4 Oct
2010
4 Oct
'10
7:20 a.m.
From: ard at p850ug1.demon.co.uk
---snip---
Much gabbing of signals with the LogicDart later, I
spotted the odd
glitch in one of the output waveforms. No idea what was causing it, but a
1nF camacitor in parallel with the 470R resistor on each phototransitor
cleared it up. And cleaered up the posiiton problem
Now 'all' I have to do is align the carriage rails properly. Iv'e
refitted the rest of the printer mechansim, and it does print, but it
fades out after few lines, to recover only wne I clean the end of the
ink cartridge. I suspect the spark is flying in slightly the wrong
direction and putting carbon ('ink') on the end of the cartridge, causing
HV leakage.
So, the replacement for the IS501 consisted of the sensor from a cheap
mouse and a 74LS14, a couple of R's and a couple of C's from my junk box...
-tony
Hi Tony
Your solution to the waveform is interesting. You added the capacitor
to fix the problem caused by the 74LS14. The problem is that the
hysteresis of the LS14 is so large that it eats up the phase
margines of the roughly sine/cosine from the optical sensors.
I've seen these used as input in mice before. I don't know why
no one ever ran a simulation or even looked at an oscilloscope
output of the mouse.
One should be using a comparitor with a small amount
of hysteresis. The LS14s have such large hysteresis that they
eat up almost all of the phase margine.
I guess it was done by an engineer that looked at the original
design and said "if I replace the compaitor with a LS14, I can
remove the feedback resistors and save a few fractions of
a penny".
Dwight