Changing photo resistor resistance range.

[Chris W]

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

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
http://thewishzone.com

 

[Larry Brasfield]

"Chris W" <1qazse4@cox.net> wrote in message news:Siaae.375$XA2.65@lakeread07...

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.

What's wrong with the following?

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

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[John Fields]

On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
<donotspam_larry_brasfield@hotmail.com> wrote:

"Chris W" <1qazse4@cox.net> wrote in message news:Siaae.375$XA2.65@lakeread07...
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.


What's wrong with the following?

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

---

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.

--
John Fields
Professional Circuit Designer

 

[John Larkin]

On Fri, 22 Apr 2005 13:31:20 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"Chris W" <1qazse4@cox.net> wrote in message news:Siaae.375$XA2.65@lakeread07...
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.


What's wrong with the following?

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



---

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.

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

John

 

[John Fields]

On Fri, 22 Apr 2005 12:06:28 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 22 Apr 2005 13:31:20 -0500, John Fields
jfields@austininstruments.com> wrote:

On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"Chris W" <1qazse4@cox.net> wrote in message news:Siaae.375$XA2.65@lakeread07...
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.


What's wrong with the following?

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



---

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.



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

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

I don't know about you, but for me, "about" is about 5 to 10% off of
nominal.

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

But thanks anyway...

--
John Fields
Professional Circuit Designer

 

[John Fields]

On Fri, 22 Apr 2005 20:09:00 GMT, Rich Grise <richgrise@example.net>
wrote:

On Fri, 22 Apr 2005 13:31:20 -0500, John Fields wrote:
On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
"Chris W" <1qazse4@cox.net> wrote in message
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.

What's wrong with the following?

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

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

Well, what if you did a little algebra:

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

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

---
Well, what about if you got your head out of your ass and changed Rt2
to 5k, like the OP asked for, and then actually solved for R1 and R2
instead of just flapping that jaw, as usual?

--
John Fields
Professional Circuit Designer

 

[Endoscope]

"John Fields" <jfields@austininstruments.com> wrote in message
news:0ifi61d12qfbpm9vslp6rqr175lh4cuuh2@4ax.com...

On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"Chris W" <1qazse4@cox.net> wrote in message
news:Siaae.375$XA2.65@lakeread07...
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.


What's wrong with the following?

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



---

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.

--
John Fields
Professional Circuit Designer

To which Barry should reply.

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

Don't hold your breath though.

DNA

 

[Endoscope]

"Endoscope" <looking@intoit.nat> wrote in message
news:SNeae.1360$p06.1085@newsfe3-gui.ntli.net...

"John Fields" <jfields@austininstruments.com> wrote in message
news:0ifi61d12qfbpm9vslp6rqr175lh4cuuh2@4ax.com...
On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"Chris W" <1qazse4@cox.net> wrote in message
news:Siaae.375$XA2.65@lakeread07...
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.


What's wrong with the following?

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



---

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.

--
John Fields
Professional Circuit Designer

To which Barry should reply.

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

Don't hold your breath though.

DNA

Oh Fuck, 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

 

[Larry Brasfield]

"John Fields" <jfields@austininstruments.com> wrote in message
news:0ifi61d12qfbpm9vslp6rqr175lh4cuuh2@4ax.com...

On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"Chris W" <1qazse4@cox.net> wrote in message news:Siaae.375$XA2.65@lakeread07...
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.


What's wrong with the following?

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



---

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.

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?

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Ban]

Larry Brasfield wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:0ifi61d12qfbpm9vslp6rqr175lh4cuuh2@4ax.com...
On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"Chris W" <1qazse4@cox.net> wrote in message
news:Siaae.375$XA2.65@lakeread07...
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.


What's wrong with the following?

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



---

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.


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?

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.


--
ciao Ban
Bordighera, Italy

 

[Larry Brasfield]

"Ban" <bansuri@web.de> wrote in message
news:7onae.814948$b5.36158831@news3.tin.it...

Larry Brasfield wrote:
"John Fields" <jfields@austininstruments.com> wrote in message
news:0ifi61d12qfbpm9vslp6rqr175lh4cuuh2@4ax.com...
On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"Chris W" <1qazse4@cox.net> wrote in message
news:Siaae.375$XA2.65@lakeread07...
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.


What's wrong with the following?

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



---

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.


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?

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.

I suggest you study this table:
http://www.rohm.com/products/shortform/26rstr/rstr_index0.html
Then come back and explain your puzzlement.

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.

I deliberately chose 5% standard values to avoid
unnecessary confusion and low SNR posts here.

Or maybe it is that you are not familiar with standard E12? That would qualify you as a bloody beginner.

Please elaborate on this concept after your review
of the E24 table at the link I provided.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[John Fields]

On Fri, 22 Apr 2005 17:52:12 -0700, "Larry Brasfield"
<donotspam_larry_brasfield@hotmail.com> wrote:

"John Fields" <jfields@austininstruments.com> wrote in message

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


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 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?

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).

---
Even with identical illumination, otherwise identical CdS LDR's will
exibit, typically, a +/- 30% difference in illumination resistance.
---

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.

---
Assume away... You know what that does, no?
---

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?

---
Who knows? Not knowing the application, certainly not I...

--
John Fields
Professional Circuit Designer

 

[Genome]

"John Fields" <jfields@austininstruments.com> wrote in message
news:78tk61t8nch0070brtv6s3dt6q1ovqba0k@4ax.com...

On Fri, 22 Apr 2005 17:52:12 -0700, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"John Fields" <jfields@austininstruments.com> wrote in message

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


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 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?



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).

---
Even with identical illumination, otherwise identical CdS LDR's will
exibit, typically, a +/- 30% difference in illumination resistance.
---

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.

---
Assume away... You know what that does, no?
---

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?

---
Who knows? Not knowing the application, certainly not I...

--
John Fields
Professional Circuit Designer

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

 

[Larry Brasfield]

"Genome" <ilike_spam@yahoo.co.uk> wrote in message
news:JEvae.9491$u5.5522@newsfe2-gui.ntli.net...
....

Barry should have corrected himself properly.

Your "properly" is a private concept.

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

No not Barry.

Your "Barry" shtick is silly.

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.

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.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[John Larkin]

On Sat, 23 Apr 2005 17:32:25 GMT, "Genome" <ilike_spam@yahoo.co.uk>
wrote:

No not Barry.

Should we make an official verb out of that?

John

 

[John Fields]

On Sat, 23 Apr 2005 10:53:59 -0700, "Larry Brasfield"
<donotspam_larry_brasfield@hotmail.com> wrote:

"Genome" <ilike_spam@yahoo.co.uk> wrote in message
news:JEvae.9491$u5.5522@newsfe2-gui.ntli.net...
...
Barry should have corrected himself properly.

Your "properly" is a private concept.

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

No not Barry.

Your "Barry" shtick is silly.

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.

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.

---
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."

--
John Fields
Professional Circuit Designer

 

[Larry Brasfield]

"John Fields" <jfields@austininstruments.com> wrote in message
newsoal619lgmagp0lnheh3rhcb64s8sk8s00@4ax.com...

On Sat, 23 Apr 2005 10:53:59 -0700, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:
"Genome" <ilike_spam@yahoo.co.uk> wrote in message
news:JEvae.9491$u5.5522@newsfe2-gui.ntli.net...
[OT banter cut.]
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.

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.
[OT banter cut.]
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."

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.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Genome]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:e78l615b7taskfsn1pk1j1j01v34vate24@4ax.com...

On Sat, 23 Apr 2005 17:32:25 GMT, "Genome" <ilike_spam@yahoo.co.uk
wrote:



No not Barry.


Should we make an official verb out of that?

John

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.

I'll add,

Stop Barrying about.

Don't know the proper term for that one.....

DNA

 

[John Larkin]

On Sat, 23 Apr 2005 20:28:42 GMT, "Genome" <ilike_spam@yahoo.co.uk>
wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:e78l615b7taskfsn1pk1j1j01v34vate24@4ax.com...
On Sat, 23 Apr 2005 17:32:25 GMT, "Genome" <ilike_spam@yahoo.co.uk
wrote:



No not Barry.


Should we make an official verb out of that?

John



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.

I'll add,

Stop Barrying about.

Don't know the proper term for that one.....

DNA

Where I grew up, we conjugated

I We

You Y'all

He, She, It They.


John

 

[John Fields]

On Fri, 22 Apr 2005 16:13:25 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Fri, 22 Apr 2005 20:09:00 GMT, Rich Grise <richgrise@example.net
wrote:

On Fri, 22 Apr 2005 13:31:20 -0500, John Fields wrote:
On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
"Chris W" <1qazse4@cox.net> wrote in message
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.

What's wrong with the following?

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

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

Well, what if you did a little algebra:

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

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


---
Well, what about if you got your head out of your ass and changed Rt2
to 5k, like the OP asked for, and then actually solved for R1 and R2
instead of just flapping that jaw, as usual?

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

--
John Fields
Professional Circuit Designer