Just some thoughts to stir the pot.

Are you making only one of these?
Are going to sell them?
PCB or hand wired?

What if there was, say 13V on the input, what would you want to do then?

Makes lot of difference on your cost vs just get it done decisions.

I have used 74HC595 to select channels. Serial in, parallel out shift 
register.  Cheap.

Siliconix (Vishay now) makes a DG4088 Ch. analog multiplexer with a 3 
bit binary input.  You could actually measure the voltage.

Maybe do a 10:1 attenuator with resistors on each input.

A 3 cent Zener if you are just detecting some voltage?

Pics are cheap, a 100 inputs would not be out of the question?

Lots of fun things to think of here!




On 5/21/2010 6:49 AM, Russell McMahon wrote:
>> Im new to PIC and electronics in general, having come from a software
>> background. Im looking to design something which can sample 90 wires for
>> the presence or absence of a DC voltage (50v, but can be taken down to
>> say 5v with a voltage divider if needed). It then needs to store a bit
>> in memory for each wire indicating if the wire was live or not and then
>> interface to a computer over USB so the computer can read the bits.
>>      
> What  they said.
> Plus:
>
> ICs to sample 100 lines are not too costly, but you MAY be able to use
> an addressed  XY or even XYZ matrix to use many less active devices,
> or to use many diodes and fewer ICs.
>
> Using XY you need 10 x 10, or in multiples of 4 say = 8x12 = 96.
>
> If you can work an XYZ addressing scheme you can get say 5x5x5 = 125
> or 4x4x8 = 128 .
>
> In an XY system an X line drives a row (say) of inputs with matrix
> points being held down (or up) by input voltages. The y colums read
> the result. This may end up with more R's and / o r diodes than are
> worthwhile.
>
> _____________
>
> Using a number of serial shft registers makes it all very easy.
>
> The CD4021B parallel to serial  is under 40 cents at Digikey in 25's.
>          http://www.fairchildsemi.com/ds/CD/CD4021BC.pdf
>
> Each  allows 8 inputs and they can be chained in series indefinitely.
>
> 12 x 8 = 96 inputs.
>
> There are various ways to get 50V to logic level input - but it should
> not take more than 2 x resistors and 1 x diode per input and just
> maybe a small capacitor too, but  could take less. They can run at up
> to 15 V supply and it MAY be worth running them at near that level and
> translating the very few control and data signals to PIC level.
>
> Depending on the tolerance and noise spikes on your 50V signals you
> MAY be able to use just 2 x resistors.
>
> What's the application?
> Sounds like telephone line sampling ? :-).
> Dim and not wholly pleasant memories from long ago stir :-)
>
>
>   Russell
> .
>    

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