18 Jan
2011
18 Jan
'11
10:51 a.m.
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.
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.
The LEDs are wired in a matrix with a ULN2804 driver to ground and a
ULN2580 drive to the +10V line. There's a 10 Ohm resistor in series with
each of the outputs of one of the drivers. This suggests a peak current
of a little under 1A. It is a low duty cycle, my guyess of 1:1000 seems
about right.
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).
Possible. But I still think dim beams are a very common problem.
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.
It may be less work in the end to make up some kind of tester to plug
into the touchscreen PCB and display the status of all the beams...
-tony