(Almost) Automatic Salad Bowl

Last summer, our friendly neighbor gave us a big orange bowl, a bag of dirt, and a packet of lettuce seeds. She asked if we could grow the lettuce for her, and bring it back to her when it was ready.  She probably had no idea, but in our small-scale gardening experiments, lettuce has been our achilles' heel.  Whatever we've tried, we've had trouble producing nice-looking heads of lettuce.  But, not wanting to be poor neighbors, we said we'd give it a try and see if we could get her some lettuce.  Unfortunately, this lettuce, like all the lettuce we've tried before, came up thin and leggy.  That's when we vowed to fix this habit of ours and grow a nice full bowl of salad for her.

A bowl of sad-looking lettuce.  Tasty, but definitely not optimal productivity, and most definitely not pretty.

Also, since we were moving away soon, we wanted to give her back her lettuce bowl.  One of the tenets of good neighboring is, of course, to return things as-good or better than they were in when borrowed.  So we decided that when we gave her back the bowl of lettuce, it would be souped up and ready to snort salads out the tail pipe for months! 

From a quick search of the internets, it seems that the most common causes of lanky lettuce are too little light, too much heat, and maybe too much water.  Apparently, having to fight against a breeze or frequent brushing (e.g. from someone's hands) can also make the lettuce grow a little stockier. So, our souped-up salad bowl should have it's own adjustable light source on an automatic timer, an indicator for when to water it, and something to provide a breeze across the growing plants.  Welcome to the (almost) automatic salad bowl.

We started out by building a box out of some wood scraps to hold the bowl and light source.

Oh good!  The bowl and sad lettuce fit in the box.

Let us (ha!) replace the sad lettuce with some happier-looking stuff that we had started outside and needed to be thinned.

Then we needed an indicator to let us know when the soil was dry.  So we found this circuit diagram, but modified it slightly (shown above) to accommodate the components we already had (or could find nearby).  The circuit works like this: because soil contains exchangeable cations, when the soil is wet, an electrical current can flow through it.  (The ions become dissolved in the water, which makes them mobile, or able to move between the probes.)  When the soil is dry, the ions are not mobile, and no current can flow between the probes.  So, in our circuit, when the soil is wet, current flows through it, bypassing the transistor and white LED.  When the soil is dry, current reaches the base of the transistor (Q1), which opens the gate for current to also flow through the LED.  Or, more succinctly, when the soil is dry, the LED lights up.  The variable resistor is used to adjust the voltage across the probes, or basically, the sensitivity of the circuit.  One disadvantage of this design is that it's always sucking a little bit of power, even when the bulb is off.  But it's much easier to notice a lit bulb than an unlit one, and the power draw is pretty small.

This is what the circuit looks like in real life.  We initially attached short probes to make sure it would work ok.  The power source is an old cell phone charger, and the fan is normally used for keeping computer processors cool.

Looks like it does!  When the probes are separated to simulate dry soil (left), the LED is lit.  When the probes are connected by a conductive medium to simulate wet soil (right), the LED goes out. Yay!

Then we replaced the test probes with more appropriately-sized ones, and mounted everything onto the box we made before.

We found out that we had to keep the probe tips pretty close together for the circuit to work properly in the soil, so we secured them with a piece of heat-shrink tubing.

We stuck the probes in the soil and plugged it in.  Then we added about a cup of water, and adjusted the variable resistor until the light went out.

Freshly watered, and the light is off.  Yay!  It works, even in dirt!

Finally, we added the light source to the tall post in the corner.  We didn't want a big fluorescent light fixture over our little box, but we couldn't find a small fluorescent grow bulb that was the right size.  We ended up settling on a non-fluorescent grow bulb (it seems to be incandescent, but the wattage is pretty low).  It puts off a lot of heat, which is unfortunate.  The fan helps to keep it a little bit cool, but if we were to do this again, we would probably look for a different kind of bulb.  Hopefully it satisfy our neighbors' lettuce capacity!

Have you built any specialized plant-growing devices?  Any ideas for how we could improve ours? (well, improve our neighbors'.)  Let us know in the comments section below!