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Water detector circuit

Discussion in 'General Electronics Discussion' started by adk315, Jan 14, 2011.

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  1. adk315

    adk315

    24
    0
    Jul 29, 2010
    I need to make this circuit a little less sensitive, the relay works fine when the probe is dipped into water, but I have a little relay chatter when the probe starts to dry. I will also need to use this circuit in both 12 and 24 vdc systems. Please review the circuit and reply with your reccomendations.
    Thank you

    Adk315
     

    Attached Files:

  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    You can start by adding the usual protective diode across the relay coil, then you'll want a just as common decoupling capacitor across the power supply.
    If that's not enough then add a (high value) resistor from the relay output and to the base of the first transistor. This provides positive feedback (= hysteresis).
     
  3. adk315

    adk315

    24
    0
    Jul 29, 2010
    Water detector

    Rescueline

    I understand the protective diode, what size capacitor would I use across the power supply and also what can I do to use this circuit for 12 and 24 systems

    Adk315
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Use anything from 0.1uF to 100uF, or even both those values at once. The only thing you need to exchange is the relay, everything else will work fine at both voltages.
     
  5. adk315

    adk315

    24
    0
    Jul 29, 2010
    Water Detector

    Resqueline

    I am still having problems with the relay chattering when the water contacts the probe. We have made your recommended changes and still have the same problem. The probe we are using is a standard Racor water probe that is screwed into the bottom of the filter bowl. The bowl is filled with diesel fuel, when we add water to the bowl the alarm relay will chatter and then stay closed, then when we drain the water from the bowl the relay will chatter then open. Do you have any suggestions on removing the chatter?


    Adk315
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,291
    2,731
    Jan 21, 2010
    Try placing a capacitor across the water sensor. Start with something like 0.1uF, and if you still get chatter, increase it in steps up to about 10uF.

    To more completely remove chatter, you would have to have this circuit triggering a retriggerable monostable. That's more difficult. Try the capacitor first.

    edit. This advice is best followed *after* you follow Resqueline's below (and incidentally originally suggested above). My advice was actually predicated on the assumption that you had already followed that advice.
     
    Last edited: Mar 15, 2011
  7. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    You could also try to introduce positive feedback (hysteresis) by means of a high-value resistor (~10M ohms) between the relay output and the transistor base.
    If it's too low resistance the circuit will never turn off once triggered. If it's too high it won't help much. You'll have to experiment a bit to see what value works best.
     
  8. adk315

    adk315

    24
    0
    Jul 29, 2010
    Resqueline

    Placing the capacitor across the water probe worked fine and removed the chatter, now when I turn on the power source the relay momentarily closes the contacts and opens, this only happens when the circuit is powered up, any ideas how to stop this?

    Adk315
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,291
    2,731
    Jan 21, 2010
    Remove the capacitor.

    If you can apply power to the detector before the relay, that too might work.

    Maybe there is a simple alternative...

    [​IMG]

    The top diagram illustrates what I recommended you do. The bottom indicates a modification which may retain most of its benefits, and should eliminate the startup issue.

    Neither diagram shows the modification suggested by Resqueline.

    Note that the top diagram also shows the placement of a diode that will prevent the rapid untimely death of your transistor connected to the relay. A 1N4001 will be fine for that (almost any rectifier diode)
     

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