I've just bought myself a small plastic covered "greenhouse" thing from the local garden centre. It's great, it does just what I need for a small garden and was cheep too. I'm using it to cultivate some seedlings that I'll be planting out in the garden, but given the unpredictable nature of the English climate, I can't trust them to be totally unprotected from the elements right now. The problem is that little pots and tubs dry out very quickly and I'll be going away for a few days soon. I need an automatic watering system that will stop things from drying out while I'm gone.

Key objectives are:

-Use stuff lying around the house
-Don't spend ages getting it working
-Make sure everything get a good soaking.
-Keep and eye on environmental considerations
-Be Safe - Don't mix water and electricity!

Step 1: The Pump and Reservoir

I bought the pump a few months ago from ebay. It's a 12v windscreen wiper pump from a ford. I bought it because I thought I'd have a go at making some kind of water feature for the garden, but never got round to it. It will be just the right size for this job. The pump is not submersible so it has to be mounted on the outside of the reservoir.
The ideal reservoir would be a large bucket, but I didn't have one lying around that I could cut a hole in. Instead I had a whole bunch or plastic plant pots that could hold about one and a half litres. These are easy to cut and disposable (or at least their primary use is not affected) if I got it wrong the first time. Their problem is that they have large drainage holes cut in the bottom, which makes them less than ideal for holding water. A simple watertight fix for this is to put a plastic carrier bag inside the plant pot and force the input pipe of pump through a small hole in the bag. As the plastic is a bit elastic it helps to make a water tight seal. The pump has a rubber grommet which goes over the input pipe to create another water seal between the inside and outside of the container. Filling the reservoir with water and placing it on a dry surface shows me that it is not leaking.

Step 2: Creating the Shower Head

Most small irrigation systems I've seen use button drippers or slow flow outputs to individual plant pots. Because I may have up to 50 or so seedlings growing, this would be impractical for me. Instead I want something like a shower from above. Unfortunately I had no spare showers to hand but I did have a plastic squash bottle. My plan is to make lots of little pin pricks in the bottle and then use the pump to not only push water into the bottle, but to cause enough pressure to force the water out in all directions. I drew some circles a round the holes with a marker pen just so I could see them easily. The water enters the bottle though a hole in the cap. As the point of the bottle is to spray water out, it's not critical that this join is water tight, but it's a 'nice to have'. I used a barrel of a ball point pen pushed through a hole in the cap and secured in place with some tightly wound electricians tape which did a good job of the seal. The end of the rubber pipe was shaved slightly and then pushed into the pen barrel. It's a good fit all round.

Step 3: Connecting the Water Pipes and Power.

The Pump came with two rubber pipes, about 30cm each. I used another piece of the pen barrel to connect both pieces of pipe so I had something longer to work with. (The pump had two outputs, I blocked off the one I wasn't using with an off-cut of the plastic bag fixed in place with a tiny rubber band, then wrapped tightly in electricians tape). This pipe connects the pump to the shower and is long enough to be useful.

The wires supplying power to the pump were about 3 inches long, so I soldered about 4 meters of speaker cable to them and again insulated with tightly wound electricians tape. I tested the pump with a PP3 9 volt battery. It worked fine, but was far from ideal as the pump probably sucks about 2-3 amps and I could hear the PP3 dying as it got warm in my hands. The 4 meters of speaker wire would allow me to place the pump in situ and run the power in through the kitchen window where a suitable power source could be connected in safety (i.e. away from all the water).

Step 4: Results... Does It Work?

Yes! Although it's not too surprising as it's such a simple system there is not a lot that could go wrong. The difficulty I have is the lack of a suitable power source. I can power it for oh... about 3 minutes using a fresh PP3 9 volt, with only a small(ish) risk of a fire due to the exploding battery. The other alternative I found was my car's battery charger which put out 12 volts with about 6-8 amps. This power solution was overkill, but as a concept proover, it worked like a dream. There was a fine spray of water that covered all the plants. The Flow was enough that I would estimate that only 1-2 minutes of operation would be enough to water the plants thoroughly.

Step 5: Improvements

It works, but it's not pretty. There is a lot more that I can do with it. To start with it's manual operation right now, and that's no good as I'm going to be going away. My plan is to find a suitable mains powered adaptor that can deliver 12v at about 2 amps and I'll plug this into a digital time switch with one minute resolution. I will need to use a bigger reservoir and I'm planning on collecting all the unused water as it falls through to the lowest shelf and funnelling it back into the reservoir. This will not only allow the system to run for longer with out intervention, but also goes some way to achieving my environmental goal of not wasting more water than I have too. As a longer term goal, I would like to have the system powered by a solar panel and stand alone 12v bike battery. When power drops off the panel (i.e. night) a custom circuit would activate the pump for a couple of minutes. If I'm honest, I'll probably end up rebuilding the system from the ground up long before I get round to making it solar powered, but it's something to think about.