Program fails when switching from USB to DC power

I put together a light dimmer that works just as I want when I have my notebook's USB cable plugged in to power my Arduino Uno but, when I unplug it and plug in my DC power module, the onboard LEDs light up but the program does not run. It seems that, during some of my attempts, the program did run so I'm not sure if it's intermittent or not. Pressing the reset button does not start it up.

It's my understanding that DC input voltage can be up to +12V DC and the two power modules I tried ate 7.5V and 5V. Also, the only data variable I declare outside startup() and loop() (sorry, forgot the actual function names), is 'int offon;'. I can measure the voltage on the board and all seems well.

What am I missing? I may be misunderstanding how the code is saved in non-volatile memory.

icantthinkofone: Understanding that DC input voltage can be up to +12V DC and the two power modules I tried ate 7.5V and 5V.

Well, there's your problem. Up to 12V, but a minimum of 6 with a real minimum of 7.5V for reliable operation.

If your 7.5volt supply isn't working, it may be that it cannot provide enough current.

If have a regulated 5volt supply, connect it directly to the 5V pin. Just don't leave it connected when you also have USB connected.

Thank you. 9V did the trick for me and I have 12V module if I need it.