Building Props for a Back-to-the-Future Time Machine - Part 3: The Speedometer

My other blog posts on BTTF props:
    Part 1: Building a Flux Capacitor
    Part 2: Building the Time Circuits
    Part 4: Building the Analog Gauges
    Part 5: Building the TFC Switch

After I built the Flux Capacitor and the Time Circuits from the DeLorean in "Back to the Future", I just couldn't stop. The next piece was the speedometer that is sitting on the dashboard of the DeLorean, which prominently displays the "88" mph to indicate the time-jump.

My initial plans were to make a simple non-functioning prop, that maybe counts from zero to 88, and after a short break, makes a countdown. Later, however, I could not resist to add a GPS receiver, so the final version is a fully functioning GPS speedometer.

Enclosure

As in my other BTTF-related builts, the case is made of 3/16" plywood with a laser cutter.

The pieces are glued and painted (with Rustoleum, gloss, almond) , and it instantly resembles the original prop.

Electronics

The electronics of the speedometer is built around an Arduino pro mini (168, 16MHz, 5V), two red 0.8" 7-segment LED displays (common anode), two shift registers TPIC6B595 (which can sink 100mA per pin, so no further transistors are needed), a Neo-6M GPS module, plus sixteen 220 Ohm resistors (8 for each display: 7 segments + decimal point).

The images do not show the Neo-6M GPS module which was an afterthought. Later, it was placed in the space on the left on the picture above. While I found many postings on the web of people having trouble, the Neo-6M module works really well for me (although I connected it via soft-serial, which people do not recommend - but I never had problems).

Of course, one needs to add the labels from the DYMO 1540 Office Mate II label maker (which I bought for the Flux Capacitor labels) - these are an essential piece for recognizing this prop. And here it is ...

... and here, working in my car.

But the usage is not restricted to the car - it also works well on my bicycle (with the battery pack placed in the cup holder).

Related

My other blog posts on BTTF-related props and pieces:
   - Building a Flux Capacitor
   - Building the Time Circuits
   - Building the Analog Gauges
   - Building the TFC Switch
   - Building a Hoverboard and Charger
   - Building BTTF clocks
   - Building a BTTF Brick Stage (featuring the smallest Flux Capacitor)

Building Props for a Back-to-the-Future Time Machine - Part 2: Time Circuits

My other blog posts on BTTF props:
    Part 1: Building a Flux Capacitor
    Part 3: Building a Speedometer
    Part 4: Building the Analog Gauges
    Part 5: Building the TFC Switch

My Back-to-the-Future obsession started while I built the Flux Capacitor. After finishing this, I knew that I also wanted to build some of the other props that are featured in the DeLorean time machine. The second most interesting prop were the Time Circuits. After a little research, I was somehow hesitant to start this, as the electronics side would become rather complex, plus some of the pieces would be hard to find. The reason for this are the alphanumeric (16-segment) LED displays on the left that display the months. Many helpful details and complexities on this issue have been investigated and documented by a fellow builder.

Then I saw the blog at the Adafruit page, where they simply use 7-segment displays for all the fields. Furthermore, the electronics setup is really easy, as they are using nine combined 4-digit 7-segment displays. Initially, I found that the Adafruit built looks a little lame, but then I concluded that much of this is due to their oversimplified enclosure. I thought that by applying my recent laser-cutter skills, I would be able to build Time Circuits, much closer to the real thing.

Electronics

I started with the electronics: From Aliexpress, I got a DS3231 RTC clock module, nine 0.56" 4-digit 7-segment LED displays, a DFPlayer Mini (plus 128MB micro SD card), and an Arduino Mega. The nine LED displays are slightly different from those used in the Adafruit blog, which are I2C based. My displays are based on the TM1637 chip, so there are no problems due to limited I2C addresses. 

There are many references on the web for a bug in the DS3231 RTC clock module (it is usually operated with non-rechargeable battery, which can be damaged by the charging circuit on the module). I used the solution described on this page and unsoldered resistor R5 on the board. It's been running fine since more than a year now with the original battery.

For the keypad, I used a cheap 12 button keypad (yes, the original prop has only 10 buttons). This is connected to an Arduino Mega analog input with a single wire, using the solution described in my previous blog.

Enclosure

Just like my Flux Capacitor, the enclosures for the Time Circuits and my other Time Machine props are built from 3/16" plywood cut with a laser cutter. The enclosure for the Time Circuits is roughly based on the size of the original prop, but slightly adjusted to fit my LED modules.

In contrast to the "flat" Adafruit version, the three rows are stacked as in the original. In between the rows, I left a tiny space (1/16") which adds a lot to the appearance.

The labels were made in scribus (open source desktop publishing software) using the Microgramma Bold Extended font, and printed on glossy 4"x6" photo paper.

Car Holder

At this point I have to admit that I do not own a DeLorean. My Time Machine props will have to operate in a Toyota Camry (I know: no style at all). After many iterations, starting with cardboard templates and various plywood versions, I came up with this shape.

This nicely fits the space above the radio and holds well due to friction.

Functions

The middle row always displays the current date and time. The top and bottom rows display dates and times that appear in the movies, or birthdates of the actors. After a random time interval (small number of minutes) to top row is copied into the bottom row, and a new random date is diplayed in the top row. The user can enter data via the keypad (which produces the DTMF tones), and there are different options:

• Entering a single digit number plays a song from the movie ("Power of Love", "Back in Time", "Earth Angel", "Johnny B Goode", the ZZ Top song, or some orchestral pieces)
• Double digit numbers are not yet used - in a future update, I plan to use these to play some of my and my family's favourite songs.
• Four-digit numbers are interpreted as years. When a four-digit number is entered, and new date/time entry appears in the top row, where the month/day and time is taken from the previous entry. And the previous entry is copied into the bottom row.
• Eight digit numbers are interpreted as month/day/year. The rest is like in the previous case.
• Twelve-digit numbers are interpreted as month/day/year/hour/minute and displayed in the top row.

If an entry is not valid (invalid number of keys, or invalid values), a buzzing sound is played.

A wonderful detail is that my code for reading the clock includes a correction for Daylight Savings Time. This makes it the only clock in our household that I don't have to adjust by hand, twice a year.

Related

My other blog posts on BTTF-related props and pieces:
   - Building a Flux Capacitor
   - Building a Speedometer
   - Building the Analog Gauges
   - Building the TFC Switch
   - Building a Hoverboard and Charger
   - Building BTTF clocks
   - Building a BTTF Brick Stage (featuring the smallest Flux Capacitor)