From: rob@despammer.com   
      
   On 12/02/2020 13:33, amdx wrote:   
   > On 2/12/2020 6:41 AM, RobH wrote:   
   >> On 12/02/2020 11:28, tabbypurr@gmail.com wrote:   
   >>> On Wednesday, 12 February 2020 10:29:32 UTC, RobH  wrote:   
   >>>   
   >>>> I could not measure the resistance of the ldr correctly as it was   
   >>>> either   
   >>>> off scale on the meter, or a negative resistance, depending on which   
   >>>> way   
   >>>> round I had the positive and common leads on it.   
   >>>>   
   >   
   >   
   >>>> The Vc of the resistor voltage in the dark is 4.8v   
   >>>> The Vc of the resistor voltage in the light is 6.8v   
   >   
   >   
   >   With these measurements is the resistor a 33k ohm?   
   >   
   >   
   >   
   >>>>   
   >>>>   
   >>>> Apologies for any confusion I may have  caused.   
   >>>   
   >>> If you're measuring an LDR as negative resistance, something is   
   >>> seriously wrong with what you're doing.   
   >>>   
   >>   
   >> Fair enough, and I may just give up with it then.   
   >   
   >   No need for that.   
   >   First make sure the ldr is out of the circuit.   
   > We want to measure the resistance of the ldr all by it's self.   
   >   
   >   When you try to measure yours (it may be different than the graph)   
   > You can't use your fingers, the meter will measure you (especially in   
   > the dark)   
   >   Make sure the ldr is dark (zero light can enter) and connect the meter   
   > in the high resistance mode, (some meters need to me set for high   
   > resistance) try measuring a 1M resistor to be sure. Connect to the ldr   
   > using clip leads or whatever you have so you don't have your fingers   
   > involved.   
   >   Record the dark resistance.   
   > Then put your light on it and measure the light resistance.   
   >   record the resistance.   
   >   
   > What are those numbers?   
   >   
   > Here's a page with a graph showing how the resistance of an ldr changes   
   > with the amount of light on it. It's about 1/4 page down.   
   >   
   >    The graph shows one that when dark was 1M ohm of resistance, and when   
   > there is a LOT of light on it, it goes down to about 100 ohms.   
   >   
   >                                   
   Â Â Â Â Â Â Â Â Â  Mikek   
      
   Yes the measurements were taken with the 33k resistor in circuit   
      
   Ok I didn't realise you meant taking the ldr out of the circuit.   
      
   The resistance of the ldr in normal daylight is about 5k ohms, and in   
   darkness , about 32M ohms. If I moved the ldr into a black area, then   
   the meter went off or out of scale.   
      
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