I’m still a few years away from retirement eligibility, but that doesn’t mean I can’t be getting ready. The problem is, it’s easy to get distracted. I was looking for a quick project and had the idea to make this retirement countdown clock, to remind me to stay focused on my retirement goals.
This is a cheap project, costs around $12 (not including the cost of the 3-D printed case), and is easy to make. The main components are:
- MAX 7219 8-digit, 7-segment LED display — you can find these on eBay for about $3.50, including shipping.
- DS 3231 Real Time Clock (RTC) module — about $2.50 on eBay. Don’t forget the 2032 coin battery.
- Arduino Nano Micro-controller — about $6.00 on eBay.
The basic design is simple, both the 8-digit LED display and Real Time Clock (RTC) module are wired directly to the Arduino.
I was initially worried that the display would draw more current than can be supplied by the Arduino, but that’s not a problem at all. Just to be safe, you’ll see in the code that I’ve reduced the display brightness to avoid any issues.
Connect the following LED pins to the Nano:
VCC -> 5V GND -> GND DIN -> D2 CS -> D3 CLK -> D4
Next, connect these RTC module pins to the Nano:
VCC -> 5V GND -> GND SCL -> A5 Note: Arduino pins A4, A5 are dual-purpose; in this case SDA -> A4 they're used for the I2C interface connection to the RTC.
Here’s a picture of the three components wired together. Because the Arduino Nano has only one 5V pin, I had to do a bit of soldering to create a Y-shaped connector cable, to provide power to both the LED display and RTC module from this single pin on the Nano. That’s the red and white shrink-wrapped cable.
I designed the 3D-printed case using Autodesk’s free 123D Design software. Here’s a series of screen shots showing how the case is designed. I start by placing a 2 mm thick back “wall” of the case. An easy way to make the rounded corners is to merge 2 mm high half-circles into larger rectangle. Once the shape is correct, you can fuse the separate pieces to make a much more complex, solid shape.
Next, add 2 mm thick rectangle case “walls” to the back piece, and add quarter-cylinder shapes, the same height as the sides, to close the edges. With these pieces aligned precisely, you can continued to fuse or merge these pieces together to make larger solids. I haven’t noticed that keeping the component pieces separate or fused makes much of a difference in how the design is printed in 3D.
The 123D Design software lets you merge pieces when exporting to .STL format, but combining pieces does make managing more complex designs easier.
Here’s the final case design. I created the face plate in the same way, but adding the embossed lettering is much easier to do using Microsoft’s excellent (and free) 3D Builder app that is included with Windows 10. You can download the case files in .stl format here.
The last step in this project was to write the Arduino sketch to run the clock. The code is almost trivial, so I haven’t bothered documenting it here. The Arduino sketch file (.ino) is available for download here.
That’s it! Post any questions to comments.