ph meter switch

Discussion in 'The Projects Forum' started by aiannar974, Jun 10, 2014.

  1. aiannar974

    Thread Starter New Member

    Mar 9, 2013
    2
    0
    Hello - I am trying to make a cirsuit that will send a signal to a switch when ph increases above a given value in the 7.6ph. With the attached circuit the millvolts from the ph probe / circuit should be about 33 millivolts. I know I will get drift overtime so I need to make sure I can vary my comparison. I was thinking of using an lm317 chip and knocking its voltage down with a reisitor to get my comparison voltage. Also, I really don't care about the exact ph reading because I am just trying to turn on an acid dosing pump. I can measure the ph seperately and then adjust the comparison to dose at a different point to maintain a wanted ph.

    So, my questions are as follows:

    Should i use an lm317, can i just compare to ground and knock down the ph millivollts with a pot? Can a pot adjust in such a small range?

    What can is use something like a CA3140 MOSFET? is it sensistve enough?

    The following are the circuits I have been referencing:

    http://www.66pacific.com/ph/simplest_ph.aspx

    http://forum.allaboutcircuits.com/newthread.php?do=newthread&f=6


    Please give me your thoughts.

    THank you,

    Anthony
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,120
    3,046
    I would use a precision voltage source, so that it's more stable with temperature and won't drift. An LM317 is pretty good, but a genuine voltage reference would be better. They're made for the purpose.

    Then I'd use a comparator to make the ... comparison of your amplified sensor voltage. This will help ensure that the output is either high or low and swings nicely from one to the other. The comparator can control a MOSFET that in turn runs your pump. (You may want have the pump be a "shot" pump that gives a squirt, waits a while, then gives another one. In other words, you don't want overshoot.)
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,387
    497
    Values in millivolts will probably be too small.

    If you keep the buffer circuit, then you can add an amplifier circuit after it, then you can add comparator circuit after that. So you would have three stages:
    Stage 1. Buffer
    Stage 2. Amplifier (Here you can raise your millivolts into volts)
    Stage 3. Comparator

    I am going to use the circuit you linked one more time. In that circuit the measurements go from -300 mV to +300 mV, that is -0.3 V to +0.3 V. Set the amplifier to provide gain of 10 (for example).
    Example:
    So say +100 mV goes in (+0.1 V), the amplifier has gain of 10, +0.1 V*10=+1 V, 1 Volt comes out. Where is adjusting 100 mV is a pain in the a$$, adjusting 1 Volt is something you can do with your fingers.
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,387
    497
    Also, you can get chip that has several op-amps on it, they are usually made in 1, 2 and 4 op-amps on one chip. If you get chip with 4 op-amps, you can use 3 of them for the circuit I described, this way you use one chip and have nice compact circuit.
     
  5. aiannar974

    Thread Starter New Member

    Mar 9, 2013
    2
    0
    All very creative. I will work on this. Thabks for the help. I am new at this, I will post back what I come up with.

    Thanks again.

    Anthony
     
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