DIY-WB LED problem (KC-5300 display)

Phil Hunter ilphayunterhay at yahoo.com
Mon Jan 21 18:02:36 GMT 2002


[snip]
> I just performed the test you described. With IC1 removed and
> in 1 and 2 jumped on IC2 (IC2 still in place), when I vary VR3,
> I get a voltage range of 0.63v - 0.85v  With the jumper removed
> and pin 1 jumpered to the sensor input, when I vary VR3, I get
> a voltage range of 1.97v - 2.65v

[snip]

> Perhaps I have the trim pots at VR2 and VR3 wrong? The
> pot at VR3 is currently labelled 502 and VR2 is labelled 254.

One way to read this is 50 and 2 more 0's, i.e. 50 & 00 or
5000, 25 and 4 more 0's, 25 & 0000 or 250K, so you have them
correct.

At this point, you should have enough info to figure out the
problem if you think about it. The pin 1 voltages don't match
what was predicted, but in the 3X amp configuration they are
about 3 times the unity gain configuration, so the sensor input
circuitry (the difference in the 2 configurations) s/b correct.
That leaves the VR3 resistor divider network or the op-amp. Notice
I said in the unity gain buffer that pin 1 follows pin 3, check
that it does, if not then the op-amp is the problem, otherwise
figure out what's wrong w/ the divider.

"Proof of this theorem is left as an exercise for the student"

Good luck, and let us know the answer.

regards,
philh
(digest)

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