Arduino resets with input voltage changes

Development Tools IDE 1.x
1

Hello, In this project that I have setup like this:

12 V, 1A power adapter
Arduino Mega + 3 other devices that uses the same line of power.

When one of the other devices changes from ON to OFF or OFF to ON the Arduino Mega resets itself.

I've used a tester and the input power only changes from 12.1 V to 11.8 V when everything is OFF.

Why is this happening and how can I avoid it? I have to use a separate line of power for the other devices?

Thanks in advance,

2

If you looked at it with a scope, you'd probably see a much bigger drop - it's looking like 11.8 only because your meter isn't fast enough to catch it briefly going (much) lower.

Can you post your schematic or a wiring diagram?

(Also, in the future, a thread like this should go in general electronics, not installation and troubleshooting - this is a power supply problem)