I have recently seen one of very interesting concepts and it's circuit diagram is present in the page: Electronic Plant Watering System. I felt that it is very useful in our real life also when we go for any tours.

Coming to my problem, I have a small in-house garden of size 130squre feet with lot of plants. I felt that this circuit is suitable for small place but not for large space like my garden. And also I think it takes lot of time to arrange this for my garden.

So, Can anybody suggest me is it suitable for 130 square feet size garden? And also, I would like to use LM555 timer instead of IC555 timer. What are the modifications to be done to this circuit to set up it to my garden? Are there any other circuits which is simple and suitable for my garden?

Kindly let me know your suggestions. Thanks in advance.

  • \$\begingroup\$ I am strongly under the impression that whoever wrote that article has no idea what they're doing. \$\endgroup\$
    – Dzarda
    May 5, 2014 at 13:16

2 Answers 2


If I were you, I'd simply trash the article you found. It might be me, but I cannot wrap my head around the circuit nor the explanation.

Some random thoughts:

  • You can approximate your garden to be exactly the same humidity everywhere at any given time. That way, one pair of probes (properly placed) can be enough.
  • Given that sensors are unreliable (they break, oxidize, etc.), you could get false reading. Redundancy in the number of sensors may work in this case. Distribute any number of sensors throughout your area of interest and implement an average/mean/median filter.
  • If however there's a lot of spatial variation (eg. damp on one side and dry on the other), you could implement a system of "local watering" - say divide your garden to 4 squares and irrigate each part separately.

I'll try describing the first option:

  1. Use an off-the-shelf comparator IC. Add some hysteresis to ensure reliable operation.
  2. Feed that to your relay. Use a N-FET or an NPN to provide the relay with enough current, if needed.


simulate this circuit – Schematic created using CircuitLab

Please excuse the crudeness of the schematic. I hope I haven't messed anything up.

Also, note that the comparator is meant to have Push-Pull output. Open-drain piece can also be used - it would require minor changes and the usage of a MOSFET instead of a BJT.

The resistor values are all empirically determined. The one unknown is R4 - should match the mean soil resistance.

  • 1
    \$\begingroup\$ I think you missed out a diode across the relay coil to protect the transistor from the voltage spike when the relay is deactivated. \$\endgroup\$ May 5, 2014 at 19:28

To specifically address your question regarding useage of LM555 and/or IC555 : Some 555 timers are bipolar (been around a LONG time) and some are CMOS versions. The main difference is power consumption. Just for functionality, any 555 timer will work. The output drive capability differences might require one to modify a load circuit. Check the data sheet for output drive capability.


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