gates for tank filling

Discussion in 'Homework Help' started by gsd, May 13, 2005.

  1. gsd

    Thread Starter New Member

    May 13, 2005
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    0
    I need to design an arrangment of solid-state gates such that a pump will start filling a tank only if it is MT (not part full) & stop when it is full.

    The tank had 2 liquid level sensors - one at the top & one at the bottom, with truth table:

    liquid absent 1
    liquid present 0

    Can anyone help???

    Thanks
     
  2. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    have you done a google search on this project? there are lots of circuits like that in the net. :D

    if you can't find one suited to your need then we'll try to come up with something.
     
  3. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    In terms of what you have described, the tank can have only 3 states of condition. Either its full, empty or part full. I understand the pump is to stop when the tank is full but what is meant by "not part full"? Does that mean empty?
     
  4. gsd

    Thread Starter New Member

    May 13, 2005
    6
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    yes, it means it has to only start when empty but to go till it is full.
     
  5. gsd

    Thread Starter New Member

    May 13, 2005
    6
    0
    Thanks - was the first thing I tried. Answered all my other questions except this one.
    If I wanted to buy something to do it I'd have no problems, but I just need to draw a diagram.
     
  6. David Bridgen

    Senior Member

    Feb 10, 2005
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    Although your truth table doesn't look complete to me, you should be able to use those sensors to do exactly what you want.

    Can you tell us what the lower sensor's output is when the liquid is at or below its level, and what it is when above.

    And what the upper sensor's output is when the liquid is at or above its level, and what it is when below.

    In other words, complete information with which to construct a complete truth table.
     
  7. gsd

    Thread Starter New Member

    May 13, 2005
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    truth table would look like:

    empty 1 1
    part full 0 1
    full 0 0

    I guess you are missing 1 0 which would only occur if there was a sensor fault
     
  8. David Bridgen

    Senior Member

    Feb 10, 2005
    278
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    ....... and which column corresponds to which sensor?
     
  9. gsd

    Thread Starter New Member

    May 13, 2005
    6
    0
    column one is for the sensor at the botom of the tank & column two for the one at the top.

    I can work out how to draw the circuit for it, the problem I am having is that either the pump starts when it is MT & stops when it is part full (using an AND gate) or the pump starts when it is MT or part full and stops when the tank is full (using an OR gate).

    The bit I don't understand is how I am meant to make it only start when the tank is MT and only stop when it is full.
     
  10. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I know one picture is worth a thousand words, but time is pressing so here's just a description.

    Try this. Use three 2input NOR gates - A, B, and C.
    Connect A and B to form a bistable by connecting A o/p to B i/p, and B o/p to A i/p. When the two spare inputs are low. the circuit will be a bistable. This circuit works on the basis that when an input is pulsed high, the pulse will flip the bistable to turn on that gate, and it will stay that way when the pulse is removed.

    Connect the bottom switch (BS) to the spare A i/p. Connect the top switch (TS) to the two C inputs tied together. C now acts as an invertor. Connect its o/p to the spare B input. Use the B output for the motor drive (high = motor on).

    Let's assume A o/p is high and B is low (motor off) and tank is draining. TS is high so C o/p is low. When empty, BS switches high, turning on A and turning off B (o/p high). Motor runs, filling the tank. BS changes to low, but that will not affect the bistable.

    When tank is full, TS goes low (C o/p high) turning on B and turning off A. Motor stops. As tank empties, TS goes high but doesn't change bistab. The cycle repeats when the tank is empty.
     
  11. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    a quad 2 input Nand Schmitt trigger will likewise do it for you. you can have the 1st nand as you reference, the 2nd nand as your full indicator and 3rd & 4th nands as your low level indicator. :p
     
  12. gsd

    Thread Starter New Member

    May 13, 2005
    6
    0
    thanks to everyone for their help.

    I am studying the subject I need this for by distance which makes it fun if you don't understand something. thank goodness for the internet!

    cheers.
     
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