From: nojunk@knology.net   
      
   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.   
      
                                             Mikek   
      
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