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I had the problem that the seed always ran out of water while I was at work. I found out that the best way to water the seeds was to spray on them with a spraybottle, so I came up with little setup:

What I used:

SprayBottle
12V Power Supply
Broken CD Rom Drive
2 Resistors
Arduino Uno
TIP120 Darlingon
Diode
Some Wood and Wires.

The main idea was to measure the moisture of the soil in the cups and then close the tray of the CD Rom to spray on the Seeds.

For the measurement I used a voltage divider circuit. I placed one wire (without isolator) on the upper side of the Cups and another on the lower side, to measure the resistance. If the cups get wet the resistance goes down. I measured the resistance in dry and wet conditions to find which resistor fits for the voltage divider. In my case I used 1mOhm. With that circuit I  measure the remaining Voltage of the 5V of the Arduino on a Arduino AnalogPin. If the value goes down, in my case under 150. I trigger the TIP 120 Transistor on an digitalPin of the Arduino to let 12V from the Powersupply flow to the Tray of the CD-Rom. The tray closes and the water is sprayed on the soil until the value goes up again. Thats it.
If you want to find out more about voltage divider and driving a motor with the TIP120 you can find plenty of tutorials on the web.

If you want I can also provide the ArduinoCode and more Infos about my project, just ask.

I hope you like my litte project, I`m really new to electronics so if I you have any advices how to make this better let me know.

Greetings Robi

Why the arduino? You could do this with a bc108 and a relay with no other parts. Im getting sick of arduino
<p>there are 1000 of ways do to this, I did it with the arduino because I could use timers and stuff (i had a clock running to not spray at night). And last but not least I had it around. Just use what you want.</p>
need to make a video....cool
That is really clever! <br><br> It occurred to me you can use a similar setup to spray a naughty cat if it jumps up on the kitchen counter using a motion sensor instead of a moisture meter.

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