Constructive day :D

Thread Starter


Joined Feb 27, 2005
Hey guys.

Well since you lot have helped me learn so much and given me some good ideas and useful information in regards to my new found learning experience I thought I'd just let you know that I have come pretty far now!!

Yesterday I finally figured out what op amps and comparators are about and throughout the day have been working on a temperature sensing circuit that works REALLY REALLY well. I used an LM339 comparator IC along with the rest of the usual bits n bobs and successfully made a four level temperature sensor. Using the four comparators inside the 339 I made four LED's light up one after the other as the temperature rises and drops. I am amazed how well it works and really pleased wit myself, heheh.

Anyway, I am MAYBE jumping ahead of my abilities here, but it doesn't hurt to ask ... I would like to wire it up to a 7 segment display (1 is fine for now) and have the display go up and down (I don't mean show the temperature, just go up and down) depending on the conditions -- Is this possible with just four comparators ? I have a sneaky feeling that it isn't but that poses the question -- how to other temperature sensors work ?? They surely can't have like 100 comparators, each one sensing 1 degrees in temperature.
Moving away from the comparator route if that isn't possible, how would I make the 7 segment display change without comparators ? If you guys know how it is done then please let me know, I don't want anyone to go to the trouble of drawing it all out for me, but maybe just give me a few pointers on how exactly temperature sensors work.

Thanks in advance and thanks for all the previous help, it has been invaluable.

- Alex


Joined Jan 22, 2004

what you have already done is a temperature to voltage conversion. what you now want is to have the actual temperature displayed using 1 seven segment but that would limit your reading in centigrade scale of only 1 to 9 degrees. it could not even display a farenheit scale.

i had that same problem before but thanks to the members they suggested i use a dot/bar display driver (LM3915) or the ICL7107 w/c i employed, i will also be trying the LM3915. however the ICL7107 is an expensive device, but on my part it's alright since am going to sell it to a client. :)

but if you would like to make a digital voltmeter then use the 7107 and at the same time use it as a temp. display. :)

Thread Starter


Joined Feb 27, 2005
Hi mozikluv,

Thanks for the input there :) I understand that a single 7 segment display would not work as an accurate temperature display, I was just wanting to see if I could get my circuit working with one just to see if I could, hehe :) I will check out the capabilities of the 3915 display driver -- I have some in my component box :) Thanks for the suggestions :D

- Happs


Joined Jan 22, 2005
Happs, congrats on getting your comparator temp sensor working, do you have a schematic you can post ?

Appears you have already duplicated some of the circuitry inside a LM3914 (3915) bar/dot display driver IC. The 3914 contains ten comparators and a stable Vref, as well as current control so resistors are not needed on the LEDs.

Since you are using comparators, the output is already digital ( high/low) so to drive a 7-segment display you'd need a decoder or more appropriately an encoder ( priority).

IC drivers for a 7-segment display (to count from 0 to 9) usually require 4 inputs, so if you go with a flash type ADC built intirely from comparators you'd need a grand total of 16 comparators followed by a 16-to-4 priority encoder circuit ( a project within itself).