HP150 Touchscreen diagnostic

On Mon, Jan 17, 2011 at 5:05 PM, Rik Bos <hp-fix at xs4all.nl> wrote: 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 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 a global fault, or perhaps they just weren't on long enough to visually register. Having a working unit to compare against could help mitigate that. I agree if you spot one or two dim LEDs and the rest bright enough to see clearly, there's a reason to investigate that specific symptom, but the absence of that doesn't tell me as much. -ethan