From: nojunk@knology.net   
      
   On 2/2/2020 11:08 AM, RobH wrote:   
   > On 02/02/2020 00:49, amdx wrote:   
   >> On 2/1/2020 1:01 PM, RobH wrote:   
   >>> On 01/02/2020 17:28, amdx wrote:   
   >>>> 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   
   >>>>   
   >>>>   
   >>>>   
   >>> Measuring the voltage between the 1ook pot and the ldr was stationary   
   >>> at just under 8v, even when the ldr was shaded.   
   >> Something wrong there! Unhook the 742 pin 2 from the connection and   
   >> put your meter at the connection. It should change voltage when it is   
   >> dark.   
   >>   What meter are you using?   
   >>   You could put two more 100k in series with the 100k pot. that will   
   >> lower the 8v, but also allow a larger swing between light and dark.   
   >>   
   >>   
   >>   The voltage at the R2 210   
   >>> resistor was 6.75v with no shading and 5.98v with shading.   
   >>> I have also found that the 10uf capacitor between pins 1 and 2 on the   
   >>> 555 does nothing.   
   >>   
   >>   If you will recall, there should not be any connection of the 10uf   
   >> to pin 2.   
   >> At this point I need a drawing of the 555 circuit you are using. Post   
   >> it where ever you can and send me the link. Once I see it, I will post   
   >> any corrections needed.   
   >>                       Mikek   
   >>   
   >   
   > There is a photo I took here:   
   > https://www.dropbox.com/h?preview=IMG_20200202_170044.jpg   
      
     I don't know why, but all I can pull up from the link are my own files.   
   Anyone know how I can view his picture?   
                                    Mikek   
      
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