From: nojunk@knology.net   
      
   On 2/1/2020 6:49 PM, 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   
   >   
   ROBH, this is exactly the circuit you want.   
   > https://www.electronics-tutorials.ws/wp-content/uploads/2018/0   
   /waveforms-tim38.gif   
   C1 is your 10uF, R1 is 100k resistor that will give you a 1 second buzz,   
   200k will be a 2 second buzz. R2 can be prety much anything but use 10k.   
   Pin 3 is your buzzer. For the switch if you don't have one, just use a   
   wire to momentarily ground pin two. If after you set this up it doesn't   
   work, take a picture and post it somewhere or send it to my email address.   
     Let me know, I want to get something working.   
                                            Mikek   
      
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