then at the output it will be equivalent to the voltage of the power source - this will be a logical unit. Therefore, VT2 opens and the relay turns on, moving the relay contacts to heating mode.
• The temperature sensor VT1 heats up as the heating circuit heats up, and as the temperature rises, the voltage at the emitter decreases. At the moment when it drops slightly below the voltage that is set on the R7 engine, a logical zero is obtained, which leads to the locking of the transistor and turning off the relay.
• At this time, the boiler does not receive voltage and the system begins to cool down, which also entails the cooling of the VT1 sensor. This means that the voltage at the emitter rises and as soon as it crosses the limit set by R7, the relay starts up again. This process will be repeated continuously.
• As you understand, the price of such a device is low, but it allows you to maintain the desired temperature in all weather conditions. This is very convenient in cases where there are no permanent residents in the room who monitor the temperature regime, or when people constantly replace each other and are also busy with work.

The operation of a gas or electric boiler can be optimized by using an external control of the unit. Commercially available remote thermostats are designed for this purpose. This article will help you understand what these devices are and understand their varieties. It will also address the question of how to assemble a thermostat with your own hands.

Purpose of thermostats

Any electric or gas boiler is equipped with an automation kit that monitors the heating of the coolant at the outlet of the unit and turns off the main burner when the set temperature is reached. Equipped with similar means and solid fuel boilers. They allow you to maintain the temperature of the water within certain limits, but nothing more.

In this case, the climatic conditions in the premises or on the street are not taken into account. This is not very convenient, the homeowner has to constantly select the appropriate mode of operation of the boiler on his own. The weather can change during the day, then the rooms become hot or cool. It would be much more convenient if the boiler automation was guided by the air temperature in the rooms.

To control the operation of the boiler depending on the actual temperature, various thermostats for heating are used. Being connected to the boiler electronics, such a relay turns off and starts heating, maintaining the required air temperature, and not the coolant.

Types of thermal relay

A conventional thermostat is a small electronic unit mounted on a wall in a suitable location and connected to a heat source by wires. On the front panel there is only a temperature controller, this is the cheapest kind of device.

In addition to it, there are other types of thermal relays:

• programmable: they have a liquid crystal display, are connected using wires or use a wireless connection with the boiler. The program allows you to set the temperature change at certain hours of the day and by day during the week;
• the same device, only equipped with a GSM module;
• autonomous regulator powered by its own battery;
• wireless thermostat with remote sensor to control the heating process depending on the ambient temperature.

Note. The model, where the sensor is located outside the building, provides weather-dependent regulation of the operation of the boiler plant. The method is considered the most effective, since the heat source reacts to changing weather conditions even before they affect the temperature inside the building.

Multifunctional thermal relays that can be programmed significantly save energy. During those hours of the day when no one is at home, it makes no sense to maintain a high temperature in the rooms. Knowing the working schedule of his family, the homeowner can always program the temperature switch so that at certain hours the air temperature drops, and the heating is turned on an hour before people arrive.

Household thermostats, equipped with a GSM module, are able to provide remote control of the boiler plant via cellular communication. Budget option - sending notifications and commands in the form of SMS - messages from a mobile phone. Advanced versions of devices have their own applications installed on a smartphone.

How to assemble a thermostat yourself?

Commercially available heating control devices are quite reliable and do not cause any complaints. But at the same time, they cost money, and this does not suit those homeowners who are at least a little versed in electrical engineering or electronics. After all, understanding how such a thermal relay should function, you can assemble and connect it to the heat generator with your own hands.

Of course, not everyone can make a complex programmable device. In addition, to assemble such a model, it is necessary to purchase components, the same microcontroller, digital display and other parts. If you are a new person in this business and understand the issue superficially, then you should start with some simple scheme, assemble and put it into operation. Having achieved a positive result, you can aim at something more serious.

First you need to have an idea of ​​what elements a thermostat with temperature control should consist of. The answer to the question is given by the circuit diagram presented above and reflecting the algorithm of the device. According to the scheme, any thermostat must have an element that measures the temperature and sends an electrical impulse to the processing unit. The task of the latter is to amplify or convert this signal in such a way that it serves as a command to the actuating element - the relay. Next, we will present 2 simple circuits and explain their operation in accordance with this algorithm without resorting to specific terms.

Circuit with a zener diode

A zener diode is the same semiconductor diode that passes current in only one direction. The difference from a diode is that the zener diode has a control contact. As long as the set voltage is applied to it, the element is open and current flows through the circuit. When its value falls below the limit, the chain breaks. The first option is a thermal relay circuit, where the zener diode plays the role of a logical control unit:

As you can see, the circuit is divided into two parts. On the left side, the part preceding the control contacts of the relay (designation K1) is shown. Here, the measuring unit is a thermal resistor (R4), its resistance decreases with increasing ambient temperature. The manual temperature controller is a variable resistor R1, the circuit is powered by a voltage of 12 V. In normal mode, a voltage of more than 2.5 V is present on the control contact of the zener diode, the circuit is closed, the relay is on.

Advice. The 12 V power supply can be any inexpensive commercially available device. Relay - reed switch brand RES55A or RES47, thermal resistor - KMT, MMT or similar.

As soon as the temperature rises above the set limit, the resistance R4 will drop, the voltage will become less than 2.5 V, the zener diode will break the circuit. Then the relay will do the same, turning off the power section, whose circuit is shown on the right. Here, a simple thermal relay for the boiler is equipped with a triac D2, which, together with the closing contacts of the relay, serves as an execution unit. The supply voltage of the boiler is 220 V through it.

Circuit with logic chip

This circuit differs from the previous one in that instead of a zener diode, it uses a K561LA7 logic chip. The temperature sensor is still a thermistor (designation - VDR1), only now the decision to close the circuit is made by the logical unit of the microcircuit. By the way, the K561LA7 brand has been produced since Soviet times and costs mere pennies.

For intermediate amplification of the pulses, the KT315 transistor is used, for the same purpose, a second transistor, KT815, is installed in the final stage. This diagram corresponds to the left side of the previous one, the power block is not shown here. As you might guess, it can be similar - with the KU208G triac. The operation of such a home-made thermal relay has been tested on ARISTON, BAXI, Don boilers.

Conclusion

Connecting the thermostat to the boiler on your own is a simple matter, there are a lot of materials on this topic on the Internet. But making it yourself from scratch is not so easy, in addition, you need a voltage and current meter to make adjustments. Buy a finished product or take on its manufacture yourself - the decision is up to you.

I present an electronic development - a home-made thermostat for electric heating. The temperature for the heating system is set automatically based on changes in the outside temperature. The thermostat does not need to manually enter and change readings to maintain the temperature in the heating system.

In the heating system, there are similar devices. For them, the ratio of the average daily temperature and the diameter of the heating riser is clearly spelled out. Based on these data, the temperature for the heating system is set. This heating system table was taken as a basis. Of course, some factors are unknown to me, the building may turn out to be, for example, not insulated. The heat loss of such a building will be large, heating may not be sufficient for normal space heating. The thermostat has the ability to make adjustments for tabular data. (More information can be read at this link).

I planned to show a video in the operation of the thermostat, with an eclectic boiler (25KV) connected to the heating system. But as it turned out, the building for which all this was done was not residential for a long time, during the check, almost the entire heating system fell into disrepair. When everything will be restored, it is not known, perhaps it will not be this year. Since in real conditions I cannot adjust the thermostat and observe the dynamics of changing temperature processes, both in heating and on the street, I went the other way. For these purposes, he built a model of the heating system.

The role of an electric boiler is performed by a glass half-liter jar, the role of a heating element for water is a five-hundred-watt boiler. But with such a volume of water, this power was in abundance. Therefore, the boiler was connected through a diode, lowering the power of the heater.

Connected in series, two aluminum flow radiators take heat from the heating system, forming a kind of battery. With the help of a cooler, I create the dynamics of cooling the heating system, since the program in the thermostat monitors the rate of rise and fall of temperature in the heating system. On the return, there is a digital temperature sensor T1, based on the readings of which the set temperature in the heating system is maintained.

In order for the heating system to start working, it is necessary for the T2 (outdoor) sensor to record a decrease in temperature, below + 10C. To simulate changes in outdoor temperature, I designed a mini refrigerator on a peltier element.

It makes no sense to describe the work of the entire home-made installation, I filmed everything on video.

Some points about assembling an electronic device:

The electronics of the thermostat is located on two printed circuit boards, for viewing and printing you will need the SprintLaut program, version 6.0 or higher. The thermostat for heating is mounted on a DIN rail, thanks to the Z101 series case, but something does not prevent you from placing all the electronics in another case that is suitable in size, the main thing is that you are satisfied. The Z101 case does not have a window for the indicator, so you will have to mark and cut it yourself. The ratings of the radio components are indicated on the diagram, except for the terminal blocks. To connect the wires, I used the terminal blocks of the WJ950-9.5-02P series (9 pcs.), But they can be replaced with others, when choosing, take into account that the step between the legs matches, and the height of the terminal block does not prevent the case from closing. The thermostat uses a microcontroller that needs to be programmed, of course, I also provide the firmware in the public domain (it may have to be finalized during the work). When flashing the microcontroller, set the operation of the internal clock generator of the microcontroller to 8 MHz.

The electronic thermostat for refrigerator will help in cases where your own (factory) thermostat is faulty or its accuracy is no longer sufficient. Older refrigerators use a mechanical temperature thermostat using a liquid or gas filled capillary.

When the temperature changes, the pressure inside the capillary also changes, which is transferred to the membrane (bellows). As a result, the thermostat turns the refrigerator compressor on and off. Of course, such a temperature control system has low accuracy, and its parts wear out over time.

Description of the operation of the thermostat for the refrigerator

As you know, the temperature of food storage in the refrigerator should be + 2 ... 8 degrees Celsius. The operating temperature of the refrigerator is +5 degrees.

The electronic thermostat for the refrigerator is characterized by two parameters: the start and stop temperature (or the average temperature plus the hysteresis value) of the compressor. The hysteresis is necessary to prevent the refrigerator compressor from turning on too often.

This circuit provides a hysteresis of 2 degrees at an average temperature of 5 degrees. Thus, the refrigerator compressor turns on when the temperature reaches + 6 degrees and turns off when it drops to + 4 degrees.

This temperature range is sufficient to maintain the optimal storage temperature of the food, and at the same time it ensures comfortable operation of the compressor, preventing its excessive wear. This is especially important for already old refrigerators that use a thermal relay to start the engine.

The electronic thermostat is a suitable replacement for the original thermostat. The thermostat reads the temperature using a sensor, the resistance of which changes depending on the change in temperature. The thermistor (NTC) is often used for these purposes, but the problem lies in its low accuracy and the need for calibration.

To ensure an accurate setting of the controlled temperature and get rid of many hours of calibration, in this version of the thermostat for the refrigerator was chosen. It is an integrated circuit, linearly calibrated in degrees Celsius, with a factor of 10 mV per degree Celsius. Due to the fact that the threshold temperature is close to zero, the relative change in the output voltage is large. Therefore, the signal from the output of the sensor can be controlled using a simple circuit consisting of only two transistors.

Since the output voltage is too low to open the transistor VT1, the LM35 sensor is turned on as a current source. Its output is loaded with a resistor R1 and therefore the current on it changes in proportion to the temperature. This current causes a drop across resistor R2. The voltage drop controls the operation of the transistor VT1. If the voltage drop exceeds the threshold voltage of the base-emitter junction, transistors VT1 and VT2 open, relay K1 turns on, whose contacts are connected instead of the contacts of the old thermostat.

Resistor R3 creates positive feedback. This adds a small current to R2, which shifts the threshold and thus provides hysteresis. The winding of the electromagnetic relay must be rated for 5 ... 6 volts. The contact pair of the relay must withstand the required current and voltage.

The LM35 sensor is located inside the refrigerator in a suitable location. The resistance R1 is soldered directly to the temperature sensor, which in turn allows you to connect the LM35 to the circuit board with just two wires.

The wires connecting the sensor can introduce interference into the circuit, so capacitor C2 is added to suppress interference. The circuit is powered by a built-in 5 volt power supply. The current consumption mainly depends on the type of relay used. must be securely isolated from the network.

The big advantage of this circuit is that it starts working immediately at the first start and does not need to be calibrated and adjusted. If it becomes necessary to slightly change the temperature level, then this can be done by selecting the resistances R1 or R2. Resistance R3 determines the amount of hysteresis.

Portable USB oscilloscope, 2 channels, 40 MHz....

In everyday life and subsidiary farming, it is often necessary to maintain the temperature regime of a room. Previously, this required a rather huge circuit made on analog elements, we will consider one such circuit for general development. Today, everything is much simpler, if it becomes necessary to maintain the temperature in the range from -55 to +125 ° C, then the programmable thermometer and thermostat DS1821 can perfectly cope with the goal.

The temperature threshold for switching on and off the thermostat is set by the values ​​TH and TL in the sensor's memory, which must be programmed into the DS1821. If the temperature exceeds the value recorded in the TH cell, the level of a logical unit will appear at the sensor output. To protect against possible interference, the load control circuit is implemented in such a way that the first transistor is locked into that half-wave of the mains voltage when it is zero, thereby applying a bias voltage to the gate of the second field-effect transistor, which turns on the opto-triac, and it already opens the VS1 smystor that controls the load . The load can be any device, such as an electric motor or a heater. The reliability of locking the first transistor must be adjusted by selecting the desired value of the resistor R5.

The DS1820 temperature sensor is capable of detecting temperatures from -55 to 125 degrees and operating in thermostat mode.

If the temperature exceeds the upper threshold TH, then the output of the DS1820 will be a logical unit, the load will turn off the network. If the temperature drops below the lower programmed level TL, then a logical zero will appear at the output of the temperature sensor and the load will be turned on. If there were unclear moments, the home-made design was borrowed from No. 2 for 2006.

The signal from the sensor goes to the direct output of the comparator on the operational amplifier CA3130. The inverting input of the same op-amp receives a reference voltage from the divider. Variable resistance R4 set the required temperature.

If the potential at the direct input is lower than that set at pin 2, then at the output of the comparator we will have a level of about 0.65 volts, and if vice versa, then at the output of the comparator we will get a high level of about 2.2 volts. The signal from the output of the op-amp through transistors controls the operation of the electromagnetic relay. At a high level, it turns on, and at a low level it turns off, switching the load with its contacts.

TL431 is a programmable zener diode. Used as a voltage reference and power supply for low power circuits. The required voltage level, at the control output of the TL431 microassembly, is set using a divider on resistors Rl, R2 and a NTC thermistor R3.

If the voltage on the TL431 control pin is higher than 2.5V, the microcircuit passes current and turns on the electromagnetic relay. The relay switches the control output of the triac and connects the load. As the temperature rises, the resistance of the thermistor and the potential at the control contact TL431 drops below 2.5V, the relay releases its front contacts and turns off the heater.

Using resistance R1, we adjust the level of the desired temperature to turn on the heater. This circuit is capable of driving a heating element up to 1500 watts. The relay is suitable for RES55A with an operating voltage of 10 ... 12 V or its equivalent.

The design of the analog thermostat is used to maintain the set temperature inside the incubator, or in a box on the balcony for storing vegetables in winter. Power is provided by a 12 volt car battery.

The design consists of a relay in the event of a temperature drop and turns off when the set threshold rises.

The temperature of the thermostat relay operation is set by the voltage level on pins 5 and 6 of the K561LE5 microcircuit, and the relay off temperature is set by the potential at pins 1 and 21. The temperature difference is controlled by the voltage drop across resistor R3. In the role of the temperature sensor R4, a thermistor with a negative TCR is used, i.e.

The design is small and consists of only two blocks - a measuring unit based on a comparator based on an op-amp 554CA3 and a load switch up to 1000 W built on a power regulator KR1182PM1.

The third direct input of the op-amp receives a constant voltage from a voltage divider consisting of resistances R3 and R4. The fourth inverted input is supplied with voltage from another divider at the resistance R1 and the thermistor MMT-4 R2.

The temperature sensor is a thermistor located in a glass flask with sand, which is placed in the aquarium. The main node of the design is the m / s K554SAZ - voltage comparator.

From the voltage divider, which also includes a thermistor, the control voltage goes to the direct input of the comparator. The other comparator input is used to adjust the desired temperature. A voltage divider is made of resistances R3, R4, R5, which form a bridge sensitive to temperature changes. When the temperature of the water in the aquarium changes, the resistance of the thermistor also changes. This creates a voltage imbalance at the comparator inputs.

Depending on the voltage difference at the inputs, the output state of the comparator will change. The heater is made in such a way that when the water temperature drops, the aquarium thermostat automatically starts, and when it rises, on the contrary, it turns off. The comparator has two outputs, collector and emitter. To control a field effect transistor, a positive voltage is required, therefore, it is the collector output of the comparator that is connected to the positive line of the circuit. The control signal is obtained from the emitter terminal. Resistors R6 and R7 are the load output of the comparator.

The IRF840 field effect transistor is used to turn the heating element on and off in the thermostat. To discharge the gate of the transistor, there is a diode VD1.

The thermostat circuit uses a transformerless power supply. Excess alternating voltage is reduced due to the reactance of capacitance C4.

The basis of the first design of the thermostat is the PIC16F84A microcontroller with a DS1621 temperature sensor with an l2C interface. At the time of power-up, the microcontroller first initializes the internal registers of the temperature sensor, and then configures it. The thermostat on the microcontroller in the second case is already made on the PIC16F628 with the DS1820 sensor and controls the connected load using the relay contacts.

The dependence of the voltage drop at the p-n junction of semiconductors on temperature is the best suited for creating our home-made sensor.

Temperature controllers are widely used in modern household appliances, automobiles, heating and air conditioning systems, manufacturing, refrigeration equipment and ovens. The principle of operation of any thermostat is based on turning on or off various devices after reaching certain temperature values.

Modern digital thermostats are controlled by buttons: touch or conventional. Many models are also equipped with a digital panel that displays the set temperature. The group of programmable thermostats is the most expensive. Using the device, you can provide for a change in temperature by the hour or set the desired mode for a week in advance. You can control the device remotely: via smartphone or computer.

For a complex technological process, for example, a steel-smelting furnace, making a thermostat with your own hands is a rather difficult task that requires serious knowledge. But to assemble a small device for a cooler or an incubator is within the power of any home master.

In order to understand how a temperature controller works, consider a simple device that is used to open and close the damper of a shaft boiler and is activated when the air is heated.

For the operation of the device, 2 aluminum pipes, 2 levers, a return spring, a chain that goes to the boiler, and an adjusting unit in the form of a crane box were used. All components were mounted on the boiler.

As you know, the coefficient of linear thermal expansion of aluminum is 22x10-6 0C. When an aluminum pipe with a length of one and a half meters, a width of 0.02 m and a thickness of 0.01 m is heated to 130 degrees Celsius, an elongation of 4.29 mm occurs. When heated, the pipes expand, due to this, the levers are shifted, and the damper closes. When cooling, the pipes decrease in length, and the levers open the damper. The main problem when using this scheme is that it is very difficult to accurately determine the threshold for the operation of a thermostat. Today, preference is given to devices based on electronic elements.

Scheme of operation of a simple thermostat

Typically, relay-based circuits are used to maintain the set temperature. The main elements included in this equipment are:

• temperature sensor;
• threshold scheme;
• actuating or indicating device.

As a sensor, you can use semiconductor elements, thermistors, resistance thermometers, thermocouples and bimetallic thermal relays.

The thermostat circuit reacts to the excess of the parameter above the set level and turns on the actuator. The simplest version of such a device is an element on bipolar transistors. The thermal relay is based on the Schmidt trigger. The thermistor acts as a temperature sensor - an element whose resistance changes depending on the increase or decrease in degrees.

R1 is a potentiometer that sets the initial offset on R2 thermistor and R3 potentiometer. Due to the adjustment, the actuator is activated and relay K1 is switched when the resistance of the thermistor changes. In this case, the operating voltage of the relay must correspond to the operating power supply of the equipment. To protect the output transistor from voltage pulses, a semiconductor diode is connected in parallel. The load value of the connected element depends on the maximum current of the electromagnetic relay.

Attention! On the Internet, you can see pictures with thermostat drawings for various equipment. But quite often the image and description do not match. Sometimes the illustrations may simply represent other devices. Therefore, production can only begin after a thorough study of all the information.

Before starting work, you should decide on the power of the future thermostat and the temperature range in which it will work. For the refrigerator, some elements will be required, and for heating, others.

Thermostat on three elements

One of the elementary devices, on the example of which you can assemble and understand the principle of operation, is a simple do-it-yourself thermostat designed for a fan in a PC. All work is done on a breadboard. If there are problems with the palnik, then you can take a solderless board.

The thermostat circuit in this case consists of only three elements:

• power transistor MOSFET (N channel), you can use IRFZ24N MOSFET 12V and 10A or IFR510 Power MOSFET;
• potentiometer 10 kOhm;
• NTC thermistor in 10 kOhm, which will act as a temperature sensor.

The temperature sensor reacts to an increase in degrees, due to which the whole circuit is triggered, and the fan turns on.

Now let's move on to the settings. To do this, turn on the computer and adjust the potentiometer, setting the value for the off fan. At that moment, when the temperature approaches the critical one, we reduce the resistance as much as possible before the blades rotate very slowly. It is better to make the adjustment several times to make sure the equipment is working efficiently.

The modern electronics industry offers elements and microcircuits that differ significantly in appearance and technical characteristics. Each resistance or relay has several analogues. It is not necessary to use only those elements that are indicated in the scheme, you can take others that match the parameters with the samples.

Temperature controllers for heating boilers

When adjusting heating systems, it is important to accurately calibrate the device. This will require a voltage and current meter. To create a working system, you can use the following scheme.

Using this scheme, you can create outdoor equipment for controlling a solid fuel boiler. The role of the zener diode here is performed by the K561LA7 microcircuit. The operation of the device is based on the ability of the thermistor to reduce resistance when heated. The resistor is connected to the electricity voltage divider network. The required temperature can be set using a variable resistor R2. The voltage is supplied to the inverter 2I-NOT. The resulting current is fed to capacitor C1. A capacitor is connected to 2I-NOT, which controls the operation of one trigger. The latter is connected to the second trigger.

Temperature control is as follows:

• when the degrees decrease, the voltage in the relay increases;
• when a certain value is reached, the fan, which is connected to the relay, turns off.

Soldering is best done on a mole rat. As a battery, you can take any device that operates within 3-15 V.

Carefully! Installing home-made appliances for any purpose on heating systems can lead to equipment failure. Moreover, the use of such devices may be prohibited at the level of services that provide communications in your home.

Digital thermostat

In order to create a fully functioning thermostat with accurate calibration, digital elements are indispensable. Consider a temperature control device for a small vegetable store.

The main element here is the PIC16F628A microcontroller. This chip provides control of various electronic devices. The PIC16F628A microcontroller contains 2 analog comparators, an internal oscillator, 3 timers, SSR comparison and USART data exchange modules.

When the thermostat is operating, the value of the existing and set temperature is fed to the MT30361 - a three-digit indicator with a common cathode. In order to set the required temperature, the buttons are used: SB1 - to decrease and SB2 - to increase. If you carry out tuning while pressing the SB3 button, you can set the hysteresis values. The minimum hysteresis value for this circuit is 1 degree. A detailed drawing can be seen on the plan.

When creating any of the devices, it is important not only to solder the circuit itself correctly, but also to think about how best to place the equipment. It is necessary that the board itself be protected from moisture and dust, otherwise a short circuit and failure of individual elements cannot be avoided. You should also take care to isolate all contacts.

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