Simple Automatic Plant Watering Circuit

The post explains a simple yet effective automatic plant irrigation system circuit which can be used for automatically sensing soil humidity and triggering a water pump when the ground gets parched below a predetermined level (adjustable).

Circuit Description

The circuit is rather straightforward and uses a single IC 555 as the main active component.Referring to the automatic plant irrigation circuit shown below we can see the IC 555 is wired in a completely unique and in the quickest possible mode.

Here it's configured as a comparator, and works better than an opamp because the IC 555 has built in opamps which are at par with any single opamp and also the output of a 555 IC is able to sink sufficient current in order to drive a relay without a transistor driver stage.

The above features particularly makes the above design very simple, low cost and yet too effective with its functions.

The pin#2 here becomes the sensing pinout of the IC, and is held at ground level via R2 which must be calculated as per the desired soil humidity triggering threshold.

The points A and B can be seen fixed inside the soil which needs to be monitored for the intended automatic watering from the water pump.

As long as the points A and B senses some level of humidity corresponding to a resistance value which may be lower than R2, the IC 555 output is held low, which in turn keeps the relay deactivated.

However as the soil tends to get dryer, the resistance across the probes starts getting higher and at some moment of time it becomes higher than R2, creating a potential below 1/3rd supply voltage at pin#2 of IC555.

The above situation instantly prompts pin#3 of the IC to become high, triggering the connected relay.

The relay activation switches ON the water pump which now starts pumping water to the particular area of the soil via a distributing water channel.

As this happen, the soil gradually gets wetter and as soon as the predetermined level is reached, the probes immediately sense the lower resistance and revert the IC ouput pin#3 to a low again switching OFF the relay and the water pump consequently.

C1 ensures a slight hysteresis in the operations ensuring that the relay triggering is not sudden or abrupt, rather it switches only after sensing a genuine response from the soil conditions.

Circuit Schematic

The above explained automatic plant irrigation circuit was successfully built and tested by Mr. Ajay Dussa.

The following images show the prototype unit and the PCB design built by Mr. Ajay.


PCB Design

Parts List

All resistors are 1/4 watt 5% CFR

R1 = 10K

R3 = 2M2

R4 = 100K

R2 = 1M preset or cermet

C1 = 1uF/25V optional for creating delay effect on the relay

Relay = 12V, 400 ohm SPDT

Supply input = 12V/500mA DC



142 thoughts on “Simple Automatic Plant Watering Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Hi Mr swagatam,
    i am new to your website and happy that there are so many good circuts,
    I am in a instrumentation field, presently on some small project which need a 3.7vdc 750mAh lithium ion rechargable battery as a back up power, so kindly let me know the one good working circuit for 1)overcharge protection 2)immediate change over 3)low battery indication. As the one like yours Automatic micro UPS circuit.but now its for 3.7vdc

  3. This is a very useful circuit sir, just made and installed it in my house, works very great. I've seen other circuits online…..but as always the best circuit with more affordability and durability is here….thank you for your innovative innovations….:-)

    • Thank you SS, I appreciate it a lot!
      By the way I have published the phase neutral indicator circuit, but it's not as efficient as the original, because due to lack of time I couldn't experiment enough with it.
      Nevertheless the results from it would be perfect.

  4. Dear sir, thank you very much…….I've seen the circuit……no problem………you may design the the buzzer indication circuit when you are free and hav time….designing this circuit for me is not as important as doing your works…..but you're more concerned about others…….you're a boon to the electronics society all over the world…..and to people like me….
    thank you very much,

  5. Sir, by the way, I wanted to use a transformerless power supply for this circuit, can I do so sir,…? I would be happy if you could suggest a transformerless power supply circuit from this blog…also please tell me the current consumption of this circuit…….thank you sir……

  6. Hi sir can you help me how to calculate R2 in the circuit dry soil=1.7M ohms and wet soil=0.7M ohms and what should I include in my design report I was designing the soil moisture sensor.Thank you your circuit is very useful

  7. hey sorry for all the questions but i could not get it working, i think my ic555 chip was defected so i build a circuit to test this other chip i had and it seems to be working!. soo im going to try once more. im confused on replacing r2 for the pot where to i place the wiper and what value of red led is that 3v?

  8. hello sir my circuit work perfectly but it soil moisture sensitivity is to high when soil moisture is less then 10% then it will be work help me i want when soil moisture is 40 % then it will work i use 6vdc relay

  9. Hello

    I have been seeing the circuits your circuits and quite impressed by it, this one is my favorite, i have built it on the breadboard and tested it works great. I didn't had 2m2 ohm at hand so i managed to combine 4 resistors 2x 1m + 2x 100k, while you said to set the moisture density threshold we need to use 2m ohm and less until the led goes off, i first used 2m pot it didn't worked, then 500k also didn't worked the used 100k it worked, measured its resistance it was 23k, the led goes off at 23k when the soil is fully wet. I have currently tested it in a small jar with soil, i wanted to ask if i were to put it in a plant pot as this one, what should be the distance between points A and B and where they should be placed in the pot so that there will be enough time for pump to water the pot/plants before the resistance decreases and it switches off.
    One more thing, somebody suggested me that a transistor should be added to at the end of the point 3 of 555 to smoothen out the relays, do u think a transistor is necessary, i don't really feel the need of transistor after all my testings. Anyway i really like this circuit. Thank you


    • Thank you very much, I am glad the circuit worked and is serving the purpose.

      The R2 needs some experimentation, if 100K is the value that works then you can use it without issues.

      you can use the circuit for monitoring the soil in a small pot also, the distance between the sensor probes should not be more than 3 inches for an optimal response

      a transistor stage may not be required because the IC itself is rated to handle upto 200mA which is well above the consumption of any standard relay

    • It will work, but not with just 2nos of 1.2 cells, you might require 4 or 5 in parallel to increase current.

      yes the flyback ferrite core can be used for the said purpose, it would in the form of E cores I presume.

    • It is in rectangular shaped, two half's of ferrite core are joined together by a piece of hard metal wire. What i have looks like this image

      Anyway if i were to wind coils on it, should i wind using the usual transformer winding method, each winding on top of one another or the HV secondary on one side and primary and feedback on the other? Would it make any difference in the power output?

  10. hello,
    I make this circuit, It is working very good.but in this circuit i want to use 12v 10amps relay for( big moter)
    in this circuit i m getting 6v output for relay, than how it is possible to connect 12v relay? So how to convert 6v to 12v(for relay),
    i think i have to connect transister for get high voltage .
    what i have to do?????

    • if your supply voltage is 12V then definitely you'll get 12V at pin3 of the IC.

      however you can connect a BC547 transistor and use the relay across its collector and the positive supply to ensure a perfect working of the relay.

      the base must have a resistor of around 10k value

  11. Hi, I am a student at and am very interested in doing research on crops in the south of the United States. The problem is, I don't have the ability to hire someone down there to water the crops, and it is too far for me to travel to. Would I be able to buy this product from you? Or how can I go about obtaining it? Thank you so much. You seem like you are a brilliant man.

    • Thanks Malorie, I am sorry I don't sell circuits here, so this may not be available from this site. However I am sure there could be other online stores who might be selling such circuits in their sites, you could probably Google it for more info……

  12. Firstly I would like to thank you very much for providing this simple and very useful circuit for students like us.
    By the way we have selected this Automatic Water Planting circuit as our project and It is working too. We have used 2M2 for R2. But the problem we are facing is our circuit sensing the moisture very quickly. Even it does not let water pump to start. When we remove the spikes from soil and short them manually, it is working correctly. But when we insert spikes in soil it senses very quickly and doesnt let water pump to start. So, please guide us as early as possible.
    Ankit Patel

    • I am glad you found this circuit useful, however I am confused regarding how the circuit is detecting the moisture without the pump switched ON? Do you mean to say it's sensing the existing soil moisture??

      In that case the soil needs to dry further…or may be the distance between the sensor rods could be increased until the the problem looks corrected.

      Another option is to increase the value of R2 to higher level until the right balance is achieved….may be a 3M3 resistor could be tried to reduce the sensitivity of the circuit.

  13. Hi Swahatham, Iam a Electrical Engineering student from Bangalore. This circuit is quite simple and very useful! So I want to build this and implement this in my terrace garden. So.. If at all I want to water more than 2 pots of plant then how do I male changes in the same circuit shown above?! Awaiting your reply Thank You! πŸ™‚

    • Hi "unknown",

      for two or more pots, you will require two separate circuits because since the two pots won't be linked so the sensors from one circuit cannot be used for the two pots….the soil must be linked with each other in order to enable sensing with a common circuit…otherwise two individual circuits would be required for each of the pots

  14. Hi sir ,i want to build this circuit but the question raises that IC 555 timer is used to get output high or low for a particular time which can be set using resistors but in this circuit we are using soil as our parameter to select time for which our timer should on ,how it is possible?
    Why there using R2 and R3 .If we are uaing R2 as our refrence resistance then why there is R2 is used?
    Sir i have question that which sensor i have to use here ?
    Which type of relay is used here? SSR or EMR?
    i have these questions ,plz sir reply me through mail as fast as possible.. Thanx for giving such a simple circuit..

    • Hi Sudin,
      It's not the resistance rather the voltage dropped by the resistance which determines the triggering point for the IC….so here the soil also leaks and provides different voltage levels depending on its wetness, which is sensed by the IC for the required triggering.

      sensor is in the ofm of stell rods.

      R3 is for allowing a time delay response in conjunction with the capacitor to the output triggering.

      relay symbol clearly suggests it is an electromagnetic relay

    • Thanx sir giving me ur valuable time .plz tell me sir how to adjust threshold level 2/3VCC Above which IC will give output low and 1/3VCC below which it will give high output ..

      • There's only one pot which you need to set for triggering the relay at the threshold….basically here the IC is configured as a comparator, the 2/3 feature is not used here.

        I have explained it in the comments and the article, you can go through for knowing more about the details.

    • Dear Nandini, ample explanation is already provided in the article and the comments, please go through it.

      all resistors are 1/4 watt 5%

      IC can be any 555 IC

      relay is 12V 400ohm SPDT

      supply voltage should be around 12V DC from any standard ac to DC adapter

  15. Hello, nice circuit! I'm looking at making a modified version of it… I'm using gravity fed water, so rather than a pump, I've got a selenoid water valve that needs to be occasionally held open. My idea is to make the circuit, the probes, the valve, and a 9v battery box all into one self-contained unit. Just put it in the garden, hook it up to the gravity feed water line, and then turn the probe on. Multiple probes can cover multiple zones of the garden, each with its' own sensing and delivery mechanism, independent of others. Can you make any suggestions to help pull it together?


  16. Hello sir this is an awesome project. I am using it for my final year project. I have a 12 volt dc pump can I use it and also I have a 220 volt ac pump can I use that one ands replace it in place of the 12 v dc without changing anything in the circuit. Also is the. R3 and R2 or pot . Thank you very much

    • Thanks Isaac, you can use any kind of pump with the relay contacts provided it is correctly wired with the specified supply voltage, and the relay contacts are appropriately rated

  17. Hell Mr Swagatam today one of the legs of my relay got broken. it took my a while to get that one so now am left with a normally open relay that i bought a long time ago and also a six pin dpt relay . i was wondering since my circuit is powering only a 12 volt dc pump can i connect the circuit direct to the pump terminal without using the relay. And if so do i have to change anything in the circuit or if i can still use the normally open relay or the six double throw relay. Please i need your help

    • Hello Kweku,

      if your pump consumption is less than 150mA then you could probably connect it directly with the IC pin#3….otherwise you may need a relay or a power transistor for the same

  18. Hi Swagatam,
    Thank you for the great and simple circuit. I am trying to run a motor in two directions to open and close a window according to heat using a DPDT relay and limit switches. I could only find thermistors in the region of 100k ohm, is it simply a matter of playing with R2 starting at 2M2 to find the right trigger value, or should a different R2 value be chosen in this case?
    I also wondered if the LED functions as a blocking diode, or if it can be removed.
    Thanks a million,

    • Tanks Brian, yes it's just a matter of playing with R2 for getting the right triggering point….2M2 is not the starting value rather an extended max range value, you can start with lower values.

      the LED is for indication purpose only and has no direct role in the circuit.

      R3 can be also removed as it along with C1 is employed just to generate a delay effect for the output triggering

  19. Hi Swagatam,
    I struggled for the last few weeks to get this circuit working with a thermistor (33k @ 25 deg). I realise this circuit was not meant for this use, but I thought I would tell you my experience anyway. So as I understand, the 555 when used as a comparator triggers at 2/3 Vcc and untriggers at 1/3 Vcc. So the thermistor, when R2 was set just right, had enough variation (to maybe 45/50 k @ 40 deg) to trigger the 555. However, with R2 in this setting, it would not untrigger, and the load remained connected. Obviously, if VR was set lower, 555 would simply not trigger.
    I then asked around, and found a dedicated comparator IC, placed a transistor in the circuit, and now it triggers and untriggers. (The voltage drop is extreme though, but I won't bother you with that!)
    This caused me to learn a lot about the 555 timer and its uses, and pitfalls I guess.
    I only tell you this with respect, and of course your circuit was never intended for this use in any case.

    I also had a question about the (revised) solar MPPT circuit. I am really intersted in this idea, but since it would be a big project for me, I wanted to ask if you are quite sure it operates as you expect. Have you tested it much?

    Thank you once again, I am really learning and getting great pleasure from your page,

    PS Feel free not to post this, or to edit it in any way.

    • Thanks Brian, for updating the info!

      yes a 555 IC will always have some hysteresis which cannot be removed completely whereas an opamp can be customized to be with or without a hysteresis, and therefore for pure comparator applications an opamp suits the best.

      The solar MPPT sections are all confirmed individually, but the entire integrated MPPT design has not been confirmed practically by me, however my brain simulation says that it would certainly work, since they have been meticulously arranged and has no space to fail.

      Having said that it's mandatory for every hobbyist to understand a given design first and simulate it in mind the way the original creator did….this can help a lot while constructing the circuit and in case something gets stuck, it also makes troubleshooting a lot easier…

      I Wish you all the best πŸ™‚

  20. Hi Swagatam, First of all thanks for very helpful knowledge sharing website.
    I have a doubt wrt above circuit.
    It seems you are mentioning about checking the resistance between two poles and LED on/off. Can you please brief how exactly this to be done?

  21. Sir, could you explain the reason for connecting pin 5 and pin 7 together through the 100k resistor? Also please explain the mode in which the timer is operating in this circuit

    • Sneha, it is included in order to generate some level of hysteresis, and make sure the output does not oscillate rapidly at the cut off thresholds. It is operating in a comparator mode

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