Thermocouple with 7-segment display

Thread Starter

slevesque

Joined Jan 31, 2023
43
Hi everyone,

I am thinking to make a small circuit which will include a thermocouple, or some temperature sensor, and a display to display the temperature value. I know using a microcontroller would be simple to do that, but I can't use one.

I was wondering if someone could help me with that. I would read the voltage value from the temperature sensor and then I would like to display it on some sort of display. I was thinking maybe a couple of 8 segment display, but there is probably something better.

My main concern here is how to translate the voltage value of the temperature sensor to the display. If anyone could help me with that I would greatly appreciate it.

Thanks in advance,
Samuel

Moderator edit: Thread title changed to 7-segment display instead of 8.
 

dl324

Joined Mar 30, 2015
16,845
Welcome to AAC!

Take a look at LM3914.

EDIT: My response was for an 8 segment display (e.g. bar graph) that has since been changed to 7 segment.
I was thinking maybe a couple of 8 segment display, but there is probably something better.
 

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Last edited:

KeithWalker

Joined Jul 10, 2017
3,063
Hi everyone,

I am thinking to make a small circuit which will include a thermocouple, or some temperature sensor, and a display to display the temperature value. I know using a microcontroller would be simple to do that, but I can't use one.

I was wondering if someone could help me with that. I would read the voltage value from the temperature sensor and then I would like to display it on some sort of display. I was thinking maybe a couple of 8 segment display, but there is probably something better.

My main concern here is how to translate the voltage value of the temperature sensor to the display. If anyone could help me with that I would greatly appreciate it.

Thanks in advance,
Samuel
The best approach would be to use a thermistor or resistive device that has a known resistive temperature coefficient (a coil of copper wire?) . Thermocouples have zero output at ambient temperature so the accuracy would vary if room temperature changes.
I built one using an old 100 Ohm copper relay coil, an LM358 and a cheep 3 wire digital voltage display from AliEx. I had to blank out the decimal point with a marker.
Thermometer.jpg
 
Last edited:

Ian0

Joined Aug 7, 2020
9,668
LM35 is probably the easiest to use with a LM3914.
1.2V full scale on the LM3914 is 120°C and the LM35 will work to 150°C
 

Reloadron

Joined Jan 15, 2015
7,501
I would start with deciding which temperature sensor you want to use. Things like temperature range, uncertainty and response time figure into the decision also the environment the sensor will be exposed to. I also assume you mean 7 segment LED displays and not 8 segment. Next once you have a sensor chosen then you can come up with a circuit designed around your sensor choice.
I know using a microcontroller would be simple to do that, but I can't use one.
So would this be a course assignment as in schoolwork? While a uC would be the more practical approach there are plenty of analog solutions again depending on sensor chosen. You may want to give some of these circuits a look and get an understanding of how they work. Again knowing temperature range and uncertainty is important and the best readout you will get with two 7 segment displays is 99 or 9.9 or .99 and that's as good as it gets.

Ron
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
Hello eveyrone,

Thanks for all your answer.

I would need a range of temperature from -10 degrees Celsius to 150 degrees Celsius. I will look at the LM35 and the LM3914 and see if I can figure out how it works.

I don't want to use a uC because I want to keep to cost of that circuit the lowest possible.

I don't need to necessary use the 7 segment, if you have another form of display in mind I would use it for sure.
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
The best approach would be to use a thermistor or resistive device that has a known resistive temperature coefficient (a coil of copper wire?) . Thermocouples have zero output at ambient temperature so the accuracy would vary if room temperature changes.
I built one using an old 100 Ohm copper relay coil, an LM358 and a cheep 3 wire digital voltage display from AliEx. I had to blank out the decimal point with a marker.
View attachment 286512
I am not sure how the digital voltage display can output the temperature. You need to set the range and then it will work? I'm not sure how to set the range too. If you could help me with that.
 

Ian0

Joined Aug 7, 2020
9,668
I don't want to use a uC because I want to keep to cost of that circuit the lowest possible.
The analogue solution will probably be the more expensive. You can get a big 32-bit micro with lots of memory for the price of a simple LED bargraph driver IC like the LM3914.
 

Ian0

Joined Aug 7, 2020
9,668
Well in that case, do you have one in mind?
Commercially, I think I’d go for the LM3914, because of the shorter design time, and not having to program it in production, and micros are still a bit scarce.
For a 1 off, perhaps still the LM3914, because of the time taken to write the program.
To tackle it with a micro, my favourite for a small analogue job is the LPC824, and they are actually in stock (it’s a miracle).
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
I' just wondering here is a uC will consume more current than the analog circuit? I would like to use the lowest current possible.
 

MrSalts

Joined Apr 2, 2020
2,767
I' just wondering here is a uC will consume more current than the analog circuit? I would like to use the lowest current possible.
When you say "8-segment display", do you mean 8-segment bar graph or do you mean a numerical 7-segment display plus decimal point?
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
Commercially, I think I’d go for the LM3914, because of the shorter design time, and not having to program it in production, and micros are still a bit scarce.
For a 1 off, perhaps still the LM3914, because of the time taken to write the program.
To tackle it with a micro, my favourite for a small analogue job is the LPC824, and they are actually in stock (it’s a miracle).
I am new to all this stuff. Can I program the micro with basic programming language like C++ or Python?
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
Well that changes things a lot. The analogue solution would be A/D converter, and 7-segment display decoder. The micro wins by a margin.
All right so if I understand correctly. I would need the thermistor and an ADC to convert the voltage value to bincary. Then I would need to pass the digital signal to the micro which will determine the temperature value and will finally power two or three 7-segment to display the temperature value.

Am I right thinking that way?
 

Ian0

Joined Aug 7, 2020
9,668
All right so if I understand correctly. I would need the thermistor and an ADC to convert the voltage value to bincary. Then I would need to pass the digital signal to the micro which will determine the temperature value and will finally power two or three 7-segment to display the temperature value.

Am I right thinking that way?
I like the LPC824 because it has a built in 12-bit ADC. You can also implement a look-up table to determine the temperature from the resistance of a thermistor, which makes the sensor much cheaper than a thermocouple or a temperature sensor IC.

But if you have a single 7-segment display, how are you going to display temperatures between -10°C and +150°C on a single digit? That’s why we all thought you were making a bargraph.
 
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