I used such a thing, several years ago, but with some CD4000 glue :

- 1st : generate a square wave (# kHz)
- apply it through a capacitor (-> no continuous voltage applied so no
electrolytic corrosion) to a "combed-shape" printed circuit
- read the rectified AC voltage (through a diode doubler)
- test (via a Schmitt trigger) if higher than the threshold
- That's all

It is installed on my rough-top for several years to close my
roof-window if it begins to rain (very usefull in summer if a storm
begins during our sleep :-)

It should be easy to do it with a 12C508 although you could hardly set
threshold (or perhaps via frequency variation ?)

Hope it could help ...
Tell us the result of your brainstorming ;-)

Jean-Yves
>
>> -----Original Message-----
>> From: Benjamin Petersen [SMTP:benjamin@MARK-INFO.COM]
>> Sent: 10. maj 1999 09:47
>> To:   PICLIST@MITVMA.MIT.EDU
>> Subject:      "Plant out of water project"
>>
>> I have an (old) idea for a new pic project.
>> I would like to attatch this device to a plant and make it beep when the
>> plant needs water.
>>
>> I was thinking about using :
>> 1 3v litium batt
>> 1 PIC12c508
>> 1 Pizo-beeper
>> 2 metal pins pluged into the plants pot (or what ever you call it)
>>
>> I would like to make some kind of A/D converter, and as far as i can
>> recall
>> it can be done with a cap and a res.
>> And something with pulsing an output and reading an input.I was thinking
>> that the water alarm should NOT beep during the night, messured either by
>> an
>> internal clock or by a photosenser. Dryness alarm should be dynamic.
>>
>> When this project is done it will ofcause be public.
>>
>> Regards
>> Benjamin Petersen
>