Moisture Sensor

This circuit indicates the level of moisture in a place due to rain or spilling of water. Green LED lights when there is moisture and Red LED lights when there is no moisture.

IC1 (741) is used as a voltage comparator. Its non inverting input (pin3) receives half supply voltage through the potential divider R2 and R3. Its inverting input (pin2) gets a higher voltage only when the sensor connected to points A and B are shorted with moisture. When the sensor is dry, the output of IC1 will be high because its pin3 gets more current than pin2. This high output makes the transistor T1 conducting and Red LED lights. At the same time T2 remains off because its base is not getting current since T1 takes all the current. So Green LED remains off. When sensor gets moisture, current flows through the sensor to the pin2 of IC1.As a result, pin2 of IC1 gets more current than pin3 and its out put becomes low. T1 then turns off allowing T2 to conduct. Red LED turns off and Green LED turns on indicating the presence of moisture.

Moisture Sensor Circuit

Touch pad can be a small piece of Strip board. Connect the adjacent tracks of the touch pad to points A and B using thin wire. Keep the touch pad in the place that is to be monitored. To test the circuit, take moist cotton and place it on the sensor. Green LED lights if there is sufficient moisture.

Circuit in Word format – Print


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About D.Mohankumar

D. Mohan Kumar M.Sc., M.Phil, is an Associate Professor from Trivandrum, Kerala, India. He has been guiding Post Graduate students in Animal Science research for the last 25 years in several premier institutions in Kerala, India. He is an avid electronics enthusiast and has been designing circuits for the past 20 years. In this period he has supported many electronics/electrical engineering students in their final year projects. He is a regular contributor to Electronics For You magazine which is South Asia’s largest circulating electronics magazine published from New Delhi. He is also contributing articles in and
This entry was posted in Detectors, Hobby Circuits, Indicators, LED, Op Amp, School Projects, Sensors and tagged , , . Bookmark the permalink.

2 Responses to Moisture Sensor

  1. shweta says:

    i tried this cct… when soil is dry the red LED glows….and when water is given it turns OFF and green turns ON. But when again when i am checking for the dry condition the green light does not get switched OFF…but red LED works fine.
    I have used 10uf 50v capacitor and 440ohm resistor with T2 in my cct.

    can i know the reason behind???

  2. where can i get this touch pad from??….i mean is it easily available at general shops which sell other components??

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