Need help in building resistivity/conductivity sensor

Discussion in 'The Projects Forum' started by i0sx0sx, Dec 1, 2015.

  1. i0sx0sx

    Thread Starter New Member

    Dec 1, 2015
    Hi guys,

    I'm very excited in building circuit and need advice/help from you guys. I can't find many circuits on this.
    I have done some research on timer circuits (555, c4060, etc) and few tiny Arduino boards and little bit confused.

    I want the circuit to work as follows,

    First, pair of probes will be connected to this circuit. And the enclosed circuit will be placed inside the alcohol fluid tank with probes injected into the fluid. From there the circuit should check the resistivity for every 4hrs and if found any then the LED should lit up. Also the checking should only done for about 1000-1500ms and not more than this because there might be chance of electrolysis. And a reset button should be there for case where resistivity is found and needs to be cured and start the timer again.

    Required resistance value should be 5v minus resistance. Could be anything from no conductivity to very close to 5V or whatever the power supply is.

    I got two Arduino based boards, one is Nano pro and other is attiny85. Will be great help if you guys can help based on the info provided.

    Need more info please ask.

    Thank you,

    Last edited: Dec 1, 2015
  2. wayneh


    Sep 9, 2010
    So, just to be clear, the probes will be in continuous contact with the solution, as opposed to detecting the level when the probes become immersed?

    Your goal is to detect "contamination" with something that increases conductivity? There is such a thing as a conductivity probe. These are used industrially and thus meant for continuous rough duty.

    An inherent problem in measuring conductivity is that the result is not intrinsic (like pH or color), but extrinsic - meaning it depends on the scale of the probes and their condition. A probe covered with bioslime won't conduct as well as a clean one.

    If you want to stick to a DC probe (AC is often used to mitigate corrosion and fouling), you could set up a comparator circuit (look up LM339 quad comparator and look at the data sheet) to trigger when the conduction to one probe is enough to raise its voltage to a preset level. There are a number of problems with this approach but it's very simple and does work.

    A normal 555 circuit won't work for a 4hr interval, but you've probably read that in your research.
  3. i0sx0sx

    Thread Starter New Member

    Dec 1, 2015

    Hi Wayne,

    Thanks for your reply. I say yes for all your questions in your reply. And also it is extrinsic.

    I will try LM339 but are there any circuits using it. I need to do this in a way later I have to interface with Ardiuno or IOT boards. And the measures and time must be as precise as possible, because as you said there might occur other reactions like corrosion if the probes are left with voltage for more time.

    This is one of the reasons I'm looking forward to make it work using tiny microcontrollers.

    Thank you