Reading temperature using LEDS and temperature sensor circuit (using opamp)

Thread Starter

gandoN9

Joined Oct 12, 2020
18
Hello,

I have a little project where i need some help.

A temperature sensor (MCP9700-E / TO) is provided, which outputs are in proportion to the temperature.

The task is to make a circuit with operational amplifiers, which shows one LED brightness temperature and a change in temperature with two more LEDs.

The "zero point" of the temperature (above which the diode starts to light) must be adjustable
potentiometer.

Right now i have made a schematic:
1602597041037.png

Am i in right direction making this schematic? Do i need to add another opamp for other 2 leds to show change in temperatures or how should i approach this problem?

What do i need to change and add to this schematic?

Much help will be appriciated.
 

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ericgibbs

Joined Jan 29, 2010
11,122
hi N9,
Have you a more modern choice of OPA, the 741 is an old specification and limited performance.

Define change in temperature, ie: up, down and what change in resolution.

E
 

dl324

Joined Mar 30, 2015
11,894
What do i need to change and add to this schematic?
You need to tell us the power supply voltage and the voltage range you expect from the sensor.

The 741 is not intended to operate from a single supply. Why are you using an opamp as a comparator?
 

Thread Starter

gandoN9

Joined Oct 12, 2020
18
In the class the opamp we have is: TS922A which i need to use.

Power supply what i have in class is able to supply 0-9V.

Voltage range for the sensor is 2.3V to 5.5V (reading the manual)

Im kinda confused, what should i do or change?
 

dl324

Joined Mar 30, 2015
11,894
So i can try to put the schematic together with breadboard in the class.
Do you have a DIP adapter? If not, it's going to be problematic to breadboard.

What a waste to use an opamp like that as a comparator. What school are you attending?
 

Thread Starter

gandoN9

Joined Oct 12, 2020
18
Hi,
Specify the range of temperatures and the required accuracy required.
E
The range for temperatures i can pick myself, its not given.

One led should show LED brightness temperature - the higher the temperature, the brighter the LED is.

And 2 other leds should show change of temp. (for example some low temp led goes on, and some high temp other led goes on)

The LED current must be proportional to the temperature

No required accuracy is given.
 

dl324

Joined Mar 30, 2015
11,894
The range for temperatures i can pick myself, its not given.

One led should show LED brightness temperature - the higher the temperature, the brighter the LED is.

And 2 other leds should show change of temp. (for example some low temp led goes on, and some high temp other led goes on)

The LED current must be proportional to the temperature

No required accuracy is given.
You started out with one comparator and one LED. Now you're up to 3 LEDs. It would be helpful if you posted the entire text of the problem so we can see what you have to work with.
 

ericgibbs

Joined Jan 29, 2010
11,122
One led should show LED brightness temperature - the higher the temperature, the brighter the LED is.
Hi,
The circuit you posted cannot do that, the output is either high or low.
It is a comparator.
E
 

Thread Starter

gandoN9

Joined Oct 12, 2020
18
Hi,
The circuit you posted cannot do that, the output is either high or low.
It is a comparator.
E
Should i use non-inverting connecting style for this led? And what about 2 other leds for change of temp. Do i need another opamp for that aswell?
 

ericgibbs

Joined Jan 29, 2010
11,122
hi,
You should note that changing the current thru an LED will not give a linear change in brightness.

First solve the problem of the increase/decrease in temperature in controlling the LED brightness, it will be a very rough approximation.

So a linear OPA set up is required for this part.

Can you do that and post your circuit.?
E
 

Thread Starter

gandoN9

Joined Oct 12, 2020
18
hi,
You should note that changing the current thru an LED will not give a linear change in brightness.

First solve the problem of the increase/decrease in temperature in controlling the LED brightness, it will be a very rough approximation.

So a linear OPA set up is required for this part.

Can you do that and post your circuit.?
E
Im not sure how i have to approach to the circuit for this single led increase/decrease in temperature. Kinda confused already. I tried some combiniations in simulator for that. But i cant get it working like that
 

Thread Starter

gandoN9

Joined Oct 12, 2020
18
hi,
OK,
Post the best sim circuit that you have created, we can check it over.

E
Sorry for late response.

So i tried this circuit:
1602759616598.png

And it worked, when temperature from sensor is raising, then my led goes brighter. Am i on the right way?

So i need The "zero point" of the temperature (above which the diode starts to light) must be adjustable
potentiometer.

Would it work if i add potentiometer in this circuit to this place?

1602760016408.png
 

ericgibbs

Joined Jan 29, 2010
11,122
hi N9,
The LED will start to light when the Vout is approx greater than 2V, this means without R5, the LM35 will have to be at approx 45C before the LED will start to light.
The OPA has a gain of approx 5, the R5 resistor adds a low voltage to the LM35's output, look at the plot I posted.

So you get a dim light at 0C and bright light at 100C.

You must tell what are the required temperature limits.
E
 

Thread Starter

gandoN9

Joined Oct 12, 2020
18
hi N9,
The LED will start to light when the Vout is approx greater than 2V, this means without R5, the LM35 will have to be at approx 45C before the LED will start to light.
The OPA has a gain of approx 5, the R5 resistor adds a low voltage to the LM35's output, look at the plot I posted.

So you get a dim light at 0C and bright light at 100C.

You must tell what are the required temperature limits.
E
Oke thanks for explaining, you have been very helpful so far.

So i added potentiometer to R1, R2 looks to be working.

The temperature limits are not given to me.

The second half of the task is to add 2 leds to to show change of temperature. For example: temperature rises - one LED lights up (for example 50degrees) and temperature is falling, 2nd LED lights up (less than 50 degrees).

Do i need another opamp for this? Or this task can be done with only one opamp?
 
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