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Voltage divider with LDR - Resistor help needed please

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picitup

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Hi Guys,

First post, what a great forum you have here :) I need some help please.

I want to use a PIC to record light levels. I have a standard Light dependent Resistor in a voltage divider arrangement, LDR has resistance between 150k when dark, 50 when very light. LDR goes to 5v, R1 to gnd, PIC to centre of both. Using 5v as reference.

I've done a fair bit of reading and played with some values for R1. For standard purposes, anything from 10k to 80k gives a good range of voltages for dark and light, whilst limiting current.

My problem is that I am interested in the lighter end of the scale, where the LDR is 50 to 200 ohms, I'm not bothered about 'the dark side' after about 1k5. I could just use R1 of 200, even 400 gives a voltage swing of over 1.1v which is enough, but I really want to reduce current as I want to run it from a battery for days or weeks.

Please can you advise me of options available to keep accuracy higher, whilst current drain lower. Do I add resistors to both sides of the divider? Do I need to involve transistors? Do I need to get cleverer with switching the voltage divider on only when I need it.

Many Thanks
 
Use the higher value resistor and an opamp to amplify the signal. You could also poll he LDR instead of running it constantly, only turning in on briefly during the actual sample period, and keep it short.
 
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