I am trying to make an automatic garden gate unlock that runs on a timer. I plan on running an arduino uno 24/7 with a software timer that activates a motor to release a bolt lock. I am going to fit all the electronics and extension cord into a waterproof junction box. Will this cause over heating? Is there a need for some kind of waterproof cooling system? Has anyone ever built outdoor electronics and ran into this problem?
The linear regulator on an Arduino works by converting excess voltage to heat. If you power it with 12V and draw a full amp from it it will turn (12-5)*1a or 7 watts of power to heat, and quickly overheat even if it's not in a box.
If you power it from 6.5V and only draw 300mA from it, it will turn (6.5-5)*.3, or a little under half-a-watt. While the Arduino is idling it will probably only draw 50mA, so less than 1/10th of a watt of heat.
For infrequent, short bursts of a few seconds you can probably get away with drawing 500mA of power without overheating.
Or skip the internal regulator and use a buck style voltage regulator. Those generate a LOT less heat.
For a project I did recently, I used an external 5V regulated supply for everything. I cut a USB cable and soldered the red and black (+5V and ground) to the 5V supply, and plugged that into the Arudino's USB connector. That feeds 5V into the Arduino.