If i am connecting arduino with HX711 weight sensor, servo, ds3231 and some other things does this matter where am i giving them 5V or VCC from ? i mean i can give it to them from different sources but the ground of all the sources need to be connected right ?
will it hurt my HX711 or will it manipulate the performance of it if i use VCC from another 5v source and just connect the ground with arduino?
Can you post a proposed circuit diagram? That way we have something to look at and not guess at what you're asking...
Mark248:
some other things .... the ground of all the sources need to be connected right ?
Not if one of those "some other things" is one of those opto-isolatable relay cards, the one with the JD-Vcc (also sometimes printed as RY-Vcc) jumpers and you take the jumper off. Then to get isolation, you must specifically NOT connect the Arduino ground to the relay card ground. The ground of the relay card's external supply goes to the realy card ground, but not to the Arduino.
So when you say:
Mark248:
some other things
... the devil is, as always, in the detail.
Basically if you have two power sources you must connect the grounds together.
See:- Power Supplies
wolframore:
Can you post a proposed circuit diagram? That way we have something to look at and not guess at what you're asking...
Yes, and in fact, if you read the following, it would vastly help you, to help us, to help you!
http://forum.arduino.cc/index.php?topic=149015.0
Grumpy_Mike:
Basically if you have two power sources you must connect the grounds together.
Not if the power sources are for an Arduino and for a relay module which need to be optically isolated.
for example in this circuit i have made a relay circuit and the ground is common with opto-coupler and arduino
Where is says "Arduino Ground", that needs to also be connected to the ground/minus on the separate 5V supply.
Or, even better, the ground on the input side of the opto-isolator should be connected to the Arduino Ground, and the ground on the Output side of the isolator, needs to be connected to the ground on the separate 5V supply.
Both will "work", but the latter will preserve the isolation provided by the opto-isolator.
In other words, in the later scenario, the two grounds are NOT connected together, as shown on the schematic.
ReverseEMF:
Where is says "Arduino Ground", that needs to also be connected to the ground/minus on the separate 5V supply.
But i am taking 12V supply and using regulator to convert it to 5,7,3.3 volt which means essentially all the voltages have same ground even if i connect that ground directly to the 12V input ground what difference will it make?
Mark248:
But i am taking 12V supply and using regulator to convert it to 5,7,3.3 volt which means essentially all the voltages have same ground even if i connect that ground directly to the 12V input ground what difference will it make?
Doh! You caught me 
Please see my updated post 
That pic is way too pale and small for my eyes.
If you want isolation, you need an external supply for the realy coil. Then the opto-coupler circuit is from Arduino 5V, through the led and resistor inside the opto, and out to the low of an Arduino digital output. If you connect Arduino and relay board grounds, you circumvent the opto-isolation.
Mark248:
But i am taking 12V supply and using regulator to convert it to 5,7,3.3 volt which means essentially all the voltages have same ground even if i connect that ground directly to the 12V input ground what difference will it make?
Wait...what?!? I don't see that on the schematic. And, I will need to see that on a schematic to know what you're talking about.
neiklot:
That pic is way too pale and small for my eyes.
I know, huh? What I do is right-click on the image, and select "View Image", then hold the Crtl key down, while I spin the mouse wheel. That will enlarge the image. Sometimes that doesn't work, if the image is, either badly compression distorted, or so small, that only "jaggies" are enlarged -- etc.
Of course this assumes you're working on a Desktop Windows PC. For anything else, I don't have a clue
That is because my whole schematic is very messy, but let me be clear, i have a 12V power supply input source from which i am using different LM317 voltage regulator to output 5,7,3.3 V which is what i need in my circuit for sensors,LCD, and stuff.
All of the grounds are connected with each other as i have this understanding that all grounds should be connected together.
BUT now that you are telling me that there needs to be ground isolation for this relay circuit too and i dont know what to do anymore with the relay circuit but i guess other things like HX711, ds3231 and all will work fine, dont they ?
Only if you need the isolation... depends on what you're using for that heater... is it also 12V DC? It's just strange when people use optoisolaters and not isolate the power supplies.
Mark248:
BUT now that you are telling me that there needs to be ground isolation for this relay circuit too and i dont know what to do anymore with the relay circuit but i guess other things like HX711, ds3231 and all will work fine, dont they ?
I'm not saying there needs to be ground isolation. I just inferring from the fact that you're using an opto-isolator, that you want isolation -- that's generally the reason for using an isolator [not always, but usually].
So, what is your reason for using an opto-isolator?
That is the problem, heater is 220V and there is only one 220V AC thing in my project and that is that heater all the other things are DC connected to one 12V input converted into different voltages.
wolframore:
Only if you need the isolation...
Which is where I came in in #2 when I was speculating that one "some other things" might be a relay requiring isolation.
Well, at this point I'm gonna have to say, I need a schematic to really feel confident in advising you. As it is, it's too nebulous to know for sure if my advice would be relevant, or even safe.
ReverseEMF:
I'm not saying there needs to be ground isolation. I just inferring from the fact that you're using an opto-isolator, that you want isolation -- that's generally the reason for using an isolator [not always, but usually].So, what is your reason for using an opto-isolator?
Yes you are right i need isolation because i don't want Relay coil EMF or whatever coming back to my circuit, basically i just wanted to copy the Relay module of arduino onto my PCB, can you tell me the solution in easy words how can i do this ?
And can you confirm me about me theory that all the DC components which maybe runs on different voltages but the Ground should be connected Like for example,
I have one atmega2560 running on 5V,
Servo 996R on 7 Volts,
LCD on 3.3V
and all of these voltages are basically regulated from a single 12V 20A supply,
I have checked Servo and it worked fine because of the Ground attached but should i check everything else too?