Before designing this, I set 4 criteria:

-- The mailbox delivery notifier had to be unobtrusive to both the postal carrier and neighbors--i.e., no wires or visible electronics inside or outside the box vault that could create anxiety. -- The state of the box contents (delivery v. non delivery) needed to be easily detected by me, but not noticeable to the neighbors or street passers-by. -- It needed to be reliable, durable, and easy to reset back to an untriggered state.
-- It needed to be inexpensive and low tech, with an uncomplicated mechanism (preferably no strings, springs, or cables)

PHOTOS COMMENTARY: Underside view of mechanism, with door at right and flag on back of mailbox at left. When the door opens, it swings down on its hinges and pushes in the plunger at far right. This action pivots the pivot rod arm which retracts the flag release latch bar at far left.

You will note there is a short ¼" bolt at the bottom right on the back side of the mailbox, which serves as a stop when resetting the mechanism. To reset the mechanism to an untriggered state, simply reach around the back of the mailbox with one hand and rotate the flag assembly clockwise until the assembly's right arm hits the bolt head, then reach under the mail box door with your other hand and pull the trigger mechanism back into position. The flag release rod easily slips back through the hole in the flag assembly and holds it in place. If you have the mechanism lined up and installed properly, you can reset the mechanism in a second with your eyes closed.

You will need to print the following to construct the assembly:

2 x latch_channel_2.stl (holds door latch and flag release in place, at opposite ends under the mailbox) 1 x door_latch_2.stl (positioned beneath mailbox to detect opening of mailbox door) 1 x flag_release_2.stl (retracts to allow flag to move) 1 x short_link.stl (connects door latch and pivot rod) 1 x pivot_rod.stl (rotates about center to convert door_latch push into flag_release pull) 1 x gear_axle_2.stl, and 1 x gear_axle_bushing_2.stl (holds pivot_rod in position and allows it to rotate) 1 x long_link.stl (you need to customize to length needed to connect the two ends of your individual mailbox--mine was 10.3 inches between the two hole centers. For my own application, I made a temporary link very quickly by drilling ¼" holes into the ends of a 1 x 11" section of ¼" plywood which was in my workshop, but had to replace it soon because it warped in the outdoor humidity.) 1 x flag_2_(1_of_2).stl (print with filament the color of the mailbox) 1 x flag_2_(2_of_2).stl (print with filament color that contrasts to the color of the mailbox) 1 x counterbalance_(1_of_2).stl 1 x counterbalance_(2_of_2).stl (holds 7 pennies to provide the weight which makes flag pivot upright when triggered) 1 x clip_post_for_flag.slt (print horizontally with support for optimal strength) 2 x clip_post_long.stl (used at both ends of the short_link connector) 3 x clip_post_short.slt 7 x eclip_long.slt (2 of these are used on the clip_post_for_flag)

You will also need to provide a ¼" hex bolt that is ½" or so long (or print the "reset bumper") to make the flag stop, and 5 #6 screws to attach the mechanism to the underside of the mailbox.

APRIL 2022 UPDATE:

This design has worked flawlessly since June 2019, except that (not surprisingly) it doesn't work if the mailbox is coated with ice from freezing rain. That being said, when the ice melts no harm has befallen the mechanism because most of the mechanism is attached under the mail box and not exposed to the elements.

I did make incremental improvements to the design in December 2019, all of which are labeled as “v2.” I replaced the square white flag with a pointed flag, because the shape change made it easier to see from a distance on dark winter days. I lengthened the counterbalance barrel so it could hold a few more pennies to compensate for the mass of the larger flag. I also created a bumper bolt that screwed into a hole at the bottom right of the back of the mailbox which created an easy-breezy stop for the flag post to hit against when you were resetting the mechanism to an untriggered position.