when using an ldr, make sure that it doesn't get wet!
i had some ldr's (different types) in use in outdoor units. they worked
well for about 1-2years, but suddenly, they started to indicate bright
sun, as soon as it started to rain. seems that the transparent plastic
cases of these units weren't best quality!
tino

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>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of William Bross
>Sent: Monday, June 06, 2005 2:43 PM
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Light level measurement with LDR
>
>
>Peter:
>
>I've been using just the LDR and a pull up resistor for years 
>with great 
>results. 
>1. Set your PIC pin for digital input.
>2. Look up the light and dark resistances of the LDR.
>3. Size the pull up to make sure that the LDRs output swings 
>well above 
>and below the hi threshold level of the PIC pin.
>4 Add a little hysteresis code to keep it in one state or the 
>other long 
>enough to keep it from 'flashing'.  I typically use about 30 seconds.
>
>Can't get any more dirt bag or cheaper than this.
>
>Bill
>
>Peter Onion wrote:
>
>>Folks,
>>
>>I want to add a light level sensor to a clock I'm building so that at 
>>night the back light is dimmed.  ( I could use the time of day, but 
>>sunrise/sun set calculations look a bit hard for a PIC (lots 
>of trig)).
>>
>>Has anyone got any simple circuit ideas to get a "reasonably linear" 
>>voltage from a typical LDR that has a log/log characteristic ?
>>
>>The **ideal** would be a linear output with Night = 0v and 
>Day = 5v to 
>>feed into a ADC on the PIC which will then program the PWM output to 
>>dim the back light.  In practise something that gives a reasonable 
>>swing over the dawn/dusk periods will do the trick.
>>
>>Peter
>>
>>
>>  
>>
>
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