18 Jan
2011
18 Jan
'11
10:18 a.m.
On Mon, Jan 17, 2011 at 5:05 PM, Rik Bos <hp-fix at xs4all.nl> wrote:
Which I suspect is one of the most common problems with the HP150
tocuchscreen. The optodevices are down little plastic tubes, so at least
for the ones along the bottom edge of the screen (LEDs for the horizontal
axis IIRC), then can easily collect dirt. Of course sticky smoke residue
can collect on any of them (no, I don't smoke, and I won't allow it near
any of my machines, but I haven't owned this machine from new).
No but in most cases the working of led's in
binary they work or they don't.
I would have to counter with two situations of non-binary-fail
specific to classic machines:
1) dirt on the LED (especially tobacco smoke residue because it's dark
and sticky and gets on everything)
2) LEDs all dim with age. I've had Sun optical mice die because the
IR LED was too dim to reflect off the pad and retain enough brightness
to be picked up by the IR phototransistor. ISTR the nominal curve is
something like a half-life of tens of thousands of hours, and a
multplexed array would leave each individual LED on for less of the
time than a continuous-on LED as one might find in a Sun optical
Yes, but a multipexed array also runs the LEDs at a much higher peak
current, which may make things worsde than you'd expect from the duty cycle.
mouse, but that's just fine-tuning the time when
they will eventually
dim to the point that detection is erratic or non-existent.
For a first-cut diagnostic, viewing the IR LED side could be useful,
but even though I do own an HP IR-LED touchscreen frame (purchased as
a loose item at the Dayton Hamfest many years ago) and have a general
sense of how it works, I wouldn't be sure what the duty cycle for an
individual LED is - if I saw none lit, I couldn't be sure if that was
I think it's a lot less than you'd expect. The LED is turned on by a
monostable on the touchscreen board (at lesat in the orignial HP150
version), and from waht I remeebr it's only one for a small fraction of
the time taken to scna that LED. And then there are 36 or so LEDs to scan
through. I would have to look up the monostable timing and the scan clock
freqeuncy if you rally want to know the duty cycle, but ti could be
around 1:1000
-tony