Thanks, Ravi, for sending me the info on your technique. It turns out that I did something fairly similar many, many years=20 ago, but for a completely different application. My application was=20 touch sensing and I used CMOS 555 timers as the oscillator sections,=20 with a 12f675 or a 16f628 as the controller. But the oscillator=20 section was the same in all versions: a 0.6" square pad on both sides=20 of a PCB with the 555 timer placed in the middle of the rear pad,=20 along with the (very) few components needed. There was no direct=20 connection between the pads - just the capacitance between them with=20 the PCB material (Fiberglass FR-4) acting as the dielectric. There=20 would then be a thin Lexan label over top of that - this label was=20 silk-screened with whatever decorations were necessary. I've spun off a separate thread talking about touch sensing. But I'm=20 thinking that some variation of a COMS 555 timer might be even lower=20 cost and possibly more repeatable than using a PIC as the oscillator. I'm being yelled at to go check something - I'll return to this=20 thread later today. dwayne At 11:53 AM 5/6/2013, Ravi wrote: >I have developed a soil moisture sensor (Photo attached). > >Super imposed on the setup is the actual sensor photo. > >The display shows the % of moisture present in the tumbler. The >oscilloscope shows the % in terms of pulse width (mS). > >So far I have not encountered any issues. From the past 4 days the % of >moisture has been decreasing everyday. > >Further tests and validations are in progress. > >Cheers > >Ravi > >On Mon, May 6, 2013 at 7:27 AM, Sergey Dryga wrote: > > > Allen Mulvey amulvey.com> writes: > > > > > > > > Thank you for your comments. I'll try to reply to several > > > people all at once. > > > > > > First, misting is not a part of irrigation. I am > > > experimenting with moisture sensors like the one below but > > > that is a totally different, and much simpler project. > > > Misting is primarily for new growth and cuttings. Their root > > > system is small if existent at all so they cannot > > > effectively process moisture from the soil. The misting > > > keeps the vegetation (above ground) from drying out. A > > > cutting with no root system can dry out very quickly, even > > > with moist soil. > > > > > > It would seem that relative humidity would be the most > > > significant factor but it is not. Temperature has a twofold > > > effect. First, as air is heated it is able to hold more > > > moisture so the relative humidity has a tendency to drop as > > > the air is heated. Secondly higher temperatures (more > > > energy) help bridge the latent heat of transition so warm > > > water evaporates more quickly than cold. A 60 percent > > > relative humidity at 40 degrees Fahrenheit has a very > > > different effect than 60 percent at 105 degrees. Both of > > > these are well within the operating parameters of a working > > > greenhouse. There is not a linear relationship. Barometric > > > pressure can also have an influence as partial pressures are > > > also involved. I was hoping to find a table which would map > > > the rate of evaporation, as closely as possible, to my > > > measurable variables. I had expected that some research > > > nursery somewhere must have already done this. > > > > > > Allen > > > > > > Moisture Sensor: > > > http://www.vegetronix.com/Products/VG400/ > > > > Thanks for explanation. As somebody suggested, look at hydorology. Al= so, > > evaporation rate is a physico-chemical problem, this might help: > > http://en.wikipedia.org/wiki/Evaporation > > > > If the goal is to keep evaporation from cuttings low, than with RH clos= e to > > 100% (but non-condensing) one can achieve that. This implies that > > temperature control is decent (no sudden drop in temperature that would > > lead > > to condensation). However, in real-world application this can be > > counterbalanced by just as rapid drop in RH, e.g. by blowing fresh air. > > > > Please keep us updated on your progress/design decisions, it is an > > interesting topic. Especially for me, since I am doing similar things > > (although not on a commercial scale, just a hobby) for my wife > > (http://imagiknitter.blogspot.com/). Eventually I owuld like to have a= l > > kinds of sensors and reporting for growing plants. > > > > Sergey Dryga > > > > http://beaglerobotics.com --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .