Changing photo resistor resistance range.

[Rich Grise]

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

and solve for R1 and R2?

Cheers!
Rich