# Changing photo resistor resistance range.

Discussion in 'Electronic Basics' started by Chris W, Apr 22, 2005.

1. ### Chris WGuest

I have a photo resistor that varies from about 400 ohms to about 15
million ohms in the desired light conditions. However I need something
that varies in resistance from 1k to 5k. Is there a way to do this with
a transistor? The circuit will always be supplied with a 100mV supply
which I suspect will effect the design of what I am trying to do.

--
Chris W

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http://thewishzone.com

2. ### Larry BrasfieldGuest

What's wrong with the following?

o
|
.-----o-----.
| |
.-.
| |
| 6.2K | |
.-. '-'400 to >1M
| | |
| | |
'-' .-.
| | |
| | | 620
| '-'
| |
'-----o-----'
|
o

3. ### John FieldsGuest

---

R1 = 6.2k
R2 = 400 -> 15M
R3 = 620

R1 (R2 + R3)
Rt = --------------
R1 + R2 + R3

6200 (400 + 620)
Rt1 = ------------------ = 875.9 ohms
6200 + 400 + 620

6200 (15e6 + 620)
Rt2 = ------------------- = 6197.4 ohms
6200 + 15e6 + 620

875.9 ohms to 6197.4 ohms isn't 1k to 5k.

4. ### John LarkinGuest

Note the op's use of the word "about."

John

5. ### John FieldsGuest

---
What makes you think I didn't?

nominal.

-12.4% on the low end and +23.9% on the high end is a little more than
that.

But thanks anyway...

6. ### Rich GriseGuest

Well, what if you did a little algebra:

R1 (R2 + 400)
Rt1 = ----------------- = 1000
R1 + R2 + 400

R1 (R2 + 15e6)
Rt2 = ------------------ = 10000
R1 + R2 + 15e6

and solve for R1 and R2?

Cheers!
Rich

8. ### John FieldsGuest

---
PS: If you're bitching about the ordering, go look up "associative
law".

9. ### EndoscopeGuest

Whoops! Sorry, Brane Fart...... Try 5K1 and 820R.....

DNA

10. ### EndoscopeGuest

Oh ****, Barry might be Trolling/Teaching. His question was......

'What's wrong with the following?'

Oh shit..... I've just given him the opportunity to write 300 lines of
explanatory dribble.

Argggggghhhhhhhhh!!!!!!!!!!!!

Kill Me NOW.

DNA

11. ### Larry BrasfieldGuest

Despite occasional intimations to the contrary, I can
do arithmetic. What is not clear to me, and behind
my question to the OP, is whether your result (and
mine) is wrong for his purposes. I took the OP's
"1k to 5k" as a given to be achieved with what is
likely a cadmium sulphide cell illuminated with a
somewhat uncertain intensity, (hence the OP's
"about" qualifiers on its resistance range). Due to
more uncertainties stemming from the cell's TC and
its replacements, I elected to provide a range that
extended a little beyond what the OP stated. The
reason for more margin at the high end is that the
cell's high resistance extreme is likely to be more
uncertain than its low extreme. And if it is what
occurs due to night illumination, the moon might
cause considerably less than the stated MOhms.

Anther potential "wrong" aspect is that the mapping
from cell resistance to output resistance may not be
what the OP needs. Again, I rely on the OP to help
solve his own problem by stating such a deficiency.

Perhaps I should rephrase the question:
In what way will the OP's application misbehave if the
above circuit is used for the light-affected resistance?

12. ### BanGuest

But why did you choose non E12 or E24 values, I can understand when you go
off ideal values, and choose the closest standard ones, but you just write
non standard values just more than 20% off.
It might be in some old bin you found only those values, ok it will work,
but to deliberatly choose some random values is IMHO an unnecessary source
of error.
Or maybe it is that you are not familiar with standard E12? That would
qualify you as a bloody beginner.

13. ### Larry BrasfieldGuest

I suggest you study this table:
http://www.rohm.com/products/shortform/26rstr/rstr_index0.html
Then come back and explain your puzzlement.
I deliberately chose 5% standard values to avoid
unnecessary confusion and low SNR posts here.
of the E24 table at the link I provided.

14. ### John FieldsGuest

---
I had no "result", other than pointing out that yours was quite a bit
away from what the OP asked for. Of course you could argue, ad
nauseam, that "about" means different things to different people and
put forth the conjecture that your results were "good enough", but who
cares?

15. ### GenomeGuest

Oh Shit.......

Leave it John.

Barry should have corrected himself properly.

Whoops!! 5K1 and 820R. Job done.

No not Barry.

And before he goes further I would just like to take this opportunity to
point out that his original answer did not include the 'caveats' he is now
giving and you are responding to.

DNA

16. ### Larry BrasfieldGuest

....
Your "properly" is a private concept.
My original question applies to your "perfect" values
as much as to the ones I posted. Going on and on
about "caveats" is pretty pointless when we have no
idea what the application requires.

I should point out that by misconstruing my added
'potential "wrong"' as a "caveat", you are missing
the point and making that obvious.

17. ### John LarkinGuest

Should we make an official verb out of that?

John

18. ### John FieldsGuest

---
Perhaps, since this _is_ seb, a more satisfactory approach would have
been to supply the OP with the tools which would have allowed him to
figure out his own solution(s)?

Y'know, "Build a man a fire and he'll be warm for a day, but set a man
on fire and he'll be warm for the rest of his life."

19. ### Larry BrasfieldGuest

That's a good point, on both the forum and approach.

For the simple resistance calculation, your depiction
of the arithmetic involved ought to suffice. Regarding
the problem of what exactness makes sense, that is
a much harder problem. I think some discussion of
the relevant issues, (preferably informed by some
knowledge of the application), is the best course.

20. ### GenomeGuest

Hmmmmm, if I can remember it right.

I Barry
You Barry
We Barry
He Barries
She Barries
We Barry
They Barry

Looks good and regular to me.