what is the knob? simple push button or encoder? what is the display type? common anode, common cathode?

 

So let's make it clear.

1) You have a 3-digit display.
2) You have a temperature sensor.
3) You have a control knob that you can turn.

What is supposed to happen?

I'm using the image below as an example. What happened is that if the temperature control on the image below turns high it displays the temperature in digits like 200 to 480 that is when increasing or decreasing. That is what I want

 

What it the temperature range—from low to high—that you want to cover, and in C or F?

C

 

C

What temperature range? e.g.: 100-400°C, etc.

Also, please answer the other questions so we can help.

 

What does it have to control?
Where does the sensor have to go?
What voltage will it have to switch, what will it have to be powered by?

Heating element

I'm using a thermocouple heat sensing circuit built using CA3140

12v

 

What temperature range? e.g.: 100-400°C, etc.

Also, please answer the other questions so we can help.

200 to 480

 

what is the knob? simple push button or encoder? what is the display type? common anode, common cathode?

At this point, the knob is only specified as a control that does something. He doesn't have any idea of what the part would be behind the function.

He's trying to reproduce part of the behavior of a hot air soldering station, though I have no idea yet what is being controlled or why...

 

very specific...

 

This is like squeezing blood...

1) We have 3-digit display.
2) We have a temperature sensor.
3) We have a control knob.

Display shows temperature: (enter your own values)
Range: 0-400
Unit: C

What does the knob do?
Turn the knob to a desired setpoint.

What does the temperature sensor do?
Sensor info is used to indicate current temperature.

How do you distinguish between setpoint and current temperature?

What makes the temperature rise and fall?

In one statement: What are you trying to build?

 

This is like squeezing blood...

1) We have a 3-digit display.
2) We have a temperature sensor.
3) We have a control knob.

The display shows temperature: (enter your own values)
Range: 0-400
Unit: C

What does the knob do?
Turn the knob to a desired setpoint.

What does the temperature sensor do?
Sensor info is used to indicate current temperature.

How do you distinguish between set point and current temperature?

What makes the temperature rise and fall?

In one statement: What are you trying to build?

I have already built a circuit that can control everything in the device below .... I only want to know how to display the temperature on led.

 

I have already built a circuit that can control everything in the device below .... I only want to know how to display the temperature on led.

How do you adjust the set point now? What is the circuit you have built?

 

How do you adjust the set point now? What is the circuit you have built?

I used potentiometer to adjust the set point

 

I have already built a circuit that can control everything in the device below .... I only want to know how to display the temperature on led.

You have to transform your numbers from whatever format the are in to seven(7) binary values for each decimal digit. If you have 1000 different decimal digits in the range of 000 to 999 you can use the same set of Boolean function for each digit. There is even a chip you can still get that will do the job for you.

Purchasing source
https://www.circuitspecialists.com/...- US&msclkid=4257b96ff2ff18b9efc4d898fe5a5da8

Datasheet
https://www.ti.com/lit/ds/symlink/cd4511b.pdf

Check out page 4 of the datasheet for the actual truth table that you can use if you want to build a 3-digit decoder with an FPGA.

 

You need to show your circuit.
Does your temperature control circuit really control the temperature or just adjust it?
If it already has feedback to control the temperature, why not use that signal to drive the display?

For some reason, you are just dripping small details one at a time and that really makes it hard for us to help you.
It would have been good to start with something like...

I have a hot air de-soldering tool with a home made temperature controller and I want to display the actual temperature, and adjust it with a pot.
"Picture of tool"
"existing circuit"
"tool power specs"

 

As a learning project I mapped a 4 digit 7 segment display without a MCU and it was a LOT of work to just cycle through the numbers much less present any meaningful data. In fact getting past the first digit while multiplexing became too much to handle so I gave up.

I suggest an Arduino and Pi Pico and I greatly prefer writing code in micropython on the Pico. Hard to find guides for Pico though learning python is an excellent resource if you have a experience with computer science and is a very powerful language for mathematics.

 

Why not use an I2C OLED display?

These would let you display the actual temp, and the set point. And if the heater is running or not.
Maybe even air flow setting?
I2C displays are quite easy to drive and very cheap. This particular one is less that $4.
They look good too.

https://makersportal.com/blog/raspberry-pi-pico-oled-display

 

first you need to decide on what sensor to use. it need to be mechanically compatible with your application (soldering station or whatever) and of course be able to operate in desired temperature range. for that application one would normally use a thermocouple. K-type is very common. next you need to condition this signal, convert it to digital form and display. you will need to do linearization unless displayed value is just for orientaiton.

there is tons of ready units that do this such as

https://www.amazon.ca/Thermometer-T...ocphy=9000826&hvtargid=pla-798154789178&psc=1

or

https://www.amazon.ca/Universal-Con...jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

if you want to build one, using MCU is a good idea, very few and small parts:
http://www.technoblogy.com/show?2G9S

 

You need to show your circuit.
Does your temperature control circuit really control the temperature or just adjust it?
If it already has feedback to control the temperature, why not use that signal to drive the display?

For some reason, you are just dripping small details one at a time and that really makes it hard for us to help you.
It would have been good to start with something like...

I have a hot air de-soldering tool with a home made temperature controller and I want to display the actual temperature, and adjust it with a pot.
"Picture of tool"
"existing circuit"
"tool power specs"

Exactly

I used the pot as temperature control and the thermocouple to sense the temperature and also for overheating

 

first you need to decide on what sensor to use. it need to be mechanically compatible with your application (soldering station or whatever) and of course be able to operate in desired temperature range. for that application one would normally use a thermocouple. K-type is very common. next you need to condition this signal, convert it to digital form and display. you will need to do linearization unless displayed value is just for orientaiton.

there is tons of ready units that do this such as

https://www.amazon.ca/Thermometer-Thermocouples-50-1300°C-Temperature-Measurement/dp/B07P8NWH67/ref=asc_df_B07P8NWH67/?tag=googleshopc0c-20&linkCode=df0&hvadid=335526108466&hvpos=&hvnetw=g&hvrand=14687228863568070777&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9000826&hvtargid=pla-798154789178&psc=1

or

https://www.amazon.ca/Universal-Controller-Temperature-Probe-Alarms/dp/B01I7MIM16/ref=sr_1_25_sspa?crid=1SJ8DKAFZCIEQ&qid=1643581556&s=industrial&sprefix=500c+thermometer,industrial,60&sr=1-25-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzNkdVN1dVT043OEpXJmVuY3J5cHRlZElkPUEwODk3MjQxM01HVVM5UVVURU5OQyZlbmNyeXB0ZWRBZElkPUEwNzg0ODM0MllYSzc2MFhJVlQ0SCZ3aWRnZXROYW1lPXNwX2F0Zl9uZXh0JmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

if you want to build one, using MCU is a good idea, very few and small parts:
http://www.technoblogy.com/show?2G9S

The k type thermocouple is already embedded in the heating core

 

You need to show your circuit.

I used the pot as temperature control and the thermocouple to sense the temperature and also for overheating

you are not answering questions. words are not a circuit...