On Tue, 11 Jun 2002, Roman Black wrote:

> Alexandre Guimar=E3es wrote:
> >
> > Hi, Roman
> >
> >     I just received and "official" answer from a membrane keyboard
> > manufacturer and he says that the resistance varies with many factors,
> > flexing of the flat cable, humidity, number of keypresses, temperature =
and
> > some other ones that I cannot remember ! The resistance can go up to 50=
0
> > ohms without the keyboard being considered bad ! The silver connections
> > develops "micro cracks" and the resistance goes up.
> >
> >     I think you cannot count on the 300 ohms tolerance of your design
> > reliably with membrane and carbon keyboards. It is a shame because it w=
ould
> > be nice to have a way to connect more keys. The method should still be =
valid
> > for the small "dome" switches that are pretty cheap.
>
>
> Excellent info Alexandre, thank you very much.
> I wasn't happy with the error margin either,
> and have started redesigning the system so that
> it will work with up to 1000 ohms per keypad
> with no error. I'll put up the schematic and
> software soon. The good news is that the new
> system will interface 12 key pad to 2 PIC pins,
> and a 16 key pad to 2 PIC pins, saving one or
> two pins over the first design. :o)
> -Roman
>
  Why do not use a standard PC keyboard, it has much many keys
and requires only 2 pic pins... available also for strong users.
The challange will be 0 pic pins keypad and 0 pins bootloader...
o|o
'-'

Vasile

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