From: nojunk@knology.net   
      
   On 2/12/2020 7:33 AM, 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 realize 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.   
      
   So know that we know the resistance in the light is 5k, we know what   
   voltage different values of series resistor will cause.   
   This it the circuit, VCC--resistor--ldr--grd   
      If you use a 5K series resistor the Vcc will be split in half, (in   
   the dark). Let's assume 9v Vcc, so 4.5v. If you raise the resistor value   
   to 10k ohms, the voltage will drop to 3.3v, 15k and the voltage will   
   drop to 2.25k, 20k and the voltage will drop to 1.8v. if you use 100k   
   the voltage will drop to 0.43v.   
     When you go to dark the ldr resistance will increase and these   
   voltages i just mentioned will increase.   
     I suggest you build that ldr resistor circuit and measure the voltages   
   in light and dark. You should see about 8.9V and 0.5v. Remember the   
   light needs to be the same as when you measured the ldr with you meter.   
      
   Here is a voltage divider calculator to show you how the differing   
   resistances will change the voltage. Put in your Vcc. Set R1 value as   
   your whatever your series resistor is, and R2 as the resistance of your   
   ldr, (in this case 5k ohms in the light.) Change values to see how it works.   
     I'm sorry if this all takes you the long route to making the thing   
   work, but I think you should understand how it works so you can optimize   
   to your lighting conditions.   
                                 Mikek   
                                 Mikek   
      
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