Thermostats are widely used in modern household appliances, cars, heating and air conditioning systems, in production, in refrigeration equipment and during the operation of 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 using 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. With the help of the device, you can provide for a change in temperature by the hour or set the required mode for a week in advance. The device can be controlled remotely: via a smartphone or computer.
For a complex technological process, for example, a steel-making furnace, making a thermostat with your own hands is a rather difficult task that requires serious knowledge. But collect small device for a cooler or incubator, any home craftsman can do it.
In order to understand how the temperature regulator works, consider a simple device that is used to open and close the damper of a mine boiler and is triggered when the air heats up.
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-axle box were used. All components were installed on the boiler.
As you know, the coefficient of linear thermal expansion of aluminum is 22x10-6 0С. When an aluminum pipe is heated with a length of one and a half meters, a width of 0.02 m and a thickness of 0.01 m to 130 degrees Celsius, an elongation of 4.29 mm occurs. When heated, the pipes expand, due to this, the levers are displaced, and the damper closes. As the pipes cool down, they decrease in length, and the levers open the damper. The main problem when using this circuit is that it is very difficult to accurately determine the response threshold of the thermostat. Today, preference is given to devices based on electronic components.
The scheme of work 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;
- executive or indicator device.
As a sensor, you can use semiconductor elements, thermistors, resistance thermometers, thermocouples and bimetallic thermostats.
The thermostat circuit reacts to the excess of the parameter over the set level and turns on the executive device. The simplest version of such a device is a bipolar transistor element. The thermal relay is based on the Schmidt trigger. A thermistor acts as a temperature sensor - an element whose resistance changes depending on an increase or decrease in degrees.
R1 is a potentiometer that sets the initial offset on thermistor R2 and potentiometer R3. Due to the adjustment, the actuator is triggered 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 surges, 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 drawings of a thermostat for various equipment. But quite often the picture and description do not match. Sometimes the pictures may simply represent other devices. Therefore, manufacturing can be started only after careful 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. The refrigerator will require some elements, and the heating will require others.
Thermostat on three elements
One of the elementary devices, by 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 pallet, then you can take a solderless board.
The thermostat circuit in this case consists of only three elements:
- power MOSFET transistor(N channel), you can use IRFZ24N MOSFET 12V and 10A or IFR510 Power MOSFET;
- potentiometer 10 kOhm;
- NTC thermistor of 10 kOhm, which will act as a temperature sensor.
The temperature sensor reacts to an increase in degrees, due to which the entire circuit is triggered, and the fan turns on.
Now let's move on to setting up. To do this, turn on the computer and adjust the potentiometer, setting the value for the off fan. At the 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 that the equipment is working efficiently.
The modern electronic industry offers elements and microcircuits that differ significantly in appearance and technical specifications... Each resistance or relay has several analogs. It is not necessary to use only those elements that are indicated in the diagram; you can take others that match the parameters with the samples.
Thermostats 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 diagram.
Using this scheme, you can create outdoor equipment to control a solid fuel boiler. The role of the zener diode 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 the variable resistor R2. The voltage is supplied to the 2I-NOT inverter. The resulting current is fed to the 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 goes according to the following scheme:
- with a decrease in degrees, the voltage in the relay increases;
- when a certain value is reached, the fan, which is connected to the relay, turns off.
It is better to solder on a mole rat. As a battery, you can take any device operating within 3-15 V.
Carefully! The installation of self-made devices 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 supply communications in your home.
Digital thermostat
In order to create a fully functioning thermostat with accurate calibration, you cannot do without digital elements. Consider a temperature control device for a small vegetable store.
The main element here is the PIC16F628A microcontroller. This microcircuit provides control of various electronic devices... The PIC16F628A microcontroller contains 2 analog comparators, an internal oscillator, 3 timers, CCP comparison modules and USART data exchange.
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, use the buttons: SB1 - to decrease and SB2 - to increase. If you carry out the setting while simultaneously 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 properly solder the circuit itself, 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 it cannot be avoided short circuit and failure of individual elements. Also, care should be taken to isolate all contacts.
Video
In everyday life and in a subsidiary farm, it is often required to maintain the temperature regime of a room. Previously, this required a rather huge circuit made on analog elements, we will consider one such for general development. Today everything is much simpler, if it is necessary to maintain the temperature in the range from -55 to + 125 ° C, then the programmable thermometer and thermostat DS1821 can perfectly cope with this goal.
Thermostat circuit on a specialized temperature sensor. This DS1821 thermal sensor can be bought cheaply in ALI Express (to order, click on the picture just above)
The thermostat on and off temperature threshold is set by the TH and TL values in the sensor memory, which must be programmed into the DS1821. If the temperature rises above the value written in the TH cell, a logical unit level 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 in that half-wave of the mains voltage when it is equal to zero, thereby supplying a bias voltage to the gate of the second field-effect transistor, which turns on the optosimistor, and the latter already opens the load control resistor VS1. The load can be any device, such as an electric motor or heater. The blocking reliability of the first transistor must be adjusted by selecting the required value of the resistor R5.
The DS1820 temperature sensor is capable of recording temperatures from -55 to 125 degrees and operates in thermostat mode.
Thermostat circuit on the DS1820 sensor
If the temperature exceeds the upper threshold TH, then the output of the DS1820 will be a logical unit, the load will be disconnected from the mains. 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 are incomprehensible moments, homemade construction was borrowed from # 2 for 2006.
The signal from the sensor goes to a direct output of the comparator on the CA3130 operational amplifier. The inverting input of the same op-amp receives the reference voltage from the divider. The variable resistance R4 sets the required temperature regime.
Thermostat circuit on the LM35 sensor
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 on the contrary, then at the output of the comparator we will get high level about 2.2 volts. The signal from the op-amp output 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.
The 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 pin of the TL431 microassembly is set using a divider across the resistors Rl, R2 and a thermistor with negative TCS R3.
If the voltage at 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. With an increase in temperature, the resistance of the thermistor and the potential at the control contact TL431 decreases below 2.5V, the relay releases its front contacts and turns off the heater.
With the help of resistance R1, we adjust the level of the desired temperature to turn on the heater. This scheme capable of driving a heating element up to 1500 W. The relay is suitable for RES55A with an operating voltage of 10 ... 12 V or its equivalent.
The analogue thermostat design is used to maintain the set temperature inside the incubator, or in a drawer on the balcony for storing vegetables in winter. Power is supplied from a 12 volt car battery.
The design consists of a relay in the event of a temperature drop and disconnects when the set threshold rises.
The temperature of the thermostat relay actuation is set by the voltage level on pins 5 and 6 of the K561LE5 microcircuit, and the relay switch-off temperature is set by the potential on pins 1 and 21. The temperature difference is controlled by the voltage drop across the resistor R3. In the role of temperature sensor R4, a NTC thermistor 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 554SA3 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 inverse input is supplied with voltage from another divider on resistance R1 and thermistor MMT-4 R2.
The temperature sensor is a thermistor located in a glass flask with sand, which is located in the aquarium. The main unit of the structure is 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 input of the comparator is used to regulate the required temperature. A voltage divider is made of resistances R3, R4, R5, which form a bridge that is sensitive to temperature changes. When the temperature of the water in the aquarium changes, the resistance of the thermistor also changes. This creates an imbalance in the voltages 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 up, and when the water temperature rises, it turns off. The comparator has two outputs, collector and emitter. To control the 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 received from the emitter pin. Resistors R6 and R7 are the output load of the comparator.
The IRF840 field-effect transistor is used to turn the heating element on and off in the thermostat. For the discharge of the gate of the transistor, a diode VD1 is present.
The thermostat circuit uses a transformerless power supply. The excess alternating voltage is reduced due to the reactance of the capacitor C4.
The basis of the first design of the thermostat is a PIC16F84A microcontroller with a DS1621 temperature sensor with an l2C interface. At the moment of power-up, the microcontroller first initializes the internal registers of the temperature sensor, and then adjusts 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.
DIY temperature sensor |
Dependence of voltage drop across p-n junction semiconductors from temperature is the best fit for creating our homemade sensor.
Andrey, perhaps the whole problem is in the KU208G triac. 127V is obtained from the fact that the triac passes one of the half-cycles of the mains voltage. Try replacing it with an imported BTA16-600 (16A, 600V), they work more steadily. BTA16-600 is not a problem to buy now, and it is not expensive either.
sta9111, to answer this question, you will have to remember how our thermostat works. Here is a paragraph from the article: “The voltage at the gate 1 is set using the divider R1, R2 and R4. A thermistor with a negative TCR is used as R4, so when heated, its resistance decreases. When the voltage on pin 1 is higher than 2.5V, the microcircuit is open, the relay is turned on. "
In other words, at the desired temperature, in your case 220 degrees, the thermistor R4 should be. voltage drop 2.5V, let's designate it as U_2.5V. The rating of your thermistor is 1KΩ, is at a temperature of 25 degrees. It is this temperature that is indicated in reference books.
Thermistor reference msevm.com/data/trez/index.htm
Here you can see the operating temperature range and TKS: for a temperature of 220 degrees, little is suitable.
The characteristic of semiconductor thermistors is non-linear, as shown in the figure.
Drawing. Current-voltage characteristic of a thermistor - site / vat.jpg
Unfortunately, the type of your thermistor is unknown, so we will assume that you have an MMT-4 thermistor.
According to the graph, it turns out that at 25 degrees the resistance of the thermistor is just 1KΩ. At a temperature of 150 degrees, the resistance drops to about 300 ohms, more precisely, it is simply impossible to determine using this graph. Let's designate this resistance as R4_150.
Thus, it turns out that the current through the thermistor will be (Ohm's law) I = U_2.5V / R4_150 = 2.5 / 300 = 0.0083A = 8.3mA. This is at a temperature of 150 degrees, it seems, while everything is clear, and there seem to be no errors in reasoning. Let's continue further.
With a supply voltage of 12V, it turns out that the resistance of the circuit R1, R2 and R4 will be 12V / 8.3mA = 1.445KΩ or 1445Ω. Minus R4_150, it turns out that the sum of the resistances of the resistors R1 + R2 will be 1445-300 = 1145 Ohm, or 1.145KΩ. Thus, you can use a trimmer resistor R1 1KΩ, and a limiting resistor R2 470Ω. This is the calculation.
All this is good, only a few thermistors are designed to work at temperatures up to 300 degrees. Thermistors CT1-18 and CT1-19 are most suitable for this range. See the reference msevm.com/data/trez/index.htm
Thus, it turns out that this thermostat will not provide a temperature stabilization of 220 degrees and above, since it is designed for the use of semiconductor thermistors. You will have to look for a circuit with metal TCM or RTD resistors.
Hello to all lovers electronic homemade products... Recently, I quickly made an electronic thermostat with my own hands, the device diagram is very simple. An electromagnetic relay with powerful contacts that can withstand currents up to 30 amperes is used as an actuator. Therefore, the homemade product in question can be used for various household needs.
In the diagram below, the thermostat can be used, for example, for an aquarium or for storing vegetables. Someone can use it when used in conjunction with an electric boiler, and someone can adapt it for a refrigerator.
Diy electronic thermostat, device diagram
As I already said, the circuit is very simple, it contains a minimum of inexpensive and common radio components. Typically, thermostats are built on a comparator microcircuit. This makes the device more complex. This homemade product is built on an adjustable zener diode TL431:
Now let's talk in more detail about the details that I used.
Device details:
- Step-down transformer for 12 volts
- Diodes; IN4007, or others with similar characteristics 6 pcs.
- Electrolytic capacitors; 1000 mk, 2000 mk, 47 mk
- Stabilizer microcircuit; 7805 or another 5 volt
- Transistor; KT 814A, or another p-n-p with a collector current of at least 0.3 A
- Adjustable Zener diode; TL431 or Soviet KR142EN19A
- Resistors; 4.7 ohm, 160 ohm, 150 ohm, 910 ohm
- Variable resistor; 150 Room
- Thermistor as a sensor; about 50 Kom with negative TCS
- Light-emitting diode; any with the lowest current consumption
- The relay is electromagnetic; any 12 volt with a current consumption of 100 mA or less
- Button or toggle switch; for manual control
How to make a thermostat with your own hands
A burnt-out electronic meter Granit-1 was used as a body. The board on which all the main radio components are located is also from the counter. The transformer of the power supply unit and the electromagnetic relay fit inside the case:
As a relay, I decided to use an automobile relay, which can be purchased at any car dealership. Coil operating current approx. 100 milliamperes:
Since the regulated zener diode is low-power, its maximum current does not exceed 100 milliamperes, it will not work to directly connect the relay to the zener diode circuit. Therefore, I had to use more powerful transistor KT814. Of course, the circuit can be simplified if you use a relay whose coil current is less than 100 milliamperes, for example, or SRA-12VDC-AL. Such relays can be connected directly to the zener diode cathode circuit.
I'll tell you a little about the transformer. The quality that I decided to use is non-standard. I had a voltage coil from an old induction meter of electrical energy lying around:
As you can see in the photo there is a free space for the secondary winding, I decided to try winding it and see what happens. Of course, the cross-sectional area of the core is small, respectively, and the power is small. But for a given temperature controller, this transformer is sufficient. According to calculations, I got 45 turns per volt. To get 12 volts at the output, you need to wind 540 turns. To fit them, I used a 0.4 mm wire. Of course, you can use a ready-made with an output voltage of 12 volts or an adapter.
As you noticed, the circuit contains a 7805 stabilizer with a stabilized output voltage of 5 volts, which feeds the control pin of the zener diode. Thanks to this, the temperature controller turned out to have stable characteristics that will not change when the supply voltage changes.
As a sensor, I used a thermistor, which has a resistance of 50 ohms at room temperature. When heated, the resistance of this resistor decreases:
To protect it from mechanical stress, I used heat-shrinkable tubes:
The place for the variable resistor R1 was found on the right side of the thermostat. Since the axis of the resistor is very short, I had to solder a flag on it, for which it is convenient to turn. On the left side, I placed a manual toggle switch. With it, it is easy to control the operating state of the device, while not changing the set temperature:
Despite the fact that the terminal block of the former electric meter is very cumbersome, I did not remove it from the case. It clearly includes a plug from any device, such as an electric heater. By removing the jumper (yellow on the right in the photo) and turning on the ammeter instead of the jumper, you can measure the current supplied to the load:
Now it remains to calibrate the thermostat. For this we need. It is necessary to connect both sensors of the device together using electrical tape:
Use a thermometer to measure the temperature of various hot and cold objects. Using a marker, apply a scale and markings on the thermostat, the moment the relay is turned on. I got it from 8 to 60 degrees Celsius. If someone needs to shift the operating temperature in one direction or another, this can be easily done by changing the values of the resistors R1, R2, R3:
So we made an electronic thermostat with our own hands. Outwardly it looks like this:
To not see the inside of the device, through the transparent cover, I closed it with tape, leaving a hole for the HL1 LED. Some radio amateurs who decided to repeat this scheme complain that the relay turns on, not very clearly, as if it rattles. I did not notice anything of this, the relay turns on and off very clearly. Even with a slight change in temperature, no bouncing occurs. If, nevertheless, it arises, you need to choose more accurately the capacitor C3 and the resistor R5 in the base circuit of the KT814 transistor.
The assembled thermostat according to this scheme turns on the load when the temperature drops. If someone, on the contrary, needs to turn on the load when the temperature rises, then you need to swap sensor R2 with resistors R1, R3.
Among the various useful gizmos that can add comfort to our lives, there are many that you can easily do yourself.
This category also includes a thermostat, also called a thermostat, a device that turns on and off heating or refrigeration equipment in accordance with the temperature of the environment in which it is installed.
Such a device can, for example, turn on a heater in the basement where vegetables are stored during extreme cold weather. From our article you will learn how you can make a thermostat with your own hands (for a heating boiler, refrigerator and other systems) and what parts are best for this.
The device of the thermostat is not particularly complicated, therefore many novice radio amateurs hone their skills in the manufacture of this device. Circuits are offered in a variety of ways, but the most widespread is the variant with the use of a special microcircuit called a comparator.
This element has two entrances and one exit. A certain reference voltage is supplied to one input, which corresponds to the required temperature, and to the second, the voltage from the temperature sensor.
Thermoregulator circuit for warm floors
The comparator compares the incoming data and, at a certain ratio, generates a signal at the output that turns on the transistor or turns on the relay. In this case, current is supplied to the heater or refrigeration unit.
Diy temperature controller device details
The temperature sensor is usually a thermistor - an element whose electrical resistance changes depending on temperature. Semiconductor elements are also used - transistors and diodes, the characteristics of which are also influenced by temperature: when heated, the collector current increases (for transistors), while a shift in the operating point is observed and the transistor stops working without responding to the input signal.
But such sensors have a significant drawback: they are quite difficult to calibrate, that is, "tie" to certain values temperature, which is why the accuracy of the homemade thermostat leaves much to be desired.
Meanwhile, the industry has long mastered the production of inexpensive temperature sensors, which are calibrated during the manufacturing process.
This includes a device of the LM335 brand from National Semiconductor, which we recommend using. The cost of this analog temperature sensor is only $ 1.
"Three" in the first position of the digital row in the marking means that the device is intended for use in household appliances. Modifications LM235 and LM135 are intended for use in industry and military, respectively.
With 16 transistors, this sensor works like a zener diode. Moreover, its stabilization voltage depends on the temperature.
The dependence is as follows: for each degree on the absolute scale (Kelvin) there is 0.01 V voltage, that is, at zero Celsius (273 Kelvin), the stabilization voltage at the output will be 2.73 V. The manufacturer calibrates the sensor at a temperature of 25C (298K ). The working range is from -40 to +100 degrees Celsius.
Thus, assembling a thermostat based on LM335, the user gets rid of the need to select by trial and error the reference voltage at which the device will provide the required temperature.
V = (273 + T) x 0.01,
Where T is the temperature of interest to the user on the Celsius scale.
In addition to the temperature sensor, we need a comparator (the LM311 brand from the same manufacturer is suitable), a potentiometer for generating a reference voltage (setting the required temperature), an output device for connecting a load (relay), indicators and a power supply.
The thermostat is an integral part of autonomous heating. will help keep the temperature in the house at a comfortable level.
Let us analyze the principle of operation of the thermostat for an infrared heater.
Should you install a thermostat for a heating radiator? In this article, we will consider the purpose of the device and the types and features of installation.
Thermostat power supply
Temperature sensor LM335 is connected in series with resistor R1. So, the resistance of this resistor and the supply voltage must be selected in such a way that the value of the current flowing through the thermal sensor is in the range from 0.45 to 5 mA.
Do not exceed the maximum value of this range, as the sensor characteristics will be distorted due to overheating.
You can power the thermostat from standard block 12 V supply or from a self-made transformer.
Turning on the load
An automotive relay can be used as an actuator supplying power to the heater. It is designed for a voltage of 12 V, while a current of 100 mA must flow through the coil.
Recall that the current in the temperature sensor circuit does not exceed 5 mA, therefore, to connect the relay, you need to use a transistor with a higher power, for example, KT814.
You can use a relay with a lower turn-on current, such as SRA-12VDC-L or SRD-12VDC-SL-C - then the transistor is not needed.
How to make a thermostat with your own hands: step by step instructions
Consider how thermostats (thermostats) are made with a do-it-yourself 12 V air temperature sensor.The device is assembled in the following sequence:
- First of all, you need to prepare the body. A used counter, for example, "Granit-1", will do.
- The circuit can be assembled on a board from the same counter. A potentiometer is connected to the direct input of the comparator (marked with a "+" sign), which allows setting the temperature. To the inverse input (“-” sign) - LM335 temperature sensor. If the voltage at the direct input turns out to be higher than at the inverse one, a high level (one) will be set at the output of the comparator and the transistor will supply power to the relay, and it will supply power to the heater. As soon as the voltage at the inverse input turns out to be greater than at the direct one, the level at the output of the comparator will go low (zero) and the relay will turn off.
- To ensure the temperature difference, that is, the thermostat is triggered, for example, at 23 degrees, and shutdown at 25, it is necessary to create a negative feedback between the output and the direct input of the comparator.
- The transformer for powering the thermostat can be made from a coil from an old induction-type electric meter. There is room for a secondary winding on it. To get a voltage of 12 V, you need to wind 540 turns. They can be accommodated if you use a wire with a diameter of 0.4 mm.
Simple homemade thermostat
It is convenient to use the meter terminal block to turn on the heater.
What should be the heater?
The power of the heater depends on what current the contacts of the relay used can withstand. If this value is, for example, 30 A (the car relay is designed for such a current), then the heater can have a power of up to 30 x 220 = 6.6 kW. You just need to first make sure that the wiring and the machine in the dashboard are capable of withstanding such a load.
Mounting
Let's consider how the device should be installed correctly.The thermostat should be installed in the lower part of the room, where cold air accumulates.
However, it is important to prevent the effects of thermal noise, which could confuse the instrument.
For example, do not place the thermostat in a draft or near electrical equipment that emits heat.
Setting the thermostat
As already mentioned, the thermostat based on the LM335 sensor does not need to be adjusted. It is enough to know the voltage supplied by the potentiometer to the direct input of the comparator.
You can measure it with a voltmeter. The required voltage value is determined using the above formula.
If you need, for example, for the device to work at a temperature of 20 degrees, it should be 2.93 V.
If any other element is used as a temperature sensor, the reference voltage will have to be checked empirically. To do this, you need to use a digital thermometer, for example, TM-902S. For accurate adjustment, the sensors of the thermometer and thermostat can be connected with electrical tape, after which they are placed in an environment with different temperatures.
Thermostat made of scrap materials
The potentiometer knob must be smoothly rotated until the thermostat works. At this moment, you should look at the scale of the digital thermometer and put the temperature displayed on it on the scale of the thermostat. You can define extreme points, for example, for temperatures of 8 and 40 degrees, and mark intermediate values by dividing the range into equal parts.
If a digital thermometer is not at hand, the extreme points can be determined by water with ice floating in it (0 degrees) or by boiling water (100 degrees).
Video on the topic
