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.

opamp

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

“HERE IN THIS PROJECT WE USE OPAMP AS A COMPARATOR”

Fig2
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 

Fig3
Bread board setup
Written by : Sourav Tamli 
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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.

 

 

64

 

/The Dark Detector Circuit
2
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.

/Results

8 7

 

The Light Dependent Resistor

Light Dependent Resistor or LDR :

The LDR is also known as the Photo-resistor or Photo-cell, a light controlled variable resistance which exhibits photo conductivity. Ideally it behaves as a wire having a zero resistance when the incident light intensity increases as well as much higher resistance value when the intensity decreases. So, we can say that under bright light, resistance is very low & in darkness resistance is very high or infinite ideally.

ldr
LDR

This image shows how LDR looks like. There are different sizes available in the market. It is made of a high resistance semiconductor. The red track on the LDR is a layer of Cadmium Sulphide (CdS). Practically LDRs are not polarized.

 

Area of Application :

There are a large numbers of applications that can be done by using LDRs such as an automatic street light which turns on itself when the sun sets, a morning alarm which senses the morning light and initiate the alarm, many security applications like the laser alarm, it can also be applied as solar tracker, a line follower robot also uses a LDR to sense the black line etc.

 

Following images will describe the variance of Resistance as well as the circuit symbol & some test result:

graph
Variance of resistance with respect to light
under light
Resistance value in front of a CFL in the order of 200KOhm
in darkness
Resistance value when the CdS track is blocked

 

–The most simple circuit that you can do using a LDR–

street light
Light controlled switching circuit

This simple circuit is the primary stage of LDRs applications later it can be modified as a automatic street light. When LDR is uncovered then LED is in OFF state and when covered LED is in ON state. So it may be a perfect system for street lighting. Here the BC547 transistor is working as a switch. So the results are quite satisfactory…

under light
When the torch is pointed towards the LDR
darkness
When the torch is removed from the LDR

So, this is how we can use LDR in a circuit. Also we can construct sensors as a physical parameter (light) is being converted to resistance value.

Coming soon with other projects bases on LDR.

Using the Breadboard

The Breadboard 

For an electronics hobbyist as well as an engineer the breadboard is the primary platform to deal with. The white dotted board is used to test electronics circuits before soldering it permanently. 

Advantages of using the Breadboard:

  1. For testing whether a circuit is working or not.
  2. Experimenting by replacing old components with a new one having different value.
  3. Breadboard reduces time of soldering.
  4. Testing different IC operations in a short time.
  5. Low cost & well available.

How a Breadboard looks like?

Breadboard
This is how the most used Breadboard looks like there are also lot of variations in sizes.

What inside a Breadboard?

 The upper & lower both two rows are connected horizontally. They are used as power supply terminals, they are not continuous. The middle portion of both supply terminals are not connected so we have to use jumper wires to make it continuous on both sides.

The other terminals below the supply terminals are connected vertically, these terminals is the prototyping terminal.

Setting up Breadboard for your very first project :

A neat & clean work on Breadboard can give mental satisfaction, make good impression, saves time also it can make all of the connections understandable. So if there is something wrong anyone can correct it easily. Use jumper wires (Single Core) to make connections. Remove insulation of the wire using a wire stripper. Jump terminals as shown below hence you get the upper and lower rows connected. So it will be easy to power components or devices from anywhere of the Breadboard.

Some tips & tricks :
  • Try to use less jumper wires as possible to make the platform neat & clean.
  • Place ICs carefully. After placing the IC on the breadboard press it’s both end and then in the middle.
  • Try to remove sufficient insulator from wire not more not less otherwise two conductors closer to each other can make short circuit.
  • Remove ICs with care. Use a needle pass it through the middle channel of the breadboard and remove the IC.

Happy Prototyping!