monitor blink black and then return the image
I do not know what could cause that. I am also concerned that the pc could be damaged.
Search your circuit for any possible short circuit. It may be that a short circuit could be causing to much current to be drawn from the usb port.
If I connect the power to the strip, and than connect the ESP to the PC, monitor blink black and then return the image.
Connecting a PC to an Arduino while it is running will reset the Arduino. Best not to connect anything once it is powered up.
PaulRB:
I do not know what could cause that. I am also concerned that the pc could be damaged.Search your circuit for any possible short circuit. It may be that a short circuit could be causing to much current to be drawn from the usb port.
there is no short on the circuit but probably I'm sharing 5V via usb from the esp and the pc.
how can I "save my PC"?
Please be specific. Which monitor "blinks black" when you plug what into what and what does it show when it comes back?
it's incredible...
I have a power strip. on this power strip is connected the monitor, the PC and the 5V power supply I use to power my leds.
now I have the PC, the monitor, the 5V power supply connected to the power strip.
5v power supply is disconnected from the arduino project I posted in the picture in the previous post.
ESP is not connected to the PC usb.
now if I connect the 5V power supply to the 2 pin connector of my arduino project,
MONITORS BLINK 
how can it be possible? it blink even if the usb is disconnected as soon as I connect the 5v power supply to the 2 pin connector
than if I connect the PC USB to the esp the PC monitor blinks again.
than the strip works correctly.
what can it be?
I add a question
Is is safe to power LEDs from an external power supply and power the esp via pc usb to use serial communication with the microcontroller?
Is this normal or we need some protection like usb isolation?
Yes, that is safe. Be sure to connect the GNDs together.
CrossRoads:
Yes, that is safe. Be sure to connect the GNDs together.
Thanks for the answer ...
But if it's safe why monitor blink when I connect the esp to the usb? 
But if it's safe why monitor blink when I connect the esp to the usb?
Once you tell us exactly what you mean by monitor blink we would stand a chance of telling you. But as far as I am concerned your explanations are not very good.
Grumpy_Mike:
Once you tell us exactly what you mean by monitor blink we would stand a chance of telling you. But as far as I am concerned your explanations are not very good.
Pc monitor goes black for a second and then return to normal.
That's all but it's a clear sign of something not working correctly. Am I wrong?
If I disconnect the 5v leds power supply I don't see any monitor blinking
Pc monitor goes black for a second
So is the monitor connected to a mains input that includes a ground connection? Have you checked that the ground in your home wiring is actually bonded to the real ground?
You could have some faulty wiring causing a problem with your earth leakage currents.
Grumpy_Mike:
So is the monitor connected to a mains input that includes a ground connection? Have you checked that the ground in your home wiring is actually bonded to the real ground?You could have some faulty wiring causing a problem with your earth leakage currents.
Yes monitor is connected to mains with good ground.
Real ground.
Then you are wrong about something, how did you check this? Have you had the monitor PAT tested recently?
As your computer does not reset there would seem to be nothing wrong with that and no danger to it. But connecting power supplies when things are powered up is a big no no in electronics as I have told you before.
Grumpy_Mike:
Then you are wrong about something, how did you check this? Have you had the monitor PAT tested recently?As your computer does not reset there would seem to be nothing wrong with that and no danger to it. But connecting power supplies when things are powered up is a big no no in electronics as I have told you before.
Home is new and the electrical wirings are new so the ground should be ok.
In Mike I trust so if you say that is ok it's ok but I still don't understand why the monitor goes black for one second.
In any case I will not connect things if pc is powered on anymore.
Thanks
but I still don't understand why the monitor goes black for one second.
The monitor goes black because something is disturbing the monitor circuit. It is not affecting the PC otherwise the PC would reset, as they tend to do at the slightest opportunity.
The problem occurs when a second supply is connected. This second supply is floating with respect to the monitor and so when it is connected the ground of the power supply and the ground on the monitor are brought to the same potential. This causes a very short inrush of current which disturbs the monitor’s operation. The result is that some protection circuit in the monitor shuts it down. Then the inrush stops as the two supplies are now equalised and the monitor comes on.
Just as a test remove the LED strip and just connect the power supplies’ negitave terminal to the rest of the circuit. It should still happen.
I didn’t ask before but I am assuming the monitor is a CRT one.
Grumpy_Mike:
The monitor goes black because something is disturbing the monitor circuit. It is not affecting the PC otherwise the PC would reset, as they tend to do at the slightest opportunity.
The problem occurs when a second supply is connected. This second supply is floating with respect to the monitor and so when it is connected the ground of the power supply and the ground on the monitor are brought to the same potential. This causes a very short inrush of current which disturbs the monitor’s operation. The result is that some protection circuit in the monitor shuts it down. Then the inrush stops as the two supplies are now equalised and the monitor comes on.Just as a test remove the LED strip and just connect the power supplies’ negitave terminal to the rest of the circuit. It should still happen.
I didn’t ask before but I am assuming the monitor is a CRT one.
if I remove the LED strip and just connect the power supply, nothing change, monitor goes black for a second.
the monitor is a new LCD connected via display port to a dedicated gpu
do you think that something like this:
should help preventing this problem and making my computer "more safe"?
is there someone who is using USB Isolator here with an ESP? Does it work well?
The thing is this.
We don't know what you are doing wrong, despite all the back-and-forth questions.
If we can ever fathom what you are doing wrong then we could tell you not to do it, but so far we can't, so we cannot recommend some random device.
So far I don't think we know exactly what your 5 V power supply is, and a detailed, precisely focused photograph of your whole arrangement in bright, uniform light would be most illuminating.
do you think that something like this:
Adafruit USB Isolator - 100mA Isolated Low/Full Speed USB : ID 2107 : $34.95 : Adafruit Industries, Unique & fun DIY electronics and kitsshould help preventing this problem and making my computer "more safe"?
No I imagine the problem will still be there because their is no indication that their is anything to do with the USB connection.
Just power the Arduino separately and try the test again.
I quite agree with Paul__B in what he says.
