Analog temperature detector using uA741 OPAMP

What is an “OPAMP”?

Fig1 (b)

An operational amplifier (or an op-amp) is an integrated circuit (IC) that operates as a voltage amplifier. An op-amp has a differential input. That is, it has two inputs of opposite polarity. An op-amp has a single output and a very high gain, which means that the output signal is much higher than input signal.


Vout = AOL[(V+) – (V-)],

Where AOL = Open loop gain of opamp



Ideal characteristics of a OPAMP:
1. Opamp has high input impedance & low output impedance.
2. Zero common mode gain or infinite common mode rejection.
3. Infinite open loop gain AOL.
4. Infinite bandwidth.

** opamp is used as differentiator, integrator, comparator, current – voltage converter, voltage- current converter, etc.

Parts List for the circuit :

1. IC LM35, Opamp (LM741)
2. Resistance- 10K, 470 ohms(2), 2K pot.
3. LED (red, green)
4. Wires
5. Bread board
6. Digital Multimeter
7. 12 v Battery


Circuit Diagram of the project & -Vcc should be grounded

Theory: This project uses IC LM35 as a sensor for detecting accurate centigrade temperature. 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. It’s 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. LM35 Operates from 4 to 30 volts. Output of IC is 10mv/degree centigrade. For example if the output of sensor is 280 mV then temperature is 28 °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 preset or potentiometer. Our aim of this project is not to construct a thermometer but to activate or deactivate a device at a particular margin temperature. For simplicity we have used 2 LEDs for indication of both low (Green) and high (Red) temperature.

Working principle: The output of IC2(LM35) increases in proportional to the temperature by 10 mV/°C , this varying voltage is feed to a comparator configuration of IC 741 (OP Amplifier). At first we set sensitivity (set a voltage by varying the 2KΩ pot) at pin no.2 . If we consider that the sensitivity voltage as V1 & The output of LM35 (pin no. 3) as V2, then we can describe easily that what is happening. If voltage V1> V2 ,then the output of the comparator at +Vsaturation , then the green LED is on and the red LED is off. When the temperature increases that the output of LM35 is also increases, after a certain time when voltage V2 cross the voltage V1 then the output of the comparator at –Vsaturation , then the red LED is on and green LED if off. When V1=V2 then the output is 0 and two LED is in off state. We have used IC741 as a non-inverting amplifier. As a comparator the output voltages will be

                                   Vout  = +Vsat   when V1>V2
                                               = -Vsat    when V1<V2
                                               =  0           when V1 = V2 

Bread board setup
Written by : Sourav Tamli 

The Dark Detector

/Basic concept

This blog will go through one of the useful applications of npn transistor & the voltage divider circuit. The npn transistor BC547 is configured to work as a switch.

So from the datasheet of BC547 transistor we can see that the maximum value of Vbe(on) is 0.7 V. That means if the base to emitter voltage is less than 0.7 V, no current will flow through collector to emitter of the transistor and it will remain off. When Vbe will be greater than 0.7 V, current will flow from collector-emitter, switching on the transistor. Now if a load is connected to the collector it will be switched on. Now in this circuit we implemented a voltage divider, the output of the voltage divider is connected to the base of the BC547. The voltage divider is the combination of a resistance and LDR.

/The voltage divider circuit
Voltage Divider Circuit
Voltage Divider Circuit


It is a very useful circuit which is implemented in different applications. The combination of R1 & R2 will affect Vout.Vout = Vin.(R2/R1+R2) so if we want to make 5V to 2.5V, R1 must be equal to R2. Here we take R1 = R2 = 1KΩ, and it gives perfect 2.5V.





/The Dark Detector Circuit
Dark Detector Circuit

Here the output of the voltage divider which is generally a combination of R1 & LDR is connected to the Base of the BC547 & a white LED is connected to the  Collector. The 220Ω resistance limits the current and prevents the LED from getting damaged.

When light falls on the LDR the resistance offered by LDR is very low. Let, R1 = 220 KΩ & R(LDR) = 4.6 KΩ. If Vin is 9V then voltage on the Base of BC547 = 0.184 V which is less than 0.7V so the BC547 is now off and the LED will not glow. Now when it is dark the resistance offered by LDR is very high in order of MegaΩ. Suppose practically R(LDR) = 150KΩ so Base voltage of the BC547 is 3.648V which is greater than 0.7V. Now current will flow from C-E of the BC547 & the LED will glow.

There are different sizes of LDR available in the market. Sometimes small LDRs don’t work properly in the circuit, so to increase the sensitivity connect two small LDRs in parallel or change the value of R1 which can be calculated from the Voltage Division formulæ.


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