Digital Wristwatch Design Part 2

In this blog post, I will deviate for the second time from Professor Foster’s portfolio bait questions and I will focus on the design of my watch using concepts learned in Praxis. If you were not paying attention last time, I’ll get you up to speed: basically Colin and I want to build our own digital wristwatches, and we are trying to adhere to techniques learned in Praxis to guide our design. The problem we are trying to solve is not on the order of anything useful to society such as improving the TTC or filtering water; it is basically to satisfy our need for a homemade wristwatch. The upside is that we will be able to find which techniques really work (or be able to say with some basis that a technique is rubbish :P ). Now as some of you have seen already, Colin and I are already quite far into the design process. Here are some of our current prototypes:

This is my working prototype. A newer version with less tape is being worked on.

This is a prototype of my prototype.

This is Colin’s prototype. Ignore any comments from him about how I alledgedly "set up" his workspace to make him look bad :P .

So how did we get to our current designs? Well, we did it through the consideration of some important design decisions on things such as the timekeeping device, the display, and the batteries used.

Timekeeping 

The most important component of any clock is the mechanism that keeps the time. In our design of our electronic watch, we considered 3 timekeeping options:

-Create a stable oscillating circuit and use a microcontroller to count the oscillations

-Use a microchip specially made to count time (RTC, or real time clock)

-Use a 555 timer

The designs will be rated by cost, size, power consumption, accuracy, and number of microcontroller pins the design takes up. Now when Colin and I decide on our designs, we didn’t really use a radar graph, but after seeing them in today’s lecture, I thought the data would be much better represented this way.

The oscillating circuit is used as a reference. It seems like it will still be a difficult decision, but it should be noted that size, power consumption, and accuracy are weighted much higher than cost. This makes the RTC the obvious winner.

Display

Another important design decision was the display of the watch. We considered designs by comparing cost, feasibility, power consumption, aesthetics, usability, and size of 3 different designs:

-LCD screen ripped from cell phone

-LED binary display

-LED numeric display

The LED binary display is the reference. In my design, I personally valued usability and feasibility much more than any of the other criteria, which lead me to choosing to use the LED numeric display. Colin weighted the criteria differently, which resulted in him choosing the LED binary display due to cost and aesthetics. I really did want to use the cell phone screen, but until I find more information on its driver, using it is not feasible.

Batteries

The last major design decision Colin and I went through was about batteries. We valued safety, reusability, size, how long it will last (its mAh), and how close it is to 4.5V. We had 3 available options:

-Use 3V total in button cells

-Use 3.7V rechargeable Li-ion

The appropriate voltage and recharge-ability of the Li-ion battery made it much more appealing than the button cell. The safety issue is a result of the tendency for Li-ion batteries to leak/explode. However, given that most cell phones/mp3 players use this type of batteries and the safety circuit the battery contains, the risk is probably minimal. In addition, the safety issue can be alleviated if the Li-ion battery is enclosed in some kind of bag or container.

Conclusion

So what have I learned so far in the design process? Here are some of the tidbits I would like to share. They were mentioned many times before in class, but it went in one ear and out the other.

-Plan ahead and consider all design options before proceeding. Preferably, use some of the design comparison methods that we used in the tutorials. I wasted a lot of time on the oscillating circuit design for timekeeping before realizing that the RTC was a much better design choice.

-Keep some sort of notebook. I really wanted to jump into the construction phase and I wrote zero documentation on any of my experiments, measurements, and ideas for a large first half of my design process. I later found out this was a huge mistake when I either forgot previous experiment results and had to repeat them or when I had a reoccurring problem but did not know exactly how I fixed the problem previously because I never wrote it down. Now that I have a design notebook, I find that it is much easier to keep track of experimental data and ideas.