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Project: Wet
Date:July 2008


This palm sized circuit board monitors the moisture level of the soil. Two pieces of copper wire are run from the circuit board into the soil near a plant. The ends of the wire have galvanized rods bolted on them which are placed into the soil (copper will corrode). If the soil is deemed to be too dry a solenoid valve will open. The solenoid valve is connected to my homes 3/4" white PVC plumbing. All I've done is extended the PVC to reach all my garden beds and trees. By drilling 1/8" holes in the PVC I've made a poor mans irrigation system which has the smarts to not waste water.


irripcbbarebonespcb 5.3415.34
atmega169atmega169pv-8au-nddigi-key 3.8113.81
P150ACT-ND150 ohm 0805digi-key 0.0270.14
EG1847-NDpower switchdigi-key 0.8510.85
uln2003adtransistordigi-key 0.3210.32
NJM#7805FA-ND5V Reg. 7805digi-key 0.5010.50
P993-ND1uF decoupledigi-key 0.0620.12
SAM1074-50-NDjtagdigi-key 0.2210.22
160-1738-1-NDwhite leddigi-key 0.2771.89
275-232reed-relayradio shack 2.9912.99
Power Supply6V Nokiathrift store1.0011.00
G1295124VAC transformerelectronic goldmine2.9812.98
1523543 position terminal blockjameco 0.4731.41


  1. schematic (pdf)
  2. src (html)
  3. src (bz2 tarball)
  4. gerbers (zip)
  5. schematic (gschem format)
  6. pcb (pcb format)

Why Homebrew?:

  • 3/4" PVC is $0.25 a foot and sold everywhere.
  • drip hose 3/4" black poly is $0.60 foot and more difficult to obtain
  • my existing plumbing in the yard was all 3/4" PVC
  • no desire to get sucked into buy emitters, adapters, and special tools
  • irrigation controllers for multizone systems can cost up to $250
  • timer based irrigation systems will water when it is raining outside
  • desire to control everything
  • I am a cheap


  • Sputter - I am using a microcontroller which means the soil moisture is being checked thousands of times a second. This causes a situation when the soil begins to dry where the solenoid is rapidly being turned on and off. I will need to add some delays or use interrupts to prevent sputter.
  • Copper Probes - I've had good luck with galvanized probes going into the earth to measure the soil moisture level. When I had used copper the probes quickly oxidized causing the moisture values to shift. Thus giving the plant more water than neccessary.
  • Location, Location, Location - Figuring out the length of probe and distance to set them apart has been tricky. Just how much water should a tree or plant get?
  • Sunlight - It is bad form to water plants during full sunlight hours. If the water hits a leaf the sunlight can cause a burn on the plant. The water also evaporates much faster during the day. On a future version I could add a photo cell to detect sunlight levels to allow watering during only in low light levels, but avoid watering during full darkness and sunlight.
  • Power - I don't like having to find 120VAC power everywhere I want to put a solenoid valve and controller. The issue is that the solenoid valves for irrigation are usually 24VAC and I've been using 120VAC transformers to get the 24VAC. In order to do standalone modules with their own solar cells I'd need a small 12VDC transformer that can convert to 24VAC. A odd device.
  • Forcasting - Even though the wet controller has the brains to not water the yard while rain is falling or recently after rain. It does not know that a storm is nearby and there is a 80% chance of thunderstorm.
  • Remote - Despite having the dry vs. wet logic I really like the idea of controlling the system over the internet. Thus allowing me to turn on or shutoff the home irrigation via my cell phone. A future version would include some sort of wireless networking to allow centralized remote management.

I have enough parts to make 12 of these boards. Each board sells assembled for $50. I can include the solenoid valve and transformer for a addition $30 bringing the total to $80.


The Grand Tour from Mikey Sklar on Vimeo.

This video shows our entire property and is nearly 12 minutes long. The irrigation system is briefly covered.