Letting the PIC be the AC source lets you choose an operating frequency
to avoid outside interference.  Also consider synchronous rectification.
 You can do it easily with a OP amp and a FET switch.  It will reduce
all sorts of interference.

Of course if you just want to know if the plant needs to be watered it
may be overkill.

Douglas Butler
Senior Engineer
Imetrix Inc.
1235 Route 28A
P.O. Box 152
Cataumet, MA 02534-0152
tel. (508) 564-6460
Fax (508) 564-6860
dbutler@imetrix.com


> -----Original Message-----
> From: Vasile Surducan [mailto:vasile@L30.ITIM-CJ.RO]
> Sent: Thursday, June 14, 2001 6:13 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: AC current
>
>
> On Thu, 14 Jun 2001, Tony Nixon wrote:
>
> > Hi all,
> >
> > Does anyone have any ideas of measuring a small AC current
> with the PIC
> > A2D?
> >
> > A friend of mine has a hydroponic garden and wants to measure the
> > current flow between 2 probes which are fed with 16VAC.
> >
>
>   I'm thinking like a pic beginner. Measuring precisely RMS
> is difficult
> ( for me ) without some electronics. But if you'll use
> chopped rectifier
> tehnique ( same as operational amplifier precisely rectifier
> ) and square
> wave pulses, as someone mentioned before, will be more easy.
> A stepup transformer can supply a stable 16V square wave. And can be
> easily driven on - off. When the pulse is on you can measure soil
> resistance. And averaging sounds much simple. When is off you can do
> something else, like offset correction of the input measuring
> amplifier.
> The only real problem is avoiding sensor electrolysis effect
> which will
> change all measured results.
> Vasile
>
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