I started experimenting with automatic watering last summer. I have my downspouts connected to 55 gallon drums. I placed a tap as close to the bottom as possible and installed a filter on the inside to keep dirt particles from going through the tap. I have an electric solenoid switch (the 110V valve from a washing machine) that controls the water flow from the tap in the rain barrel to perforated plastic tubing that trickles water to my garden.

The solenoid switch is controlled by a timer and I can control the amount of water this way. As long as there is some rain every couple of weeks, the barrel is never empty. I can fertalize the garden by simply adding soluable fertalizer to the barrel.

I'm still trying to come up with a humidity/moisture sensor that will prevent the solenoid switch from coming on if the soil is already wet enough. Does anyone have any ideas?