Power related question from an ultra noob

Hello everyone,

fisrt project, first post.

I made this little arduino project where the arduino convert the distance from a sonar into a byte and converts it to an artnet value (dmx over ethernet). Works wonder but something strange and scary happened: when we ended the tests I disconnected the power supply (9v 650mA as recommended) and my colleague asked I connect the box again to show the result to someone, I did but the power supply light started breathing. I disconnected everything fearing something wrong might be happening. This morning I re-connected it using the same power supply but with the arduino out of the box, sure enough the power supply light started breathing again, same for RX, TX, L and ON on the arduino. Disconnected again, connected using the USB cable, ready to send my sketch again, everything was fine, no breathing, nothing, everything worked as expected. Then I connected using the power supply again and everything was fine.

I am pretty scared of using that during a show because that would screw up the event as the sensor will be high up in the air and won't be connected to a computer. If it does that someone has to use a lift to reach the box then get the arduino out, connect using USB then try wwith the power supply again...

I can't believe I am the first one to experience this, anyone has an idea about what happenned and how to prevent it?

arduino leonardo
ethernet shield
9v 650mA power supply

thanks to those who will help!

A link or picture for the supply would be useful.
What do you mean by "breathing"

Sounds like the on board regulator is shutting down. Running 500mA through it with a 9V might be an issue.

I don't know what the sonar module draws but the Ethernet shield is on the order of 200-250mA. Plus there is already a bunch of heat generated by the Ethernet shield.

If you have USB and 9V connected, it is very possible your board is actually being powered by USB which provides up to 500mA and doesn't using the on board regulator.

If you have a multimeter, measure the Vin and 5V of the Arduino when connected with external supply. Look to see if the supply is sagging or if the 5v regulator is shutting down.

Also make sure your supply puts out 9V DC and not 9Vac.