Soil Moisture Sensor Circuit Explained

Soil Moisture Sensor Circuit Explained

May 3 2009
Update 2011-09-05: I have created a new blog about electronics only – check it out here: Electronic measurements, NSLU2 and soil moisture sensor

The 1-wire soil moisture sensor circuit I use in my garden is designed by Eric Vickery from http://www.hobby-boards.com.

(c) 2006 Hobby Boards Designed by Eric Vickery
Title: Moisture Meter 3
Rev: 3
Date: 10/27/2006 06:11:30p

(Click the picture for a larger version.)

Here’s a bit of explanation on how the humidity sensor circuit works.

The circuit is connected like this:
- Watermark soil moisture sensor: Pin 4 and 5
- 1-wire network: Pin 1 and 2
- 12 volts power supply: Pin 1 and 3
and the circuit works like this:
1. Watermark sensor gets wet
2. The time constant of oscillator IC 555 changes
3. 555 IC oscillates faster and its supply current goes up
4. Current in resistor R2 goes up causing the voltage across R2 to go up too
5. The value of the current register of IC DS2760 changes
6. Current register of IC DS2760 is read via 1-wire network
In my case I got the following values during calibration. I let the Watermark sensor dry in the wind and recorded the current register value, and afterwards the sensor was soaked in a bucket of water:

Dry = -0.2368
Wet = -1.400

These numbers form the 0 and 100 % limits of my soil moisture readings.
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14 Responses to “Soil Moisture Sensor Circuit Explained”
1. Dennis Hall said on July 13th, 2009 at 11:13 pm
  
  Quick question. What type of data is provided for the output? Is it serial or a square wave? Please let me know, I want to send the output to a transducer or a pizzo for a simple alarm when the saturation point is reached. Thank you for your time and consideration in reviewing my request.
  
  Sincerely,
  
  Dennis Hall
  technician1965@hotmail.com
2. Thomas W. said on July 14th, 2009 at 1:48 pm
  
  @Dennis: As I see it the “output” of the circuit is a software register.
  I think what you need is an OpAmp Schmitt-trigger measuring the voltage across R2, and removing U1, R1, C3 and D1, but I’m not sure if that’s what you want?…
3. braulio said on March 9th, 2010 at 4:51 am
  
  Hi, I want to do this circuit, but in dallas website, the ds2760 is no loger available, I could change the ds2760 to ds2762? thanks, best regards
4. Thomas W. said on March 9th, 2010 at 1:29 pm
  
  @Braulio: I think you can use DS2762 instead as the manufacturer Maxim states, “DS2762 is an update to DS2760″.
5. RD Pawar said on July 27th, 2010 at 11:46 am
  
  I require to measure water content at root zone of plants. What material shall be used as a probe. whether still wires will work.
6. Thomas W. said on July 27th, 2010 at 2:02 pm
  
  @RD Pawar: Please take a look at this post: http://happyfarming.com/2008/12/23/soil-moisture-sensor/
7. riteeka said on April 1st, 2011 at 7:20 pm
  
  I want measure soil moisture n give its output to controller.I had made one circuit but it is giving me output even at low humidity.I want to compare present moisture value with threshold value.how can I do that?.thank you for giving your time and consideration.
  
  sincerely,
  
  Riteeka
8. Thomas W. said on April 5th, 2011 at 5:25 pm
  
  @Riteeka: What type of controller do you want to use, and what does your circuit look like? Do you have a schematic? What sensor are you using?
9. mpho said on June 15th, 2011 at 10:51 am
  
  hi Tom,its Mpho in Botswana,one country in southern Africa..i need some assistance from you,i want to design a circuit which control the moisture content,pliz send me some possible circuit diagrams
10. Thomas W. said on June 27th, 2011 at 9:55 am
  
  @Mpho: What device / component will you be using to control the water flow? Some kind of pump or valve? I plan on using an electric drill pump (Gardena, 1,700 l/h @ Ø 13 mm hose, 3 bar) connected to a sprinkler or soaker hose depending on the crop (some crops are sensitive to water on their leaves due to blight.)
11. hazil said on January 20th, 2012 at 5:25 pm
  
  sir..how i want to control the soil moisture sensor?can i use the microcontroller?can u recommended what type of microcontroller?my project is about solar system for irrigation that using soil moisture sensor..
12. Thomas W. said on January 23rd, 2012 at 11:11 am
  
  @Hazil: I think you can control 1-Wire devices with the Arduino board. I’m using NSLU2, but a SheevaPlug or FitPC should be able to control it to.
  http://diagramtips.com/2011/08/31/automatic-temperature-measurements-using-nslu2/
13. hazil said on January 23rd, 2012 at 2:07 pm
  
  sir, can relay switch and timer can be added to this circuit?
14. Thomas W. said on February 3rd, 2012 at 12:38 pm
  
  @Hazil: Yes – the voltage across R2 could be measured by the input of an opamp comparator and then a switch actuator could be added like in this circuit: http://66pacific.com/ph/phschematic.htm (Valve solenoid = relay coil)
  An extra 555 IC can work as a timer IC: http://www.kpsec.freeuk.com/555timer.htm#monostable
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