water level sensor

Discussion in 'General Electronics Chat' started by payal.kellyyy, Mar 28, 2011.

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  1. payal.kellyyy

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    Mar 28, 2011
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    Hii everyone I am really new to this forum I am having a trouble regarding water level sensor project of mine.please find enclosed the circuit for the same .
    In this circuit can anyone tell me what is the function of the electrolytic capacitor connected across the sensor probes.Also is it possible to decrease the value of this electrolytic capacitor and compensate the same for a feedback resistor across the op amp.Waiting for your reply.Need help guys urgently.Please guys check this new link for updated circuit diagram for water level sensor corrected one .
     
    Last edited: Aug 15, 2011
  2. R!f@@

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    The Capacitor C2 is used to to reduce fast voltage fluctuation. In other words the charge helps to reduce output switching when the water level changes due to sudden movement of the container. Thus it prevents annoying triggering.

    The capacitor will charge to the desired voltage only if the water level reduces slowly thus giving actual level of the water.

    The resistor value depends on the probe resistance or the type of probe used. It determines the triggering point. If you need to change this you need to compensate the change by changing the reference value set by the resistor on the other input of the opamp. Which can be either R3 or R4.

    From the looks of it, the reference is set at mid point of Vcc.
     
  3. SgtWookie

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    It's a poorly designed circuit.

    The LM741's output swing won't go lower than about -V + 2v, so the output transistor will always be turned on. It needs to be a different opamp that has an I/O range that includes the ground rail.

    The inverting input to the opamp is tied to +12v via a 270k resistor. I think whomever drew the schematic was relying on the leakage of the 10uF 50v cap to act as part of the voltage divider.

    If such a circuit relies on the conductivity of water, and the water is pure, the circuit won't work as pure water is an insulator.
     
  4. SgtWookie

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    It is the way the 741 opamp was designed. If you look in the datasheet, you will find out that it's output range is very limited. It is supposed to be used with a dual-rail supply (both positive and negative supplies).

    An LM358 dual opamp would work. It was designed to be a single-supply opamp. The unused opamp should be wired as a voltage follower with it's output connected to the inverting (-) input, and noninverting (+) input connected to ground.

    If the output of the opamp cannot go below ground+2v, then the transistor will always have current flowing through it's base, and will be sinking current through it's collector.

    Well, the term "rail" when used in electronics is kind of an anachronism; people used to start drawing schematics by making horizontal lines for +V, ground, and -V, then start connecting components between these power lines. It started resembling railroad tracks, so they started calling them power "rails".

    But I digress. The LM741 is a dual-supply opamp. If an opamp was designed as a single-supply opamp, it will say so on the first page. Look up the datasheet for an LM358 and you will see.
     
  5. KMoffett

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  6. KMoffett

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    Again, there's no magic value for the choice of the hysteresis band range. The voltage output of your electrode configuration will vary depending on the conductivity of the water and the depth of the electrode at a given water level. You need to do some measurements to see how much voltage variation "you" can tolerate before the comparator changes states. This is going to be an experimental task.

    Can you post a schematic of the exact circuit you are going to use?

    Ken
     
  7. bertus

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    Apr 5, 2008
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    Hello,

    I merged you threads AGAIN.
    Please use the [​IMG] button.

    Bertus
     
  8. Adjuster

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    The comparator requires a reference voltage on its inverting input, at least big enough to overcome voltage offsets and any interfering signals which might be picked up on the wiring. The reference might be a few hundred mV to a few volts positive of ground. A potential divider powered from the same supply that powers the sensor should be adequate.

    The non-inverting input needs to present a load resistance to ground, as otherwise even a tiny amount of leakage current in your sensor will trigger the comparator. Adjust the load resistance according to the sensor resistance, so that with water present the non-inverting input voltage will more than the inverting input reference value - perhaps about twice as much.

    I would not be too sure that the resistance of your sensor would be as low as 1kΩ, unless you are using salt water. You would be better to make measurements of the actual sensor. Allow for a bit of deterioration of the electrodes, which will tend to corrode and / or get dirty. This may raise the resistance.

    You seem to have done something odd with one of the DC balance inputs - not very clear which from your diagram. Please refer to the datasheet for correct connections. http://www.national.com/ds/LM/LM111.pdf
     
  9. Adjuster

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    Someone with more patience than myself may be willing to draw out a circuit for you, but I note that you have been asking similar questions before. You also had more complete circuits earlier - first with a reference derived from the supply, then something with an internal reference. Why start again from the beginning?

    I should correct my earlier comment about the detector input: if the level detector electrode works by providing a path to ground, then a load resistor to ground is only required if the maximum input voltage would otherwise be too high for the IC.

    As to your questions: a capacitor between the inputs may help, particularly if the input wiring is long. The balance inputs are used to minimize input voltage offsets, but your application does not require much precision. The strobe function allows the circuit to be shut down. You do not need these functions, so do not connect anything to these inputs.

    A stabilized reference voltage is usual, but often a band-gap device is now preferred to a zener. The most stable zener voltages are around 6V: if this is too much for your application, reduce it with a potential divider. In any case, your circuit may not really need a stabilized reference: if you get the reference by a potential divider connected to the same supply as the sensor, the effect of supply voltage changes via the two paths should largely cancel out.
     
  10. KMoffett

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    Is R6 supposed to be your water sensor?

    Ken
     
  11. Adjuster

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    Yes, this cannot work. There is no reference voltage, and no input voltage. The comparator inputs are simply connected to ground via resistors.

    At one input, you could add a potential divider giving a few volts output, with sufficiently large output impedance so that the level will drop substantially when the water resistance is added in parallel.

    At the other input, there should be another potential divider, giving a lower voltage than the first, but preferably with a similar output impedance. It may be helpful to make this divider adjustable for setting the threshold.
     
  12. KMoffett

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    Let's step back a bit. What do you want the circuit to do...the big picture!
    It's often easier for us to help solve your problem, than help solve your solution.

    The comparator circuit is OK if you want to detect when the water reaches a high level. It's not a good circuit if you want to control the level.

    Ken
     
  13. payal.kellyyy

    Thread Starter New Member

    Mar 28, 2011
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    Dear kmoffet,

    R6 is supposed to be my water resistance .I have to actually make a water in fuel sensor .the sensitivity lower than 1

    This is the technical specs for my project.I have to detect the presence of both water and fuel (diesel).I am supposed to make the circuit for the same using LM311 comparator .Hope this information can help you in solving my problem.
     
    Last edited: Apr 27, 2011
  14. payal.kellyyy

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    Dear Koffet,

    The output of my sensor is connected to a relay which will trip as soon as it will detect presence of water in fuel .I am really new to handling the project I need a lot of ur assistance .If any more information is required for this project apart from the technical specs in my previous thread kindly let me knw...
     
  15. KMoffett

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    Is the assumption that water in a fuel container will separate from the fuel and pool in the bottom of the container?. That sense electrodes be placed in the bottom of the container? If the water is sensed, the circuit will trigger the relay...momentarily?...latch on?

    Ken
     
  16. KMoffett

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    OK, see if this works for you. Same assumption as in #25.
    R3/R4 set the trip voltage at 1/2 the supply voltage...6V...the values are not critical, just R3=R4.
    R1 and the probe resistance (<100KΩ) produce the voltage level for detecting water. R1 is slightly greater than the probe resistance when it touches water. It depends on the conductivity of the water.
    D1/D2/R2/C1 protect the input from excessive voltage...like static.
    D3 protects the IC from relay flyback voltage.
    The lower set of relay contacts and SW1 provide output latching when water is momentarily detected. Delete them if latching is not needed.

    With this design hysteresis is not an issue.

    Ken
     
  17. payal.kellyyy

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    Mar 28, 2011
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    Dear Koffet
     
    Last edited: Apr 27, 2011
  18. KMoffett

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    1. What is the voltage rating and resistance of your relay coil?
    2. What is the voltage measured between common and pin-3?
    3. What is the voltage measured between common and pin-2 with the electrodes out of the water?
    4. What is the voltage measured between common and pin-2 with the electrodes in of the water?

    Ken
     
  19. payal.kellyyy

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    Mar 28, 2011
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    kl;huuuujklbuyhkihuj
     
    Last edited: Apr 27, 2011
  20. KMoffett

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    My circuit as drawn, meets your second requirement at the output, pin-7. Well almost. The output-low saturation voltage of the LM311 is nominally 0.7V, not 0.0V. Not sure why they would want a high output of "Vcc-0.7v". The relay contacts can provide Vcc or floating and 0.0v. Adding a diode in series with the Vcc contacts would provide "Vcc-0.7V". But more importantly, have you made my circuit work?

    Ken
     
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