This is my first instructable post, not sure if this will garner any interest, but if interested I'll post more details.
I decided to build something for the vegetable garden. I used a similar circuit from the LM317 datasheet to charge the battery. The solar panel is a 12v, but they usually run from 16v - 20v so the drop from the charging circuit isn't too bad.
The Solar panel charges the battery fairly well. The arduino monitors the moisture sensors and opens a solenoid to water the garden as needed. The arduino also controls some LEDs and monitors the nrf24l01.
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Step 1: Inside Then Panel
I usually build my own Arduino for projects like this. I opted to connect three moisture sensors, to three different zones in the garden, I first used the regular ones off ebay, but those deteriorated after a week. I decided to make my own with some nails, worked very well, and was pretty easy to do.
The Arduino draws the power off the battery through a 3.3v voltage regulator. The Arduino controls the solenoid by a darlington pair transistor.
Step 2: Wireless
Yes, I added a remote control to the garden to trigger the solenoid at will, using two nrf24l01. I did that for two reasons. One, because I wanted to play with a transceiver module, and two, I have a cat that likes to use the garden too, and I wanted to change that behavior.