From: rob@despammer.com
On 11/02/2020 15:17, RobH wrote:
> On 11/02/2020 14:04, amdx wrote:
>> 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
>>>
>>>th
>>>>
>>>>
>>>>
>>>> ------
>>>> 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
>
> Mmm, strange that, but yes when the light shines on the ldr.
> I have the led lighting up when light shines on the ldr, but I am not
> sure where to connect the buzzer to. Connecting it to pin 3 on the 555
> it buzzes with a low pitch and the led lights dimly, then when I shine
> light on the ldr the buzzer really buzzes and the led is in full
> brightness.
Oops, wrong circuit above, please ignore as that is the schmitt trigger
circuit you sent me.
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