Clocks are one of the main themes in "Back to the Future" - and also some of my favorite items for building replica props. Previously, I built three of those clocks, the one in the Time Circuits, the Harold Lloyd clock (as seen in the beginning of part I in Doc's house), one based on a photo featured in part III.
Part I of the movie also features another important clock which is part of the "Twin Pines Mall" sign (which, after Marty changed the timeline, became the "Lone Pine Mall").
I had this idea quite some time ago, and the electronic parts all at hand, when a posting of another build in the "Back to the Future Forever" facebook group inspired me to get this started.
Electronics (I)
The electronics is built around an Arduino Nano.
The time is displayed on a 8x32 LED array, based on the MAX7219 driver IC - this looks rather close to the display in the original movie prop. The differences are that in the original movie prop the colon is not exactly aligned with the grid of the dot matrix, and the "AM"/"PM" displays use a smaller dot matrix). But for my purposes this is close enough.
For the real time clock (RTC), I am using the module DS3231 which I also used previously in the Time Circuits. This clock is very precise and it runs for years with a CR2032 battery. There is, however, one problem with this modul, related to the charging circuit which is supposed to be able to charge a rechargeable battery. As discussed in detail on this web page, the charging circuit would do damage to both a rechargeable battery or a non-rechargeable battery. So, the suggested solution is to simply remove a resistor (as indicated in the image) and to operate the module with a (non-rechargeable) CR2032 battery - and that's what will I do.
I also add a light dependent resistor (LDR) to the circuit, so the clock can adjust the LED intensity based on the surrounding brightness.
My test-setup (using an Arduino Uno) works fine.
Enclosure
The LED matrix defines the size of the unit - the enclosure is scaled accordingly to 5 1/8" x 21". The outer part of the enclosure is built like a picture frame using 3/4" thick and 3 1/2" wide pine boards, cut with a miter saw, glued, and sanded.
The additional pieces are cut with a laser cutter from 3/16" plywood and clear acrylics.
For the first time, I used the program inkscape, so I could cut the letters for "Twin Pines Mall".
Everything is painted using acrylic paints.
This is the enclosure.
Electronics (II)
The electronics is soldered on a 6cm x 8cm PCB. First comes a sketch...
... and then the soldering.
Everything is connected...
... and inserted into the enclosure.
The Final Unit
Here it is: my "Twin Pines Mall" clock (with two Lego figures as a reference).
