Homemade Trailer With M105A2 Bed ==> Trailer Electrical Box

This page details my attempts to make an electrical/electronic/utility box for my M105A2 trailer.

The trailer, at this point, is done and runs good and has everything electrical that is required (5 lights and electric brakes). However, it is not yet wired and this box represents three purposes:

Provide electrical connections for lights, brakes, breakaway brake actuator, and 110V smart battery charger for the actuator's battery.
Provide space for gloves, chains etc that can be locked with a padlock.
Provide LED lights that would show connection status and brake status.

For the box, I am using an old Square D box that housed a size 1 contactor or something like that. I cleaned it with a wire brush and painted with "cold galvanizing" coating McMaster item 7725T2, which is zinc based cold galvanizing coating. Then I painted it with one coat of army green (matching camo pattern of the trailer bed). I will apply another coat once I am done wiring the box.

It has a rubber seal and lockable locking handle. It is waterproof (weatherproof).

The box will have two compartments, bottom one for electronics/electricals and the top one, which is just an area for whatever I need to keep there, like chains, tiedowns etc.

Electrical connections will be made by means of a terminal block, for neatness and security.

LED Lights

I will use three LEDs to indicate trailer status.

GREEN LED -- Whether trailer is electrically connected to truck and supplied with power
RED LED -- Whether RIGHT brake is actuated (wire comes from pigtail on axle)
RED LED -- Whether LEFT brake is actuated (wire comes from pigtail on axle)

All LEDs are equipped with 500 ohm, 1/2 watt series resistors.

I may wire the input line through a diode, and run the LED status indicator off the truck side of this diode, so that it does not glow when the trailer is not connected, due to backup battery installed on the trailer.

This way, I can get the trailer's status simply by looking at the rearview mirror, and could see brakes actuation status when I tap on brakes on engage brake controller. That's an extra safety addition and provides some peace of mind.

Truck connectors

This box will provide two connectors:

Standard four blade connector for lights only (under 3,500 lbs GVW only)
7 Blade RV plug with wiring for brake controller

The rationale for it, which reasonable people may disagree with, is to be able to tow light loads with vehicles other than my pickup with its brake controller (to be installed).