From: rob@despammer.com   
      
   On 12/02/2020 15:20, amdx wrote:   
   > 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 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   
      
      
   There was no page which you referred to in the previous that I could see   
   any link to, and there is no link to a voalage divider page if that is   
   what you mean.   
      
   All I want to do now is add a buzzer which will work when the led comes on.   
      
   Thanks   
      
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