Need help with simple water level gauge

Discussion in 'The Projects Forum' started by Bridges-PdP, May 2, 2011.

  1. Bridges-PdP

    Thread Starter New Member

    Dec 3, 2008
    6
    0
    Hi,

    I'm in Haiti and I'm trying to build a simple level indicator for a water reservoir. The reservoir is about 9ft. deep, but will always have about 10" of water in the bottom.

    I'm trying to make something that uses 5 or 6 wires of varying lengths extending into the reservoir in such a way that when I press a momentary switch a LED will come on if the tank has two feet of water or 2 LEDs will come on if the tank has 3ft of water, 3 LEDs if the tank has 5ft, 4 LEDS at 7ft and 5 LEDs at 8ft. I have a pretty good variety of resistors, a handful of 5mm Red LEDs, a push button NO switch and terminal(barrier strip) some perf board, 5 colors of #14 wire. I understand that the wires will deteriorate over time. Can someone jot a diagram on a napkin and take a picture of it for me?

    Thanks so much,

    Scott
    Port-de-Paix, Haiti
     
  2. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    It won't work. Water isn't that good of a conductor to make a dependable current at the level required for an LED. You have to use transistor amplifiers to assure good performance.
     
  3. Bridges-PdP

    Thread Starter New Member

    Dec 3, 2008
    6
    0
    Thanks for your help!

    When you say transistor amplifiers, do you mean some type of IC or something I might be able to do with basic npn or pnp type transistors?
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,771
    930
    Why electronic? This is a perfect fit for an ANALOG mechanical system. Torsion spring, round indicator card and a float on the end of a weighted rope. The rope is pulled downward as the float goes lower and the round wheel turns( the rope is wound around its axle and the spring rewinds it as it rises). A cover with a single opening lets the user 'see' a number through the opening that corresponds to the level of the water.
     
  5. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    I mean basic npn or pnp transistors, probably. Water will not carry 10 or 20 milliamps to run an LED unless you use quite a lot of voltage or put salt in the water. You need to design an amplifier stage that will sense very little current and "switch on" so the LED's can get enough current, limited by a resistor. A J-fet would be very good because its gate current is in the neighborhood of nanoamps. A standard transistor will need maybe dozens of microamps. An LED needs about 10 milliamps. You can see that each of these statements describes about a hundred to a thousand times as much current as the sentence before it.

    The first thing to do it set up a test and see how much current you get at the voltage you are willing to use. Then see how much current flows just because the sensor is wet after the water level has dropped. Then figure on using low leakage capacitors to keep random noise from giving a false "on" condition.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    Is this an open reservoir? What's wrong with the tried and true painted-stick? That's how every U.S. river is monitored for flood stage. No moving parts, as precise as you like, no operator training needed, no power needed.
     
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