From: nojunk@knology.net   
      
   On 2/10/2020 10:19 AM, amdx wrote:   
   > On 2/10/2020 10:02 AM, Evgeniy wrote:   
   >> Hello, amdx!   
   >>   
   >> On Mon, 10 Feb 2020 09:39:54 -0600   
   >> amdx  wrote:   
   >>   
   >>>>   
   >>>> Ok, I boobed by putting x2 incorrect resistors. Doh!!!   
   >>>> I used x2 47k resistors in error and have changed them to the 10k   
   >>>> resistors.   
   >>>   
   >>>    Those are just a voltage divider, setting the voltage on Pin 2. I   
   >>> suspect you could you could use a resistor value up to 500K and have   
   >>> little change in the voltage on Pin 2.   
   >>>     I found the input resistance of a 741 is 2Mohm, so, I did the   
   >>> calculations, if you use two 10k resistors and the voltage at Pin 2 is   
   >>> 4.5v, changing them to 500k will only shift that voltage by 0.5v.   
   >>> It would raise Pin 2 to 5v.   
   >>>    (Side no, there may be different style 741s, with different input   
   >>> impedance, I don't know.)   
   >>>   
   >>   
   >> It's not a good idea to set huge resistance on high-impedance imput:   
   >> AMP can receive radio noise on input. 10K is enough normal resistance, I   
   >> think. With R1 and R2 47K it should work too, but I do not prefer such   
   >> resistance: too high in high-impedance input.   
   >   
   > All that is possible, 500k is not huge, It would actually be 250k to   
   > ground, but, I was trying to teach him that, he is just setting the   
   > voltage on pin 2 with those resistors. The ratio is important not the   
   > absolute value.   
   >   
   >   
   >   
   >   It looks like that there   
   >> was a problem with badly connected R2, because when leds are off voltage   
   >> on pin 3 should be below ~6V (voltage on point A), but he had:   
   >>>> The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v   
   >>>> approx, and when Hi the voltage is 10.6v, and again does fluctuate upto   
   >>>> 10.9v.   
   >   
   >   I think he is just letting light leak into his ldr giving him that   
   > fluctuation.   
   >                                         
   Mikek   
   >   
   >   
   >>   
   >>   
   >>   
   >> ------   
   >> With Best Regards,   
   >> Evgeniy Shtrenyov   
   >>   
   >   
       Rob, I still wonder,   
     Do you want the the buzzer to buzz when light shines on the ldr?   
     Your previous circuit was opposite.   
                                   Mikek   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)