Ive been doing some research into possible solutions and have found  
references to  a MAX4626 "analogue switch" which to me looks like a  
solid state relay - it says its the size of a grain of rice.

What Im contemplating is using 90 of these arranged in a matrix with 9  
columns and 10 rows per column.  Each column would share an output pin  
from the PIC which I shall refer to as "column select" This would be  
wired in parallel to the COM pin of the MAX4626.  The IN pin would go  
to the wire I want to sample which has a voltage divider on it to drop  
it to 5v.

The PIC then tests each column in turn by switching on the appropriate  
column select wire, thus applying 5v to the COM pin of all the  
MAX4626s in that column.  If the wire being sampled is live it will  
put 5v on the IN pin and the solid state relay will close.  The  
prescence of 5v on the NO pin of the 4626 will indicate the wire  
sampled is live.  This will allow me to sample the 90 wires using   
only 19 i/o pins on the PIC.


Does that sound reasonable?

Thanks
Andrew

Sent from iPhone

On 25 May 2010, at 12:00, Oli Glaser <oli.glaser@talktalk.net> wrote:

>
> ----- Original Message -----
> From: "Justin Richards" <justin.richards@gmail.com>
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Sent: Tuesday, May 25, 2010 10:52 AM
> Subject: Re: [PIC] Using PIC to sample 90 wires
>
>> Indeed.
>>> But sometimes vivisection can be educational.
>>> Andrew posted, then responded to input  2 days later and that was  
>>> ~ 2
>>> days
>>> ago.
>>>
>>> I thought the subject was worth talking about in its own right as it
>>> explores what a beginner sees as easy and why, what people  
>>> perceive a
>>> beginner will perceive and why, what people see as trivial from  
>>> their
>>> own perspectives, and how people analyse the technical aspects of  
>>> such
>>> issues. Whatever.
>>>
>>> I know which solution I'd prefer :-)
>
>
>> I agree totally.
>>
>> I learn so much from other peoples questions and the answers that  
>> follow.
>>
>> At times there are questions that are asked that I would like to  
>> ask but
>> dont ask as I really dont have an immediate application and enjoy the
>> various posts that follow.
>>
>
> I also agree fully. Discussions like these can generally enlighten all
> involved directly, plus the people that read them. It's always nice  
> to hear
> others views on things, how they apply themselves to a problem. This  
> is what
> interests me most - the application of "knowledge" rather than the
> "knowledge" itself. Great for people who may be mostly "self taught"  
> and may
> rarely have the benefit of fellow students/tutors/colleagues to pick  
> up the
> practical stuff from, the "real world" things that books often miss  
> out. In
> short, it's all good stuff.
>
>
> -- 
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