3 Oct
2010
3 Oct
'10
12:21 p.m.
Some days ago I was asking about a source for an IS50 OPIC optical sensor
used in the shaft encoder of my Olivetti Sparkjet printer. And it appears
this device is unobtainium
In an earlier message tonigth I mentioned I had recently bought a cheap
PS/2 mouse, and that it is now very dead.
These 2 events are connected...
The reason I boguth a cheap mouse is that it was optomechancial, rather
than optical. And the reason it was PS/2 is that that was the cheapest one
I could find. The salesdroid in Maplin thought I was craxy when I daid I
didn't care what the interface was. On the other hamd, trying to ecplain
classic computing to him was going to take too long, so I simply said that
PS/2 was what I wanted...
And of coruse the reason I wanted an optomechancial mouse is that it
contains a pair of dual phototranssitor sensors to sense the motion of
the encoder wheels.
Of ocurse I took the mouse apart. It was very cheaply made, with the
slotted disks and spindles moulded in one piece, then clipped into
'bearings' moudlded in the base of the mouse. Ouch. But I didn't care
about that. I soon desoldered the sensorts which appear to be dual
phototransistors with a common collector connection. And the pitch of the
slots in the mouse's encoder disks was very close to that in the disk in
my Sparkjet printer.
Next job was to carefully remove and dismantle the encoder in the
Sparkjet. I desoldered the OPIC, which shed a couple of pins in the
process, but I cared more about the PCB. I also removed the dual
resistor assockated with the OPIC> I then fitted one of the mouse sensors
in place of it, common collector to the +5V tack. And with a little cut
and muper, I routed the 2 emitter connections to pads that had held the
dual resistor. I carefully ressembled the sensor and fitted 10k
resisotrs in placeof the dual resistor assembly. That would do as an
emitter load for testing. Turning the spindle produced a change in
voltage, but it was close to +5V all the time. Dropping the resistors
would help, in the end I foudn that 470 ohms (yes, that low) was ideal. I
added a 74LS14 schmitt trigger IC on a bit of stripboard to clean up the
signals, conneted the outputs to the pins on the connector to the
printer's main board and gave it a try.
Using the logicDart I could get a display of the wavefortms, and by
adjusting the encoder PCB position I got a pretty good pair of quadrature
signals. Doing the self-test on the printer got the carriage jiggling
about is it should do (the printer was dismantled so that there was no
platten, HV generartor, carriage rails, stc, so I couldn't see if it was
preinting anything, but I coudlsee the montion of the carriage drive).
Alas evey so often it would miscount and the carriage would jump in position.
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
4 Oct
4 Oct
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
1:42 p.m.
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 don't think this is the same problem. Without the capacitors, and with
the 470R resistors, I was getting waveforms with roughtly 1:1 mark-space
ratio, and while not exactly in quadrature, pretty darn close. The only
problem was that every so often (perhaps 1 in every 10000 pulses), there
would be a glitch in one of the waveforms. It's not supply relateded, and
I couldn't repodcude it by, say, jiggling the encoder disk.
But that glitch was taken by the elctronics as a valid pulse which upset
things.
AAdding the capacitors hasn't noticeably changed the mark:space ratio or
the phasing. But it's got rid of the glitches.
A lot of optical mice use the '14 and the output signals look pretty good
to me.
-tony
2:59 p.m.
From: ard at p850ug1.demon.co.uk
Hi Tony
It looks good as long as you are going one direction. Try one tick
backwards and you'll understand, right after the other input has
flipped. You'll get and error count. I won't happen all the time,
just when it stops in that small window of the cycle.
And, yes, I know a lot of mice use the LS14. I've even tinkered a
few into working with them in them. Those that have a LM339 or
similar work fine. It should have about 10% or less hysteresis
feedback. Most LS14s are in the order of 30% or more. Try
the LM339 and resistors.
Dwight
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 don't think this is the same problem. Without the capacitors, and with
the 470R resistors, I was getting waveforms with roughtly 1:1 mark-space
ratio, and while not exactly in quadrature, pretty darn close. The only
problem was that every so often (perhaps 1 in every 10000 pulses), there
would be a glitch in one of the waveforms. It's not supply relateded, and
I couldn't repodcude it by, say, jiggling the encoder disk.
But that glitch was taken by the elctronics as a valid pulse which upset
things.
AAdding the capacitors hasn't noticeably changed the mark:space ratio or
the phasing. But it's got rid of the glitches.
A lot of optical mice use the '14 and the output signals look pretty good
to me.
-tony
5 Oct
5 Oct
10:56 a.m.
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.
=20
I don't think this is the same problem. Without the capacitors=2C and wit= the 470R resistors=2C I was getting waveforms
with roughtly 1:1 mark-spac=
e=20
ratio=2C and while not exactly in quadrature=2C
pretty darn close. The on=
ly=20
problem was that every so often (perhaps 1 in
every 10000 pulses)=2C ther=
e=20
would be a glitch in one of the waveforms.
It's not supply relateded=2C a=
nd=20
I couldn't repodcude it by=2C say=2C jiggling
the encoder disk.
=20
But that glitch was taken by the elctronics as a valid pulse which upset=
=20
things.=20
=20
AAdding the capacitors hasn't noticeably changed the mark:space ratio or=
=20
the phasing. But it's got rid of the
glitches.=20
=20
A lot of optical mice use the '14 and the output signals look pretty good=
=20
to me.=20
=20
-tony
=20
Hi Tony
It looks good as long as you are going one direction. Try one tick
backwards and you'll understand=2C right after the other input has
flipped. You'll get and error count. I won't happen all the time=2
just when it stops in that small window of the cycle. And=2C yes=2C I know a lot of mice use the LS14.
I've even tinkered a
few into working with them in them. Those that have a LM339 or
I'm harldy and expert on mice, but of the ones I've seen :
Modern ones (like the PS/2 mouse I took the dual phototransistor from)
just feed the imputs into some microcontroller thing. I have no idea what
the input characteristics are, and there's no trivial way to measure them
[]1]. So no idea what the hysteresis is.
[1] One way to do it would be to feed a pair of quadrtature signals into
the chip with said chip connected to a host expecting a PS/2 mouse, and
adjust the high and low levels of each signal in turn until it just stops
working. Rather more work than I want to do for this :-)
Older ones, which used chips I can recognise, and for whcih I may even
have official schematics/service manuals use some kind of TTL or
4000-series CMOS schmitt trigger chip. I[ve not seen one that doesn't.
The '14 in variosu forms is common. Maybe it is a bad design really
(althohgh as I said, I think that depends on the outputs of the sensors)
but it seems to work well enough.
similar work fine. It should have about 10% or less
hysteresis
feedback. Most LS14s are in the order of 30% or more. Try
the LM339 and resistors.
_If_ I havr problems, then that's what I'll do. So far it seems to be
working fine. I have now alinged the carriage [2] and it prints as well
as I'd expect a Sparkjet ot print. In other wods it's legible, but
nowhre near letter quality.
[2] You need to get the axis of the ink cartirdge level with the top of
the paper guide electrode, and a consistent distance from it across the
paper. If the cartridge is far too low, it doens't print at all. If it's
just to lwo, the ink scatters from f the edge of the guide onto the paper
and it looks weird. If its' mucth too high, you get ink depositied o nthe
front of the cartridge, and the printing fades out after a frw lines.
What I did was make a special tool. A 4mm rod (same diameter ad the glss
tube in the ink cartridge) with a point on one end (point on the axis of
the rod, obviously).I fitted that into the carriage in place of the
cartridge (printer power off and casing removed) and adjusted the
position of the carirage rail assembly so that the point was just running
among the top of the elecrode, and just touching a sheet of paper fed
round the platten. On the second attempt, it printed fine....
-tony
5143
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5145
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