Arduino R2 UNO Losing programming after disconnecting from PC

Hey all, I have an old R2 UNO which I've written a simple sketch for a tinkerkit DMX shield using examples from here, when I upload my sketch or the examples they work perfectly as expected however when I unplug the UNO from my laptop and power it back up it does not output DMX.

To try and diagnose this I have flashed the Blink example onto the UNO and that works perfectly after power cycle.

With my sketch loaded and the Tinkerkit DMX Shield removed I can see that the "L" Led flashed quickly for a second after powerup I don't know what that means however with the shield in place it doesn't appear to flash at all.

If anyone has any idea as to why this is happening I'd be most grateful.


Since the blink sketch is retained, you can be sure that there is nothing wrong with the Arduino.

That your sketch demonstrates some issues, there must be something wrong with it.

I polished up my crystal ball, but all I see is an only I Love Lucy rerun. It's in color, though. Weird.

The built in LED is flashed by the Uno bootloader. If it is not flashing it could mean that the bootloader is not running. That could be because the processor is not getting power or is being held in reset.

Thanks for your reply, why do you suppose the bootloader isn’t running when the shield is connected but runs fine without it?

I’ve been using Arduino for a few years now and never come across this before.

Break out your DMM and check that the processor is getting power (Vcc around 5V) and that the reset pin is held high (not in reset).

Thanks for that, VCC is 5v and reset is also 5v

I have discovered that if I power on the Arduino with shield and hit reset it powers up properly running the program.

Very odd