From: nojunk@knology.net   
      
   On 2/1/2020 10:56 AM, Ralph Mowery wrote:   
   > In article , rob@despammer.com says...   
   >>   
   >> No, I'm still here reading the posts.   
   >>   
   >> As I am a hobbyist with electronics and use youtube for most of my   
   >> answers, I am very interested in why some posters says it doesn't or   
   >> shouldn't work. I know that you can get a flashing led with just a 555,   
   >> but I'm completely ignorant whether an op amp is also needed or not as   
   >> the case may be.   
   >> I'd do some reading up on the 741 and watch some videos about it.   
   >>   
   >> I see the point of what some people say about the author not having a   
   >> clue etc, as I am very aware that that is also the case with other   
   >> topics on youtube.   
   >>   
   >> I only posted my question because the buzzer didn't work originally in   
   >> the way it was postioned on the schematic.   
   >> I know that it does work now, but I am also looking at or for other   
   >> similarish circuits.   
   >> I have found something on the hobby-circuits site which I'll put   
   >> together when I have all the parts.   
   >>   
   >>   
   >   
   > The way it is suppose to work is the 741 is set up as a voltage   
   > comparater.  When the voltage on one of the input pins is larger or   
   > smaller than the other, the 741 output changes from low to high.  The   
   > 741 is not a good device to use and a few more resistors should be used   
   > to get it to work consistnatly.  The transistor seems to invert the   
   > signal and drive the led.  Then the 555 gets triggered to send an output   
   > voltage to the buzzer for a period of time.   
   >   
   > The 741 is usually an amplifier and there are dedicated comparators that   
   > work much better.   
   >   
      
     I have not put any thought into the 741, I concentrated on getting the   
   555 to work properly first.   
     The OP pointed out the the 741 and transistor did not pull pin 2 low   
   enough to trigger the 555. So I don't doubt some 741 problem.   
     I wonder if it's possible to trigger the 555 directly from the   
   resistor/ldr connection and skip the 741/transistor circuit all together.   
     Hey ROBH,   
     When you light the ldr, what does the voltage drop to? (at 100k at ldr   
   connection)   
   You might need to keep adding 100k resistors in series with you 100k pot   
   until you get it to drop to just under 3 volts.   
     This might not work after you connect it to pin 2, but it is worth a try.   
   If it doesn't work at least you will know why, even if we have to tell   
   you. IF you do this give us light and no light voltages and then the   
   same when connected to pin 2 of the 555.   
                                  Mikek   
      
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