Calculate divider calculation

[Spadez]

Hi!

I want to use an LDR to get a reading on the PIC via A2D converter. Here are my measured resistance levels of the LDR:

Dark: 65K Ohm
Normal light: 3.98K
Very bright: 1.8K

If I want to get a voltage range of 0V through to 5V to the PIC from the LDR, I know I need a voltage divider circuit, but how do I work out the resistance of the static resistor, and is there an online calculator to simply this?

Ive looked but I couldnt find what I needed,

James

 

[MikeMl]

First, without using an opamp, starting with only a single supply voltage of 5V?, there is no way to get a signal that swings from 0V to 5V. You could get maybe 1.5V to 3.5V. Is that enough?

What supply voltages do you have to work with?

 

[Spadez]

Hi Mike.

15V comes onto the board, which is stepped down to 12V, then 5v. It would be the easiest to use the 5V line.

I think a swing from 1.5V to 3.5V would be fine.

 

[ronv]

You won't be able to get 0 to 5 volts but you can center it with a couple of calculations. Try 11k.

 

[Spadez]

11K looks like it will give me a range of 4.29V to 0.72V which is pretty good, with a 5V input voltage. Thank you