26 Jul
2013
26 Jul
'13
2:47 p.m.
The original circuit has a voltage divider for the LDR
formed with
the 18.2K resistor in the lower half and the LDR to +5V, feeding the
negative input of the 741, with the positive input fixed at 2.5V. The
result is the 741 (comparator) will trip when the LDR resistance
passes through 18.2KOhms.
Agreed
[...]
In your simulation, you used resistances for the LDR
of 74K dark and
I fial to see why you need to 'simulate' this circuit. The actual
comparator circuit, whether the origianl HP one iwth no feedback or the
intended modification is simple enopug hthat you can understand it in
your head.
The dificult part is the sensor itself. Yes, you can measure the ligth
and dark resistances (and if they really are 74K and 4K then it should
work with an unmodified comparator circuit). But I don;t think you cna
simulatre the sensor more than that. There are too many unknowns.
4K illuminated. That should have worked just fine with
the original
HP circuit, although it wouldn't work with the way you have actually
modded the board.
Where are youre results/descriptions of this? Rik has desciribed what he
did, you have (quite probably correctly) found errors in it. But I don't
see your work published anywhere.
I wrote it because it's a working (tested)
solution for an actual
problem.
AND you can't just replace the CDSe type with a normal off the
shelf LDR
that doesn't work, I tried..
So be careful before just shouting something without testing it!
I'm not shouting, I am being careful - far more than you, and I have
repaired and done a fair degree of experimentation with these sensors
(in one instance replacing the lamp/LDR pair with an IR LED/
phototransistor pair, and it didn't require any changes to the
comparator circuit). I haven't counted how many items I have reverse
engineered, but it's
many dozens, over a hundred I expect depending on what level of
Is that all :-)?
-tony