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How to Make a Soil Moisture Tester at Home with a Single IC 741

A very simple mud or soil moisture tester circuit can be built by using a single opamp and a few passive components, let's learn the details through the following article.

Circuit Objective

After water and sunlight the earth or the soil is the next most important natural gift that this planet has provided us with, without which persisting of the living beings could never be possible.

Soil produces plants, and plants supply us food. However plants need a well watered soil, or in other words plants or crops cannot survive without an optimal supply of water to the soil in which they grow.

Therefore testing the correct soil moisture becomes a crucial aspect in order to cultivate healthy crops without wasting excess water.

A simple soil moisture tester circuit explained can be used by anybody who may be interested to check or monitor the moisture level of a given area of land and ensure the correct amount of water supply to it, either manually or automatically through the same circuit.

So we have both the options available with this circuit, it enables the user to test the level of moisture of the soil, and if required make the unit an automatic soil moisture level controller by connecting a motor pump with the attached relay contacts in the circuit.

Simulation and Working

Let's see how the circuit is designed to function:

Soil Moisture Tester at Home with a Single IC 741

Referring to the circuit above, the design makes use of a single IC 741 opamp comparator for the required testing function.

The pin3 which is the non-inverting input of the opamp is used as the main sensor probe with respect to the other probe connected with the ground.

The moisture level present in the soil develops a resistance across it which increases with a decrease in the moisture level and decreases with an increase in the moisture level, meaning a wet soil will have a much lower resistance compared to a dryer soil.

This aspect is exploited in the design the probes are used to test the soil resistance between pin#3 and ground of the comparator IC 741.

This soil resistance forms a potential divider with the 100K resistor connected across the positive supply line and pin#3 of the IC, and the potential difference developed here in response to the soil moisture level is compared by the potential at pin#2.

Pin#2 potential is determined by the setting the shown 100k pot. Thus this pot is effectively used for determining or verifying the exact moisture present in the soil.

If the soil moisture produces a lower resistance at pin#3 than the set level at pin#2, the output at pin#6 is rendered low, meaning when the soil is relatively wet the output of the opamp shows a zero volt, while in case the soil condition develops a higher resistance (dry condition) then the output of the opamp goes positive, triggering the connected transistor and the relay.

In other words, the output of the opamp and the relay stay switched OFF as long as as the soil moisture level is more than the threshold set by the pin#2 pot, and vice versa. Therefore a relatively wet soil will keep the relay switched OFF, and a dry soil will switch it ON.

The LED compliments the relay action and illuminates whenever the soil is dry than the desired set level.

This pot needs to be appropriately calibrated with a dial and then the various points across the dial marked as per the predetermined moisture content of a sample soil collected inside a container.

Once this is done, the calibrated pot can be used for checking any soil by simply inserting the shown probes into the soil, and by adjusting the pot until the output is rendered a high (LED ON).

How to use the Circuit as a Soil Moisture Controller

As explained above, once the pot is set to a desired value, whenever the soil moisture goes below this set level, the relay is instantly activated.

In the switched ON position the relay contacts join the N/O contacts, and these contacts could be wired to a water pump and its power supply in series, so that whenever the relay clicks, the motor pump is activated and the soil begins getting the required water supply until its moisture level is restored to the desired optimal point.

At this level the opamp detects the condition and quickly changes over to a zero logic at its output, switching OFF the relay and the motor, the water spraying is consequently stopped.

The above action keeps repeating by testing the soil moisture and applying water accordingly, in an entirely automated way without any manual intervention.

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. Dear Sir I Want to make An Circuit That I can Easily Connect A mp3 out to A Mobile Speaker In . But Sir I Am Afraid If The Mobile Speaker Get Damage ?? Sir please Help Me

  2. Dear Saroj, it is not possible to use the mobile speaker with an external music input and this is not recommended.

  3. ...there's no external port in mobile phones which will allow it to connect with external inputs

  4. Sir I Am Sorry I Couldn't Clear You . Actually I Want To Connect A Speaker with My Siren Out Put . nowadays In Market Very few Speaker Are Selling Called Mobile Phone .
    Thank You Sir

  5. sorry, I still did not understand your requirement, do you want to drive an external speaker by providing a music input from your cell phone? in that case you will require an amplifier in the middle.

  6. Sir
    Actually I Want To connect A Sound Box With my Siren Out Put. Then my soundbox willget damage ???

  7. can you show how a mini 12v pump can be integrated with this circuit to water plants that are in pots?

  8. If you have an amplifier inside your soundbox then there will be no can go ahead with it.

    If not then use an amplifier in between the sound box and the siren input

  9. if the siren signal is already amplified then make sure the sound box speaker is rated to handle the siren wattage.

  10. the relay connections will be similar to the following circuit:

  11. How can i understand the Adjustment ? Is my speaker okay for Received signal from siren out . my siren ciruit work in 9v

  12. add a volume control to the system and then slowly increase it, if the speaker distorts then its too high for the speaker

  13. Sir Please Help Me How Can I Control Bolume ??? Sir Please Describe Me .

  14. Sharoj, show me your siren circuit then I'll tell you how to fix a volume control

  15. My Alarm Circuit Work on 9v Battery. And Have a mini Speaker In this Circuit.
    I want to replace The Speaker And Connect the Cable To Mini Speaker.

  16. please put the image on "sharing" mode, because I am not able to be see it at the moment.

  17. Sir Can You Please Provide Me Simple Microphone Amplifier Circuit

  18. Sharoj, presently I do not have in my website, but you can find many such circuits online.


    My Alarm Circuit Work on 9v Battery. And Have a mini Speaker In this Circuit.
    I want to replace The Speaker And Connect the Cable To my Speaker.

  20. Sir I Want To Drive A Relay From getting Easy Signal From Ultrasonic Sonar. Abmnd i want to make a easy Circuit. Not Complex
    Please Help ame Sir

  21. Sharoj, it will be a complex circuit as far as I think....cannot be an easy one.

  22. What's that 2k2 ohm resistor in the circuit diagram sir? Is that a typographical error? What should it be? 22k ohm or 2.2k ohm resistor? I hope i could get a reply from you ASAP because I am needing this in my project to be submitted next week. Thank you very much sir!

  23. Sir, did you get my message?

  24. Andrea, 2K2 is another way of writing 2.2K


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