Hi, I noticed a strange behavior from one of my Uno's yesterday. Apparently it only works after it has a sketch uploaded into it. As long as it stays connected via the usb (i haven't tried powering it through a battery at the same time) it works well as it should.
However, as soon as i disconnect it (so it powers off), then it stops doing anything useful i can understand.
Any ideas on why this is happening? 
Thank you.
Not sure what is happening but people often say this and it always turns out to be something else that they are doing wrong and nothing to do with the arduino board.
kuruki:
Hi, I noticed a strange behavior from one of my Uno's yesterday. Apparently it only works after it has a sketch uploaded into it. As long as it stays connected via the usb (i haven't tried powering it through a battery at the same time) it works well as it should.
However, as soon as i disconnect it (so it powers off), then it stops doing anything useful i can understand.Any ideas on why this is happening?
Thank you.
So you power it off by unplugging it from USB and you somehow expect the board to operate without any power?
retrolefty:
So you power it off by unplugging it from USB and you somehow expect the board to operate without any power?
..... 
Grumpy_Mike:
Not sure what is happening but people often say this and it always turns out to be something else that they are doing wrong and nothing to do with the arduino board.
Even simple sketches like "Blink" or simple print outs don't work. I first noticed it when trying to make two nrf24l01 antennas to communicate and i would only get signal in my receiver just after uploading the sketch in the transmitter and never after having unplugged and plugged the power (usb cable in our case).
What happens when you have a running sketch and then you press the reset button?
Grumpy_Mike:
What happens when you have a running sketch and then you press the reset button?
just restarts as it should. While pressed does nothing. (i think that's what it's supposed to do)
The problem still remains... After unplugging and powering up again it seems like it "forgets" the sketch i have uploaded...
The problem still remains... After unplugging and powering up again it seems like it "forgets" the sketch i have uploaded
Well no it doesn't as I said before. The program runs from flash memory and that is non volatile.
Is it a "real" Arduino of a ripoff clone?
I have an Uno R2 with a similar affliction. Upload sketch, works fine, disconnect, reconnect and it's the bootloader blink that starts running. I changed processors in an attempt to fix it. After posting on here one of our mods drew my attention to a post detailing a modification to the board necessary to correct this.
However, I don't know if the OP has an R2 or not!
dannable:
I have an Uno R2 with a similar affliction. Upload sketch, works fine, disconnect, reconnect and it's the bootloader blink that starts running. I changed processors in an attempt to fix it. After posting on here one of our mods drew my attention to a post detailing a modification to the board necessary to correct this.However, I don't know if the OP has an R2 or not!
R3 here. I am thinking of replacing the atmega chip and trying again with the different one.
Grumpy_Mike:
Is it a "real" Arduino of a ripoff clone?
i'd call it a clone, which was working well so far whatsoever...