Four large tomato plants on the balcony, record temperatures on the forecast and the 2 week trip to Norway starts tomorrow... what to do?

A drip watering system is not sufficient as the tomato pots are quite big. I needed more power...

Rummaging through my part box, I found a simple enough combination of parts to hopefully keep my plants alive. And it was simple enough that I even got enough time to write this Instructable. =)

Step 1: Parts

YL-38 Hygrometer (a few bucks on ebay)
Small submersible pump (I used a DC30A-1230)
fitting plastic hose
Relay
Power supply
BEC (or some other component to generate 5V from the input voltage)
cable binders
Bucket

All in all maybe 20$ in parts.

Step 2: Circuit

The circuit is very simple:

The YT-38 has a pot with which you can set when the digital output switches. To make things easy, the output is high when the contacts are open (like, not in wet soil) and closed when the degree of conductivity set by the pot is reached. So we only need to attach a relay to it to switch the supply voltage to the pump.

If you use a pump that runs on 5V you still should use the relay, as the YL-38 can't handle this much power drawn over the digital pin.

As I am in a hurry, I assembled the circuit on a breadboard and put that into a plastic bag to protect it from the rain.

Step 3: Waterworks and Installation

My pump is submersible so I put it into a big plastic bucket with water under my tomatoes. I found a fitting pipe in my wine making supplies, measured it to length, put one end over the pump nozzle and constricted the other with a cable tie.

My tomatoes are in two plant pots, side by side on my balcony. I routed the hose from the reservoir up across both pot. Then I cut another hole into the pipe, just over the center of the first plant pot. I fastened the end of the host to the second pot with a wooden peg an more cable ties.

Finally, I routed the power cable through my balcony door to the psu.

Step 4: Testing and Calibrating

I plugged the sensor into the soil under and a bit to the side of the first hole and switched everything on. The pump started up, and the water went where its was supposed to.

I calibrated the pot by putting it into different parts of the plant pot, measuring different 'wetness levels' of the soil. I decided for one slightly dry spot (the flower pot is deep) and calibrated the pot just to switch on the pump.

Done! Now my parents only need to refill the bucket every few days and I'll be eating ripe tomatoes when I'm back! =)