Fork me on GitHub

Project Notes

#429 3x7 Pomodoro Timer

A Pomodoro timer wire sculpture using the Boldport 3x7 display and an ATmega328.


Here’s a demo of it running a 5-minute countdown (I would be surprised if anyone watches the whole thing without skipping to the end!)..



Over the years, I’ve become habituated to working in a Pomodoro style - make the day a series of tasks worked on in short blocks of time, with regular breaks. But I’ve never actually used a timer - just relied on my internal clock to work in roughly 1 hour increments.

As I was building the Boldport 3x7, it started to appeal to me as a very nice display to use for a non-distracting Pomodoro timer.

The sketch proved to be quite simple, using the LRThreeDigits library for driving the 3x7 display.

After breadboarding the idea my first thought was to make a PCB … but as there’s been a bit of Mohit Bhoite fandom in the Boldport Club recently, I was drawn into a another copper-wire sculpture. Not very ruggedized, but it does look interesting!

Now for the true test - is it actually useful? Well, I’ve started using it for real as my pomodoro timer and so far so good.

Note: the two left-most digits are minutes, the last digit is tenths of minutes. This is actually why I built my 3x7 with the yellow digit on the right;-)

Design Concept

There’s a few things I set out to achieve:

  • the 3x7 displays minutes in two digits, and tenths of minutes on the 3rd digit
  • the pomodoro countdown runs from at most 95 minutes, but default to start at 55 minutes (my preferred time block)
  • the Arduino measures reasonably accurate time for the countdown, but I’m not going to be upset if it is a little off (less than a minute)
  • before starting the count, two buttons can be used to increase or decrease the countdown respectively, in increments of 5 minutes
  • when the countdown has completed, the unit will flash for a period of time
    • a button press resets the app for another countdown
    • if no input, go to sleep
  • if sleeping, a button press wakes up and resets the application

Measuring Time

How to measure time with an Arduino? We could:

  • use the millis() function
  • use an external real-time clock
  • use interrupts to measure increments of time

Given that timing requirements are not critical, I’m going to start by trying to base things on millis(). This counter eventually rolls-over, but since that is after ~50 days, I’m ignoring this for now.


Buttons are connected to pins 2 and 3, using the built-in pullup resistors. Hardware interrupts on the pins are used to trigger related functions in code.

These buttons handle “up” and “down” adjustment of the countdown duration before it starts (in 5 minute increments).

Once the countdown has started, pressing either button will cancel/reset the counter.

If the application has gone into sleep mode, either button can be used for wake-up.

Pin Connections

In order to reserve pins 2 and 3 for buttons with hardware interrupts, the 3x7 is connected from pin 4 to 13:

3x7 Pin Arduino Pin Port
Digit 1 Sink Pin 4 PD4
Digit 2 Sink Pin 5 PD5
Digit 3 Sink Pin 6 PD6
Segment g Pin 7 PD7
Segment f Pin 8 PB0
Segment e Pin 9 PB1
Segment d Pin 10 PB2
Segment c Pin 11 PB3
Segment b Pin 12 PB4
Segment a Pin 13 PB5

Note: this requires v1.2.0 or later of the LRThreeDigits library.

Sleep Mode

If there has been no button input for 5 seconds after the countdown is complete, the program puts the processor into SLEEP_MODE_PWR_DOWN. Since there are no additional peripherals to power, the current draw is very low in this mode. I haven’t measured it accurately yet, but according to a USB power meter it is below the ~1mA resolution of the reading.

Breadboard Prototype

To test things out by plonking an ATmega328 on a breadboard..




Wiring Up

I may do a PCB one day for a more ruggedized version, but I’m on a copper wire construction binge at the moment. I don’t have a detailed design - just some very rough sketches in a book (mainly to make sure I didn’t get my pin connections all mixed up). The actual design just came together by eye and a bit of patience;-)

The finished base ready for testing. The USB mini connector is for power only (I need to pull the chip if I want to reprogram).



With 3x7 mounted:


And powered up:


Next steps - I’m thinking about adding a mount so that I can hook the unit to the top of my monitor. Later…

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.