18 Jan
2011
18 Jan
'11
12:38 p.m.
On 2011 Jan 18, at 10:18 AM, Tony Duell wrote:
LED's typically run at a higher peak current for visual displays where
the objective is to bring up the average energy output to achieve the
same 'brightness' for perception by the human eye. In an application
such as that at issue though, the instantaneous output from the LED is
what matters to the opposing phototransistor/diode. The 'normal' LED
current may well be enough (or even more than enough). I wouldn't
expect even a 1:1000 duty cycle to be an issue. The peak current
requirements could go either way, depending on the sensitivity of the
detectors.
As to the original question, what about just hacking up an IR
phototransistor/diode or an IR receiver/amp module to a scope, manually
run it past the LED holes looking for activity (the modules may not
provide much indication of relative output though, just
presence/absence).
Similarly, power up an IR LED and run it past the holes of the
detectors and look for activity/change on the detectors data line.
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