Possible over-current issue in Arduino transistor circuit?
You would be better off using a logic-level MOSFET for the transistor. Even a small SOT-23 part such as the AO3400A can switch several amperes. The 1N400x diode is fine.

The LM7805 will also get quite hot, and needs a heatsink. A good switching 5V supply would be better- run cooler and waste less energy.