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L33, Temperature Sensor Using IC741

IC LM35 used as a sensor IC for detecting accurate temperate in centigrade. Linearity defines how well over a range of temperature a sensor’s output consistently changes. Unlike thermistor, Linearity of a precision IC Sensors are very good of 0.5°C accuracy and has wide temperature range. its output voltage is linearly proportional to the Celsius (Centigrade) temperature.

The LM35 is rated to operate over a -55° to +150°C temperature range.It draws only 60 µA from its supply, it has very low self-heating, less than 0.1°C in still air. Operating voltage of LM35 IC’s is 4 to 30 volts.

Output of IC is 10mv/degree centigrade for e.g. If the output of sensor is 250 mV. then temperature is 25 Degree C. so by using a Digital multimeter we can easily calculate the degree temperature. For trigger point you should set the voltage of pin 2 of IC 741 by using variable resistor or potentiometer.

The aim of this project is not to construct a thermometer but to activate or deactivate a device at a particular margin temperature. 2 LED is using in this circuit for indication of both low (Green) and high (Red) temperature.


The output of IC2 increases in proportion to the temperature by 10 mv/degree. This varying voltage is feed to a comparator IC741 (OP Amplifier). The op-amp is one type of differential amplifier. It has two input inverting (-) and non-inverting (+) and one output pin. In this circuit. IC741 is used as non-inverting amplifier which means pin 3 is the input and the output is not reversed. This circuit amplifies the difference between its input terminals. As a comparator, bi-stable output of an op amplifier is as follows.

Part list-

  • IC = LM35,

  • IC = LM741

  • Resistor = 10K

  • Resistor = 470 ohm

  • Potentiometer = 2K

  • LED 2pcs (Red and Green)

  • 9V Battery with Cap

  • Switch

  • Connection wire

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