From: nojunk@knology.net
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
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