Re: Changing photo resistor resistance range.

"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:0ifi61d12qfbpm9vslp6rqr175lh4cuuh2@xxxxxxxxxx
> On Fri, 22 Apr 2005 10:27:02 -0700, "Larry Brasfield"
> <donotspam_larry_brasfield@xxxxxxxxxxx> wrote:
>
>>"Chris W" <1qazse4@xxxxxxx> wrote in message news:Siaae.375$XA2.65@xxxxxxxxxxxxx
>>>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@xxxxxxxxxxx
Above views may belong only to me.


.

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