Alexandre Guimar=E3es wrote:
>=20
> Hi, Roman
>=20
>     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.
>=20
>     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

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads