Help with simple circuit/opamp project

Discussion in 'The Projects Forum' started by bears13, Feb 19, 2013.

  1. bears13

    Thread Starter New Member

    Feb 19, 2013
    1
    0
    I am working on a project in which I have to attach a hydrocarbon sensor to a microcontroller in order to read in the voltage and know if the sensor has detected something. I have attached the circuit for the hydrocarbon sensor. As you can see there are two resistors in parallel so the voltage will be constant. We need to figure out what supporting circuitry will be needed in order to read a voltage and amplify it so that our microcontroller can read it. We were thinking of connecting an opamp or two to amplify our signal, but we are not sure how to connect them to the sensor.
    We are really confused and need a good, easy-to-understand explanation. Any help would be appreciated.
    Thanks!

    http://s.eeweb.com/members/laura_webb/discussions/2013/02/19/COBRA-MODEL-100-HC-schematic-1361297798.pdf
     
  2. tindel

    Active Member

    Sep 16, 2012
    568
    193
    It looks like the HC sensor resistor will go as high as open circuit and as low as 10kohm. put this in parallel with 330kohm resistor and you get an equivalent resistance between 10k (no hydrocarbons) and 330k (lots of hydrocarbons). You can supply the equivalent resistance with less than a 12V source and sense the current and that will tell you how many hydrocarbons you have.

    That's what I get out of that spec sheet anyway. It's not very descriptive - maybe a phone call to the manufacture would help ;)
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,061
    I don't think you need anything more than a resistor voltage divider. Check the input impedance of your µC. If it's >10X higher than the typical resistance of the sensor, you should be able to measure voltages with little disturbance of the reading by the µC itself. If not, then a voltage follower op-amp circuit may be called for. There are op-amps available that have very high input impedance. You may not need anything exceptional, but that is an important factor in selecting the right amp.

    Your µC needs to see a voltage, so you need to convert the changing resistance of the sensor to a changing voltage. Putting a known-value resistor in series with the "unknown" resistor, and measuring the voltage drop across that known resistor, allows you calculate the current by Ohm's law and then the unknown resistance. (You need to know the total voltage drop, also, either by measuring the 12V supply or by measuring the drop across the sensor as well as the known resistor.) You'll then want to apply a formula to relate the measured resistance back to hydrocarbon level. It may be best to establish this by experimentation, rather than theory.

    The manufacturer can likely supply you a typical application circuit.
     
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