A few details of sensor construction need to be pointed out. In order to prevent electrochemical degradation of sensitive lines exposed to outdoor air and moisture, the circuit presents no DC voltage to them. Electrical current that flows between the two interleaved sets of lines is pure AC because gate A output is always inverted with respect to its input (thus there is no need for a capacitor connected in series). The actual shape of the signal is square, not sinusoidal, but it is AC nevertheless. As for the ideal material of which the exposed lines should be built, bare copper is not the first choice. Despite the fact that we prevented its forced electrochemical degradation, copper has a natural tendency to slowly cover itself with a thin film made of copper-sulphate if exposed to ambient air. This is the result of chemical reaction between metallic copper and hydrogen-sulphate gas found in trace quantities in the atmosphere, and that is what gives for example old church roofs a distinct green patina. The reaction is self-limited as after the first layer of green substance covers the metal, no further sulphatation takes place (in contrast to iron rusting, which eventually eats the metal entirely). Unfortunately, this compound is electrically non-conductive and even a thin film made of it would degrade the sensitivity of the circuit significantly.

In order to prevent sensor deterioration, we spent quite some time thinking about the method of its prevention. Obviously, it would be ideal if the exposed lines could have been made entirely of platinum or similar precious metal, but the cost of such approach would be prohibitively high. An interesting solution might have been covering the copper by a very thin layer of gold or other worthwhile metal such as nickel or chromium, using an electrochemical treatment that we imagined we could have been able to perform ourselves. But as it soon turned out, that was not such a straightforward process as it seemed. One of the problems was the necessity to work with fairly nasty and poisonous chemicals that one would be practically unable to aquire if not belonging to the circle of professional galvanizing workshops. But even if a good-willing craftsman could have been persuaded to treat the sensor surface in a one-shot fashion, that would have required spending a lot of time and perhaps a not so small amount of cache from our part. Finally, there were some subtle lesser known quirks of galvanization able to influence the long term stability of such a sensor: simple gilded copper surfaces gradually loose their oxidation resistance due to difussion of copper atoms into gold, nickel/chromium and similar multi-layered structures sometimes react unfavorably to water due to effects of varying electronegativity, especially in the presence of microscopic surface scratches, and the list goes on.

After a great deal of mental energy spent on this topic, we calmed down and decided to tin the copper using the common tin-lead soldering alloy. One of the benefits of such simple processes is that we had all the necessary equipment at hand and that the thickness of the protective layer could be rather high. Curiously, as part of our search we discovered that, although tin itself is not immune to oxidation, the resulting tin oxide is actually electrically conductive! Tin oxide is in fact the optically transparent material used for connecting microscopic pixels in LCD screens to their driver electronics. So there we were - despite giving up on fancy electrochemistry, humble tinning turned out to provide adequate protection for our contraption. We reasoned that sensor surface should continue to operate undisturbed even when oxidized, and the fact that the device is doing fine after two years of continuous operation seems to support the validity of our expectations.

The basic concept of sensing the presence of moisture and liquid water as presented here provides quite a lot of room for further improvements and applications such as measuring soil moisture etc. We will welcome the response from those crafty readers that found this article a call for testing the circuit themselves and conceptualising other new devices upon it.