Making a diagram of a car radiator temperature sensor

Automatic thermostat for car engine cooling

Many, even most, passenger cars have an electric fan in their engine cooling system, periodically blowing air through the engine cooling system radiator. Some cars have a temperature sensor switch on the radiator which is preset to a certain temperature at the factory, and when it is reached it closes the contacts that bring current to the electric motor relay on the radiator.

Others use a common motor temperature sensor, which is a thermistor, and the decision to turn the motor on or off is made by the car’s ECU (Electronic Control Unit).

In both cases there is the same problem – the temperature threshold of the fan is not regulated depending on the weather, time of year, mode of operation, coolant used, or just the driver’s desire. As a result, in summer the car overheats and can boil, and in winter the heater does not warm up well. In addition, there are a lot of problems when you change from one type of coolant to another.

In modern cars where the decision to turn on the fan is made by ECU based on the resistance of the temperature sensor, the problem can be solved by making changes in the ECU firmware, but it is expensive and not always possible. In cars with temperature switch, it is possible to replace one temperature switch by another, for a different temperature, but it is a time consuming process and it is not always possible to find a suitable sensor.

But, in fact, it would be desirable just to be able to twist some screwdriver and adjust required (or desirable) temperature of cooling system fan. It is clear that you can solve the problem with an ordinary thermostat circuit, where the temperature information can be taken from the temperature sensor. This can be the same sensor that communicates with the ECU, or a sensor on the arrow temperature indicator, it all depends on the specific car, or rather, its scheme.

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Thermostat scheme

There are a lot of schemes of thermoregulators in amateur radio literature, so, without claiming to be original, here is the scheme, which I personally collected for my car. As I said above, the circuit is almost typical. It consists of comparator on op-amp and two circuits, which set voltage on its inputs.

The voltage at the non-inverting input is set with the trimmer resistor R2, and the voltage at the inverting input is taken from the engine temperature sensor, which is a thermistor that, together with other parts of the car circuit, forms a temperature-dependent voltage divider.

Schematic diagram of a thermostat for switching on the engine cooling in a car

Fig. 1. Schematic diagram of the thermostat for switching on the engine cooling in the car.

At the output of the circuit there is a switch on transistor VT1, its collector is connected to the winding of the relay controlling the electric fan. And power to the circuit is supplied from the car ignition switch output, so that power to the circuit is supplied only when the ignition is on. This is necessary because when the ignition is off, the voltage to the temperature sensor circuit usually does not come, respectively, the voltage on the temperature sensor drops to zero, regardless of the value of the temperature.

Circuit operation

With trimmer resistor R2 you set some voltage on pin 3, which corresponds to the temperature of the fan.

When the coolant temperature is lower than the set temperature, the resistance of the temperature sensor is high, and the voltage on it is significantly higher than the voltage on pin 3 A1. Therefore, the output of the operational amplifier A1, working as a comparator, will be low voltage. Transistor VT1 will be closed and no current will flow to the fan relay winding.

Since a common operational amplifier type KP140UD608 is used here as a comparator, the minimum voltage at its output is slightly different from zero, so to improve the closure of the transistor VT1 in the chain of its emitter included two diodes type 1N4004. If this is not enough during adjustment, the number of diodes must be increased.

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When the coolant temperature reaches and exceeds the set temperature, the resistance of the temperature sensor is low, and the voltage at it is lower than the voltage at pin 3 A1. Therefore, the output of the operational amplifier A1 is high. Transistor VT1 opens and starts current to the winding of the fan relay. The trim resistor R2 is a multi-turn resistor.

Making a temperature sensor with our own hands

Thermo-sensor, assembled with your own hands, can bring undoubted benefit, both in the home and the household. The ambient temperature controller will turn on or off the fan, heater, water heater, underfloor heating and many other devices in the house, heat or ventilate the greenhouses in time. With a minimum of experience with tools to make the temperature sensor with their own hands is not very difficult.


Principle of operation

The idea of creating a temperature sensor is that it uses an electrically conductive element that changes its resistance under the influence of fluctuations in ambient temperature. Such element is a thermistor.

The principle of variable resistance is that when heated, the resistance decreases and the current flowing through it, changes its characteristic. This process is reflected in the operation of the application circuit, which turns the corresponding devices on or off.

Making a simple temperature sensor

Before making a temperature sensor, you need to prepare the following:

  • 12 V power supply;
  • A fan (12 volt computer cooler);
  • VDR1 thermistor (10 kOhm);
  • variable resistor (10 kOhm);
  • IRFZ 44 field effect transistor;
  • breadboard;
  • wires;
  • soldering iron and solder.


After preparing the above materials and tools, proceed to soldering the simple circuit.

  1. Connect the plus terminal of the power supply with a wire to the input contact (+) of the cooler;
  2. The three pins of the field effect transistor are soldered with wires as follows: “source” with the cooler, “gate” with the thermistor, “sink” with the variable resistor.
  3. The wires connect the free contacts of the thermistor with the “+” of the power supply, the variable resistor with the “-” of the same block.
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Pin assignment of a field effect transistor


Test the thermostat in the following order:

  • a burning match or a lighter is brought to the thermistor and the cooler should start;
  • When it cools down, the fan should turn off;
  • If the circuit does not work, you need to recheck the soldering and contacts.

Schematic diagram of the thermostat

TR – thermistor, K – cooler, R1 – variable resistor, FET, AB – 12 V battery.


In this case a thermistor is used, the resistance of which is 10 kOhm at an air temperature of 20 ° C. When it gets warmer the resistance drops. It is necessary to adjust the variable resistor to switch on the cooler at the moment of sensor’s heating. By selecting several positions of the variable resistor rotary knob, the desired effect is achieved.

Thermal sensor on germanium diodes

A feature of germanium semiconductor diodes is their high sensitivity to changes in air temperature. Therefore, these radio components can be used as thermal sensors when they are turned on in reverse.

Germanium diodes

Their use is due to the strong dependence of reverse current on ambient temperature. This feature of diodes is used in a simple cooler speed regulator circuit.

Diagram of temperature regulator with germanium diodes

Germanium diodes connected in parallel (3-4 pieces) are included in the reverse direction in the base circuit of the compound transistor. Their glass housings can be mounted directly on the cooler without any heat sinks. Resistor R1 protects the transistor from thermal breakdown, and R2 determines the threshold of the regulator. If the fan does not turn on when the room temperature is exceeded, the number of diodes must be increased. When the cooler starts to rotate the blades at high speed the number of radio diodes should be reduced.

Using the Arduino temperature sensor

To build a temperature sensor based on an Arduino microcontroller you need to prepare the following:

  • Arduino UNO;
  • connectors;
  • circuit board;
  • DS18B20 digital module (range -56 to +1250 C).
Battery charger from computer power supply

The DS18B20 digital temperature sensor is a device that not only signals when a set temperature threshold is exceeded, but can also memorize measurement values. The sensor chip has three output contacts which are “+”, “-” and a signal wire. The waterproof version of the temperature sensor is used to measure heating of water or liquids.

You can always buy the temperature sensor as well as the Arduino board from online stores. The digital module is connected to the Arduino via GND channels and the Vdd output is connected to 5V, Data to any Pin. For a better understanding the wiring diagram of the DS18B20 digital sensor to the Arduino is shown in the photo below.

Diagram for connecting a temperature sensor to an Arduino


Depending on the purpose of using the ambient temperature meter to make it yourself, you can choose the most acceptable and cost-effective option. For cooling power-consuming boards it is enough to use a simple circuit with a cooler. But to work with ventilation and heating equipment you will need a more complex system using microcontroller Arduino and factory-made temperature sensors.

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