As a huge fan of the EEVblog I came around EEVblog #513 - Mailbag several months ago. In this episode Dave get's an awesome time circuits clock out of the movie Back to the Future from the guys at Shackspace, Stuttgart, near my hometown Tübingen. They documented everything on GitHub, so check out their awesome project.
So I wanted to have also one of these and started designing my own version. I did a few different design decision compared to their ansatz:
- I wanted to use a embedded linux system to the drive to clock, so that I can easily control everything over WiFi. I went for an Raspberry Pi 2 as they're quite popular and easy to handle.
- Instead of multiplexing all the LEDs I went for an LED driver per segment. This makes everything a little more costly as you need a few more of those chips, but the driving is much less time critical which is an advantage if using a non realtime operating system such as Linux. Also we don't have to care about any multiplexing issue while filming or photographing the clock.
- The original clock uses fourteen-segment displays for the month display and seven-segment displays for the rest of the digits. I opted for sixteen-segment displays for all digits as this makes it easier to display alternative text on the display and also it was very difficult to find fourteen/sixteen/seven-digment display which match in their look.
As the three rows of digits consists of different color the resistors which set the LED's current have to be different. To the LEDs in intensity I assembled the first board with three different colored digits. In the next step I could assemble three complete boards.
As I soldered them all by hand I was expecting some issues with the boards and therefor I hooked them all up and tested them with different patterns I implemented in the software. The all-LEDs-on test was also a good way to get first measurements for the design of the power supply board. I figured out that a voltage of 2.5V is enough for all the LEDs to light up perfectly and do not waste to much energy on the LEDs drivers as they have to burn all energy which is not used by LEDs.The last thing was the design of the case for the clock. I decided to go with a custom build case with parts out of the laser-cutter at the local fab-lab Die Werke. My first approach was to test build something out of MDF to see how everything works. The MDF case consists of several layers out of 3mm MDF which are glued together. The first layer provides the visible front, the second holds the colored windows out of acrylic glas and nuts to screw the PCBs to. The rest of the layers provide an outer ring as side walls. The labels on the front were engraved with the laser-cutter.
Originally the MDF case was designed as a prototype but I liked to look, so I decided to keep it and build a second clock out of acrylic. As the laser-cutter cannot cut through metal and I wanted to have the metal look as in the movie I bought some black acrylic which has a very thing layer of aluminium on-top of at Innograv. The rest of the design was very similar to the MDF prototype except that the walls are not layered but solid pieces and the labels are printed with an old-school DYMO M-10 Label Embosser.
So the clocks are now almost finished, except for the power board which is still missing, but the PCBs are already on the way to me.