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Planning « Kevin's Projects


My goal with this project is to create a working digital clock styled after the time circuits shown in the movie Back to the Future. If you haven’t seen that movie, you should. Robert Zemekis and Bob Gale created a modern masterpiece back in 1985. It still holds up, almost 20 years later. In fact, you should go watch it. Right now. Seriously, step away from the computer and watch it. I’ll wait.

All right! Fantastic movie, right? I don’t care what anybody says, Huey Lewis rocks. You’ll probably have to watch it again to catch all the little time-based subtleties they put in there (like Lone Pine Mall…), but first I want to turn your attention to the Time Circuits. These were installed in the dashboard of the Delorean, and consisted of LED displays in metal housings with some old-school stick-on labels. They look like this:

Time Circuits

An exemplary specimen of 1980s mad scientist chic.

In this project, I am going to build a clock that looks (roughly) like that. It won’t be exact of course… notice that the time circuits in the movie are three separate displays in a stair-step configuration. The enclosure I’m envisioning will have all three displays flush to one another, so it will look good hanging on a wall.

Here are the design requirements for my Time Circuits Clock:

  • The green row (Present Time) will be a working digital clock, showing the current date and time.
  • The red row (Destination Time) and the amber row (Last Time Departed) will be user-selectable dates/times.
  • The circular LEDs between the hour and minute displays will blink at a rate of 1 Hz.
  • The month displays will be 16-segment alphanumeric displays, and all other displays will be 7-segment numeric displays.
  • The whole thing will run off a single Arduino Uno.
  • It will be enclosed in an aluminum case.
  • It will have stick-on labels.

When I first thought this project up, I sat down and did a little design work and tried to work out the scope of the project. Here’s what I came up with:

Time Circuits Clock Sketch Page

My initial planning for the time circuits clock.

As you can see, I initially thought that I could run each of the 3 month displays off a single MAX7219 IC, but that turned out not to be practical, as you’ll see later.

So the first thing I did was to source the LED displays themselves. I wanted them to be pretty big, so they were easy to read, and I needed them in three colors: red, green, and amber. It is of course getting harder and harder to find 16-digit alphanumeric displays, since most text in devices is now done on an LCD screen instead, and it was even harder to find 16-segment alphanumeric displays the same size and color as the 7-segment displays I could find online. I finally decided that in order to get them to match as closely as I can, I needed to order both types of displays from the same company… so I hit the internet and found a company in China that still makes these displays in the colors and sizes I needed. This company was willing to ship me a set of 16- and 7-segment displays in three colors, so these arrived on my doorstep:


LED Displays

Shipped all the way from China.

The company I bought them from only made common anode displays, so that’s what I got. That decision had some complications down the road, which I’ll get into in another post.

Once I had the displays, I made sure I had correct pin diagrams, and did some quick back-of-the-envelope power calculations:

LED Display Pin Diagrams

The pin diagrams for the dual 7-segment and 16-segment LED displays. The diagrams that came with the parts were incorrect, so I worked them out with a jumper wire and a limited current source. My initial power usage estimates are also seen below; I calculated a weighted average of 5 segments per digit over the range of numbers (0-3 for days or 0-5 for minutes, etc.) used by each display.

Holy amperage, Batman! I don’t think I can use a standard 9V battery to power this thing. Also, I’m anticipating a lot of soldering, since that all adds up to a whopping 366 LEDs, plus ICs and other assorted components.

Should be an interesting build.


Categories: Time Circuits Clock


  1. How do you plan to control all 366 LEDs with the arduino’s 13 pins? I2C? Shift Registers?

    Mark / February 27, 2013 / 6:33 pm
  2. I’ll be farming out individual display control to a bunch of MAX7219 and MAX6969 chips, which speak serial and can be chained together on a bus. At most I should use 6 Arduino pins to run 2 data buses.

    Kevin / February 28, 2013 / 8:55 am
  3. Not 0.8″ real size is 0.56″ dual 7 segment. Present time for part 3 black background green segment, Destination time & Present time have black background & white segment! Month is painted plexi with black spray. Rear side on month plexi is 2 fuse style lamp bulbs! 🙂 Ask me for more info 🙂
    Part 1 & 2 have 0.56″ 7 segment! 🙂

    Tommy / April 16, 2013 / 2:45 pm
  4. Have you managed to resolve the Common Anode issue? Also, don’t the 7219chips need 3 arduino pins each?

    Mark / May 24, 2013 / 3:33 pm