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Temperature display with ICL7107CPL


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Hi MP,
Thank you for the hint. First i must say, i am at very beginning level of electronics, so please excuse me if i can not understand even simple stuff :-[. i am building this circuit for two reasons. one is to check temperature sensors that made up with LM20 semiconductor and the other reason is to read the temperature from the sensor.
So as i said to get some understanding of the circuit i built the test circuit that was on the ICL7107 data sheet.  Couple of modifications added which you can see in the attached schematic. The scale i selected was 2V. This circuit works very well and it display the output voltage from the LM20. But i want to see the actual temperature instead of the voltage. so i need help modifying this to display the temperature. lowest temperature I'm going to measure is 0

Temp_Display.pdf

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You can invert the voltage with an inverting op-amp, ICL7660, or any other charge pump circuit.
You will need to use positive and negative supplies on the 7107 to read the negative voltages. (Your attached schematic looks ok for this)
In regards to the question about "Common", you can float this voltmeter. The input and the power supply do not have to be connected to common.

Have you seen the thread in Projects Q/A about this chip and the two projects we have on this site that use this chip? You should go through those threads and look at the projects for additional information.

MP

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Ok here is my full schematic including the power supply part. so far i got all this built on a temporary copper strip board. Please let me know whether that LM7805 needs filter capacitors. I am using a standard 9V battery for this circuit.

Thanks again MP for your support on this and yesterday i read all the articles related to ICL7107 posted in this website. most of them were voltmeter projects. still i can not figure out how to display 30.0 when it is actually reading 1.515V. Do i have to do anything with the Vref?

Thank you,
Gayan

Temp_Display.pdf

post-25686-14279143272562_thumb.jpg

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Hi Gayan,
I missed your question earlier about the meter displaying 30, etc. R4 should give you enough adjustment to trim the voltage scale, but not the actual range of the meter. If you are using the 2 volt scale, you are not going to display a 30. You will need to change the input scaling so that it is incorrect, then change the reference pot, R4, to match your temp sensor scaling. Look at the voltmeter in the projects section that has a selection switch for range on the input. Here is the link:
http://www.electronics-lab.com/projects/test/007/index.html
This should give you the idea.
I know all of the other posts are about voltmeters, but that's all this chip does. Your application is actually a voltmeter scaled out of adjustment. All of the design discussions are certainly valid. Good to hear you read up on these. I know there were quite a few pages.
Caps will help keep the regulator from oscillating. I don't know how well regulated your supply is up to the 7805, but if the reading fluctuates too much, adding a cap on each side of the regulator will take care of this.

MP


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Why is that some of the segments of led display brighter than other segments? is that because i don't have two capacitors around the voltage regulator? if i do need filter capacitors what are the best values. When i serched internet i found a schematic with 100n capacitors, will that be ok for this?

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That shouldn't cause come of the segments to be brighter than the others. This could be due to the displays you are using or you might have damaged them. Some designs do not use the diodes to drop the voltage to the displays and some do. You might have a display that requires more current and the diodes might be dropping too much. Another thing to look for is the wiring to the segments. Each of the segments is lit from a pin on the 7107. You should check the wiring since this is a perfboard setup.
If you use capacitors, 100n should be ok. If you go with bigger in the input of the regulator, it is ok as well. I recommend the use of capacitors simply because you are not only filtering the electronics from the supply when you use capacitors, but you are also filtering the supply from the electronics. This is a problem when using digital chips since they all incorporate some type of switching or clocking actions. This clocking action can move through the supply lines from component to component without the capacitors to smooth out the supply lines.

MP

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Hi MP,
Thank you so much for the reply and i learned a lot from you in this project.
I went through the following links to get some understanding about the scaling. Both links relates to two projects that list under the project section.
http://www.electronics-lab.com/projects/test/014/index.html
http://www.electronics-lab.com/projects/test/007/index.html

Now could you please correct me if i'm wrong again.
(1) If the input to the pins 30 & 31 (IN HI, IN LO) is between 0V-2V then i select the 2V scale. if it is between 0V-20V then scale is 20V. I hope i'm right so far.

(2) output voltage from temperature sensor is between 1.160V to 1.864V. so it means the input to the ICL7107 is less than 2V. In that case i select 2V scale, right?

(3) Now i want to display 0.0 on the display when it reads 1.864V and display 60.0 when it reads 1.160V. What i confused about is you said i can not display 60.0 on the display if i selected the 2V scale right? but the input is still under 2V. Or are there two scales for display and the input? Could you please kindly explain me about this part again?
Thank you
Gayan

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Gayan,
I think you will have a problem with the decimal point since a display of 60.0 would normally be done on the 200V scale. If you float the input and make the reference voltage adjustable, you should be able to come up with a scale that allows you to go from 0 to 60 with your input voltage that is less than 2V...that is, if you invert the voltage first so that you are going in the correct direction when the temp sensor gets hotter. Let me know if this is not clear. I will expalin further about floating the voltage, etc.

MP

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Hi MP,
I got good news for you. spending more than four hours yesterday i finally managed to get the temperature reading on the display. As you said i played with the trimmer until i get a closer temperature reading and also i added another trimmer to the input as well. because i wanted to input 1.864V to the IN HI pin of the ICL7107 IC. That is the voltage for 0.0

post-25686-1427914327369_thumb.jpg

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Hi Gayan,
This is an LED meter with up to 19 LEDs draining the battery with 8mA each. Therefore the little 9V alkaline battery's life will be as short as 1 hour at which time the 7805 regulator won't regulate anymore because the battery's voltage will be less than the minimum of 7V that is needed. The circuit won't warn that its readings are wrong.

Use a low-dropout regulator then the battery will last a little more than 3 hours.

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Hi Gayan,
It looks fine. That is exactly what I was describing. Actually, it looks like you saved a step by connecting the sensor to the floating "In Lo" so that inverting the signal was not required. Good work!
In regards to the battery draw, there isn't much you can do about it. But I think you already know this draws a lot from a battery since you have already used this setup with another sensor. Will you eventually connect this to a power supply? If not, you might want to add some type of time-out circuit that turns off the displays after a pre-determined time. This is the source of your battery drain.
Regarding the capacitors: Use tantalums for that size.
Again, congratulations on your success.

MP

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Thanks MP & Audioguru for your comments. I think  I have to change the power supply design. I might use a DC adapter (close to 9V DC output), because 1 hour battery life probably not enough. Also note that this is not a permanent on circuit. It is only used to test a temperature sensor (which made up with LM20) and get the temperature reading. It take only takes less than 30 seconds for each. And also I preferred a portable device so i can test pre-installed sensors  without a AC wall socket. But doesn't matter, i will use a DC power adapter.

Now it is little bit easier to draw the PCB when i remove the battery from the enclosure. With the battery i only can have a PCB size of 60mm x 55mm. Without battery it is 87mm x 55mm. I'll post the finished PCB layout if someone interested in building this project.

OK i already got some tantalum capacitors 0.22uf/35V and 0.47uf/35V, i hope i can use these. I'll use .47 before 7805 and .22 on the output side of the 7805. please correct me if I'm wrong.
Thank you all again,
Gayan

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Gayan,
These values should work fine. You can also use combinations of capacitors to equal a different value if you wanted.
A lot will depend upon how smooth the DC is from your power supply. If you have problems, go to a larger value on the input of the 7805 to smooth out the signal better and a smaller value on the output if needed for better filtering of the higher frequencies.

MP

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