Construction

Lighting

The project was housed in a 50 litre transluscent plastic box, similar to a recyling box. The lights were mounted on the underside of the lid and bolted to a wooden sheet on top, to give it some rigidity. I had a 12v LED spotlight which I mounted in the centre, as I was not too sure how much light the tubes would produce.

As it turns out, the flourescent tubes gave ample light, especially after the inside of the box was covered in aluminium foil to reflect as much light as possible. A terminal strip was mounted on the top of the lid and the wires from the lights brought out there.

Watering

The box was filled to a depth of approx 12cm with general purpose compost and a simple drip-feed network snaked over it. The end of the feed pipe was brought out via a connector at one end of the box near the top. Wires were soldered to two galvanized nails that were pushed into the soil about 5cm apart. Originally, I used stainless steel bolts but found the resistance was too high. The galvanized nails gave a resistance of just under 10k in dry compost which fell to less than 1k when wet (experimentally tested in another container). Hopefully, the galvanized nails will not rust and change their resistance. The wires from the soil moisture sensor were brought out through the side of the box near the watering tube. Bringing these items out through the side of the box enabled the lid to be removed easily without disconnection.

I tried various mechanisms to water the plants. Originally I used a 25 litre reservoir into which I had mounted a 12V electric valve (spare from the rocket launcher project). However, the valve only dripped water when activated, it obviously needs suitable water pressure to activate it properly. I tried it on a mains water hosepipe and it worked as expected. However, I do not want to be tied to mains water so abandoned that idea. Next I tried a submersible pump (rescued from the bottom of the water butt - and used to water the garden). This worked admirably but was a little clunky to move around. Eventually I settled on a old car windscreen washer pump as that also has a water reservoir so can ensure the pump is kept primed. This was a much neater solution although I did not have a syphon-lock valve so had to be careful to site the reservoir below the soil level to prevent it auto-syphoning. This is a high throughput pump so I decided it should not operate for more than 2 seconds at a time and adjusted the software accordingly (more on that later). The reservoir is about 1.5 litres so holds a reasonable amount of water.