Water Detector

Discussion in 'The Projects Forum' started by jmeredith, Nov 22, 2009.

  1. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    Hey guys can anyone help me with a water detector included is the wiring diagram, the only thing is I am not sure I have R2 hooked up correctly.

    Is R2 supposed to be a potentiometer? I couldn't find any 1M pots, so to test I just put in a regular 1M Ohm resistor

    other than that everything else is wired up per diagram... I am not getting any closure on the relay with the sensor probes in water...

    what am I doing wrong? I do have a couple of 100k pots, maybe I can switch out?

    I rebuilt this circuit without pcb and without the relay, it seems to function correctly, but when I add in the relay, things go south...

    thanks,

    -Jason
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    What are the specifications for the relay you are using?

    The circuit you are building is taken from "Getting Started in Electronics" by Forrest Mims.

    The relay that he suggests to use has a 500 ohm coil resistance.


    hgmjr
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    The transistor does not have enough gain.

    Try using a Darlington configuration.

    Like this:

    [​IMG]

    If that isn't enough gain, another stage could be added; collector resistor would need to go to +5v though.

    I've been a fan of Forrest Mims III's books since they first came out. They are not completely error-free, however nearly all of the circuits will work if assembled properly.

    He did a couple of generations of electronics enthusiasts a big favor by writing all those books.
     
    Last edited: Nov 22, 2009
  4. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    Yes this is from his book.

    here are the specs for the relay I am using:
    -Maximum switching power of 10 (watt/VA)
    -Maximum switching voltage of 60 VDC, 120VAC
    -Maximum switching current of 0.5 A
    -Maximum initial contact resistance of 150m ohms

    and it looks like its a 250 Ohm (thats whats written on it anyway)

    http://www.radioshack.com/product/index.jsp?productId=2062478#tabsetBasic



     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    You need to get the 1 Meg potentiometer to give you the range of adjustment that will be needed to set the sensitivity of the detector.

    hgmjr
     
  6. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    Yes, his books gave me hours of enjoyment (read:frustration) as a youngster... I havent picked up a soldering iron in 10 years... I have forgotten most of what little I learned back then.

    He lives just up the road from San Antonio if I remember.

    I will give the darlington a try.

    thanks!

    -jason

     
  7. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    Wookie, I was not able to find a 1M Potentiometer I have 1K, 10K, &100K pots can I substitute the 100K where the 1M is and up the fixed resistor value?

    -jason
     
  8. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    Well crap! radio shack didn't have any 1M pots :( back to the drawing board...


    Thanks for all the help!

    -Jason


     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    There is always Digikey if you don't mind ordering online.

    hgmjr
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Wait a minute.

    Go ahead and use your 100k pot.

    If the relay still doesn't close, you need still more gain in the transistor stage.

    Pure distilled water is a very good insulator. You may very well need more gain.

    Also, you may have mis-identified the leads to the transistor(s). Please triple-check them.
     
  11. hgmjr

    Moderator

    Jan 28, 2005
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    You could check the Radioshack for a 500K potentiometer. Then you could use a 470K resistor in series with it to get you close enough to the 1M. This would give you a reduction of adjustment range.

    hgmjr
     
  12. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    Will do thanks for all the help!


    I'm using 2N2222 Transistors, pin 1(bottom pin IF flat side is on left IF viewed from top) is emitter. that is what should be going to ground right?

    What's the difference in a 2N2222 and a 2N3904 I see the 3904 has a 350mW Dissipation and the 2222 has 1.8W Dissipation... Wait a minute

    As I am looking on the packaging I see the small print that says "Viewed from bottom" Hmm, OK maybe I have the Collector and the Emitter Swapped.



    -J

     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    That will definitely keep things from working.

    hgmjr
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    You have to view them from the bottom.

    Actually, TO-92 packaged devices all have a power dissipation limit of 625mW. It's a function of the package, not the individual transistor.

    2N3904/PN3904 transistors are rated for 200mA maximum collector current, but the realistic maximum is 100mA.

    2N2222/PN2222 transistors are rated for 800mA maximum collector current, but the realistic maximum is 400mA.

    There you go - that's why I suggested "triple checking".

    It's very easy to mis-read datasheets.
     
  15. hgmjr

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    Jan 28, 2005
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    I agree that it is very easy to misread a datasheet.

    It never hurts to review your circuit hookup whenever the circuit does not work.

    hgmjr
     
  16. hgmjr

    Moderator

    Jan 28, 2005
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    The purpose of the diode is to protect the transistor from the flyback voltage that can occur when the transistor is switched off.

    Look closely at the orientation of the diode. It is reverse-biased when the relay is powered.

    hgmjr
     
  17. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    well, I am really glad the fate of the world doesn't rest on my ability to put together simple circuits.. I have been trying this silly thing for several hours... I have been able to get my transistors hot... but thats about it.

    I am going to bed, and shall try again tomorrow hopefully some rest will help...

    ahhh I am beginning to remember why I stopped doing this so many years ago. even the small victories are hard to win.

    thanks everyone for the help...

    -Jason
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    It's OK - if you find yourself getting frustrated, give it a rest and do something else for a bit.

    It'll be there when you get back, patiently waiting for you. :)
     
  19. hgmjr

    Moderator

    Jan 28, 2005
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    Don't get too frustrated. Sometimes it helps to set a problem aside and come back to it later. You can hit it fresh tomorrow.

    hgmjr
     
  20. hgmjr

    Moderator

    Jan 28, 2005
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    Good luck with with your water detection circuit troubleshooting today.

    hgmjr
     
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