TI MSP430, LEDs and low-power!


Because this is a project I particularly like, I had to talk a lot about it so I divided it in several parts.

  1. What is it?
  2. The electronics
  3. PCB design
  4. The software

What is it?

In one sentence, it is a coin-cell powered wristwatch that uses 16 LEDs to display the time, features a capacitive touch button and a 30 days battery life and is built from 3 sandwiched PCBs.

Reading the time

Touch the “Touch me” button and one of the 12 red LEDs lights up, displaying the hour. After one second, it turns off and another red LED turns on, showing the minutes in increments of 5, like an analog clock is divided in 12×5 minutes. The orange LED that lights up in one of the 4 cardinal points let you know minute-precision time. If the top orange LED is on one minute has been spent in the 5-minute range indicated by the red LED. The right one is two minutes, three for the bottom and four for the left one.

After quickly getting used to it, you can ignore the orange LED and read the time with a 5-minute precision, or use the orange LED if you want minute precision.

Project origin

Two years ago I read an article on Hackaday about Mats Engstrom and his watch, DaTajm. The device used a resistive touch sensor and was made out of 3 sandwiched PCBs! How cool is that? LEDs, low-power, real-time clock and advanced PCB design! Perfect project for the end of the semester.

Wristwatch
3 PCBs and 4 screws, that’s it!

The day after, I decided to start a similar project, but with the LEDs embedded inside the PCBs, shining through the top one and using a capacitive touch sensor, to add a bit more challenge. I wasn’t sure of the battery life I could achieve with a capacitive button, as is requires an excitation circuit, compared to the resistive button, that is passive and can trigger an interrupt. This Application Note from Texas Instruments describes how they built a system with two capacitive sensors, drawing a whooping 1µA average, using the integrated CapTouch capability of their low-power MSP430 series MCU. This was my green light!

One thought on “Capacitive LED wristwatch

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