7 segment digital display thermometer

Discussion in 'The Projects Forum' started by chrischrischris, Mar 8, 2012.

  1. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Hi.
    I've been trying to search for a circuit diagram for a 7 segment digital display thermometer that can be made without having to do any programming. It only needs to be accurate to about 1 degree C. Does anyone know if this is possible without programming?
    Chris
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. soda

    Active Member

    Dec 7, 2008
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    Hi

    This is what i'm using.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    You can use an LM35 and ICL7135 ADC.
     
  5. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Hi Soda.
    Can I use your suggested layout with 3 individual seven segment displays - first one for minus, second and third for degree celcius?
     
  6. Wendy

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    Mar 24, 2008
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  7. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Mr Chips

    Sorry I have limited knowledge here,so I'm trying to decifer your suggestion. It looks like the LM35 is a centigrade linear temperature sensor and the ICL7135 can output the reading. However it hooks up to a 7 segment led display. Correct?

    I'm wanting to connect to 3 single digit 7 segment led displays (6"). Can I use this setup? If so, is there a circuit I can see?
     
  8. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Thanks Bill.
    I didn't realize I had any replies to this thread from a while back as I didn't get any email responses. I assumed no one was answering. I just happened to go back to this thread now and noticed replies.

    Bill, I still am interested (as a plan "b") in my last question in that thread on the possibility of using the cheap ebay digital thermometer and attaching 3 larger seperate led segments in lieu of the smaller 3 digit unit. Any chance on the info I've uploaded?

    When you said "yes" to just wire up the 7 segment displays the way the multiplex is wired, does this mean that I would wire all "a" segments together, all "b" segments together etc. Then wire my linked "a" for example to the "a" on the small thermometer? If so, what about the commons? What do I do with those?
     
    Last edited: Mar 31, 2012
  9. Wendy

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    Mar 24, 2008
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    OK, the first question is what the displays you are using. You can tap off of the resistors on the chip side, these will be logic levels. You just need to determine what the polarities are.

    Oddly enough, you have posted a possible schematic. I suspect you will need transistor drivers on the other side too though, to increase the current through whatever type of displays you will be using.

    You will need to make these circuits as high impedance as possible though, since we don't know how close to the limits the existing circuit uses.

    So, what are the odds this is your circuit?

    [​IMG]
     
  10. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Bill,
    I'm assuming the 3digit display in the thermometer is common cathodes as when I put gnd to each segment pin they light up. Hence when I tap off the circuit, I was looking at passing the signal of each segment via a high voltage/current Darlington arrays (ULN2003a) to handle the higher voltage/current to the new LEDs (as I am with the other circuits on the soccer score board).

    The digits I'm using are common anode 6" (should arrive from OS this week or next). Forward current per segment is max 60mA, voltage range 26V to 15V. I'm aiming at 24V if possible and lower current to make the digits last longer.

    My thermometer does seem to be close to the circuit I posted. Go figure. However my circuit only has 1 IC.

    Should I start by: a) linking all the "a"s on my digits, then "b"s, etc, b) pulling off the existing 3digit display, c) connecting each output to a darlington array, d) connecting the other darlington pin to my 3 digits (where the "a"s are joined, etc)? The only issue I would then have is the commons. I don't know where to connect them on the small board? Any hints?
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    You have 7 digital signals, pins a-f, and 3 multiplex signals, call the D1-D3. All of them are digital in nature (though without the resistors the signal levels on the display is going to be awful low).

    Basically you just want these signals to turn on/off transistors to control much larger current levels.

    What you need to do is establish what the signal levels you have to play with on the 10 outputs, then design transistor drivers (or solid state gates) around them. Since I don't have the circuit in front of me, I can't tell you what the logic levels are, and so can't come up with drivers.

    I would suggest probing around your circuit to determine what these levels are.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    Here is how I was thinking of doing it on a stadium circuit for myself. I never did build this sucker myself, but you will note I used transistor drivers on both sides. The display was a major simplification, but it is the currents that matter.

    As a bonus I'll include my mechanical drawing for the large digits. The spacing on the marks was 1" .
     
  13. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Bill - success. Thankyou.

    I ripped off the 3 digit display from the temperature unit, had 12 pins, found a site that listed what pin 1 to 12 were, compared notes to what I found "a", "b", etc to be. I then connected the 3 single digits as shown below. The commons were pins 8, 9 and 12. Connected via resistors and bingo. Next to upside to the larger digits when they arrive. As for transistors between - will do when I put it together. Thanks again.
    3 digit wiring.jpg
    3 digit details.jpg
     
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