Reading LM35 with AD0804

Discussion in 'Circuit Help' started by Aperson, Nov 21, 2011.

  1. Aperson

    Aperson

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    So i want to grab the analog signal of the LM35 and convert it to digital with the AD0804 with this diagram http://www.youtube.com/user/maximomorales#p/c/264B666DBE3B03DB/0/BJ_1i9qKZog (step by step build http://www.youtube.com/watch?v=1S3jBn8taXI) but it's not working at all.

    All the leds simply go on so it reads 1111111 even with no input and nothing changes them significantly. I measured the voltage on the INPUT pin of the AD and it gives out 5....is this normal?? shouldn't it be 0 or something because it's an input pin , not an output pin?

    Also on the diagram i don't understand what the micro pulser is doing...is it switching the thrid pin to ground?

    I'm really new to this (i am also supposed to be on engineering not on electronics lol) and i am getting kind of desperate with this thing -_-

    Any kind of help would be appreciated D: (should i post pics of my circuit or something?)
     
    Aperson, Nov 21, 2011
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  2. Aperson

    Aperson

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    Aperson, Nov 21, 2011
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  3. Aperson

    duke37 VIP Member

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    I can't look at videos but, looking at your diagram, you do not have any capacitors to bypass the power supply.
    Looking at the national data sheet it seems as if the LM35 is prone to oscillation. They show the device connected to an A/D convertor with a resistor and capacitor damper to stop oscillation. Digital devices can produce lots of rubbish on the supply lines.
    The voltage on the input pin of the A/D is defined by the LM35 unless something is faulty. If you disconnect the LM35 and connect the input pin to ground, you should get zero out..
     
    duke37, Nov 21, 2011
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  4. Aperson

    davenn Moderator

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    hi Aperson

    now home from work and with a decent internet connection, I was finally able to see the videos. As with your diagram in your second post therer's definately no capacitor bypassing done of the PSU, as Duke37 commented about in your drawing.
    This is really essential. if you are using a 7805 type voltage regulator as in the videos, you should have a 100uF in the input to the regulator and a minimum of 7V there. Preferably 8V to 12V to ensure that the regulator produces a stable 5V output.
    On the output you should have a 10uF electrolytic or tantalum capacitor.
    Both the 100uF and the 10uF should be mounted as close as possible to the 7805 reg.
    For good practice it is normal to have a 0.1uF or 0.01uF right at the Vcc pin of the chip.
    All those cap values and locations mentioned are the good standard practice for digital circuits. have a good look through old computer boards and you will find 0.1 or 0.01 decoupling caps on the Vcc rails of virtually every chip.

    Now fiollow Duke's other comments re the connections between the LM35 and the ADC0804 and see how you get on :)
    report back with hopefully good news

    cheers
    Dave
     
    davenn, Nov 21, 2011
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