The automatic temperature control system is a very essential feature of a factory or an industry. In most of the case the temperature plays a vital role in the process of manufacturing or the process carried in that factory or industry.
The most common and simplest way of controlling temperature is by using a fan which is automatically connected to a network such as it is switched on when the temperature of the surroundings increases.
The change in temperature of the surroundings can be sensed with the help of thermistor or a sensistor. These electronic components sense the temperature of the surroundings. When there is a change in the temperatue tempetature then these electronic components start to conduct the electric current. This is the main principle of the automatic control devices. These devices can be extended to an extent that we can set the temperature when the fan should rotate.
Th1, the 50K thermistor, is a standard type. This 50K was measured at exactly 25 °C and with 10% tolerance. The resistance increases as the surrounding temperature decreases. . Another name for this thing is 'NTC'. NTC stands for "Negative Temperature Coefficient" which means when the surrounding temperature decreases the resistance of this thermistor will increase.
P1 is a regular Bourns trimmer and adjusts a wide range of temperatures for this circuit. R1 is a 'security' resistor just in case the trimmer pot P1 is adjusted all the way to '0' ohms. At which time the thermistor would get the full 12 volt and it will get so hot that it puts blisters on your fingers. R3 feeds a bit of hysteresis back into the op-amp to eliminate relay 'chatter' when the temperature of the thermistor reaches its threshold point.
Depending on the application and the type you use for Q1 and Re1, start with 330K or so and adjust its value downwards until satisfied. The value of 150K shown in the diagram worked for me. Decreasing the value of R2 means more hysteresis, just don't use more then necessary. Or temporarily use a trimmer pot and read off the value. 120K worked for me. Transistor Q1 can be a 2N2222(A), 2N3904, NTE123A, ECG123A, etc. Not critical at all. It acts only as a switch for the relay so almost any type will work, as long as it can provide the current needed to activate the relay's coil. D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering.
Here I have modified the circuit by replacing IC741 by IC555. These two ICs have the similarity to amplify the given input signal when it is operating under certain biasing conditions. Hence the circuit has been changed accordingly to have same output but by using 555 timer instead or f using 741 operational amplifer.
Working of 555IC:
Comparator 1 has a threshold input (pin 6) and a control input (pin 5). In most applications, the control input is not used, so that the control voltage equals +2/3 VCC. Output of this comparator is applied to set (S) input of the flip-flop. Whenever the threshold voltage exceeds the control voltage, comparator 1 will set the flip-flop and its output is high. A high output from the flip-flop saturates the discharge transistor and discharge the capacitor connected externally to pin 7. The complementary signal out of the flip-flop goes to pin 3, the output. The output available at pin 3 is low. These conditions will prevail until comparator 2 triggers the flip-flop. Even if the voltage at the threshold input falls below 2/3 VCC, that is comparator 1 cannot cause the flip-flop to change again. It means that the comparator 1 can only force the flip-flop’s output high. To change the output of flip-flop to low, the voltage at the trigger input must fall below + 1/3 Vcc. When this occurs, comparator 2 triggers the flip-flop, forcing its output low. The low output from the flip-flop turns the discharge transistor off and forces...
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