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MQ 137 nh3 sensor circuit?

  1. Jun 19, 2012 #1
    hi everybody.
    a friend of mine needs to make an ammonia sensor from the mq137 sensor, but we couldn't find a circuit anywhere. can any of you guys help us out? we need it to trigger a relay when a preset threshold is reached. thanks in advance!
  2. jcsd
  3. Jun 19, 2012 #2


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    Staff: Mentor

    Do you have the datasheet? The manufacturer most likely publishes an application circuit, either in the datasheet or separately.
  4. Sep 30, 2013 #3
    This might be a bit late for your needs, but I came across your post just today, and have a circuit for you which might benefit others as well who are searching for this topic. The sensor you have is a simple resistance device whose resistance decreases when it senses the gas its designed for. By using a load resistance (sometimes included on a provided sensor board), and taking the voltage across it, you can apply that voltage to a LM311 comparator with a reference voltage divider on its other input. When the sensor voltage exceeds that set voltage, the output of the 311 goes low and energizes a relay per your request. The relay coil needs to be a low current type since the 311 is rated at 50 milliamperes. You could use a solid state relay to control a 110VAC circuit if desired or you can control a larger relay using a p-channel MOSFET to drive its coil. This same circuit will work with the line of similar sensors which include methane, propane, and hydrogen sensors. Figaro was the original manufacture for these, but Hanwei makes some knock-off types that are similar.

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    Last edited: Sep 30, 2013
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