Around March of 2014, out of the blue, SparkFun started selling these beauties.

Those are 4-digit 7-segment LED displays in a DIP6 package. Tinkerers and enthusiasts everywhere let out a collective shriek of delight, throwing fistfuls of money at SparkFun, depleting their initial stock in less than half an hour. One early commenter summed up the feeling perfectly: “No idea what I’ll use it for, but don’t care: shut up and take my money!”

After a couple of weeks, I was finally able to buy some for myself.

My first official project incorporating this component is what I’ve been calling the Bubble Clock Watch. It draws inspiration from SparkFun’s BigTime Watch kit, but it is meant to be smaller, and is based around the surface-mount TSSOP MSP430 instead of the through-hole DIP Atmega328.

The Design

The design incorporates one MSP430g2231 microcontroller with an external 32kHz crystal, along with one surface-mount 74HC595 output shift register to drive the display. The clock is powered by a single CR2032 battery, and should last at least 5 years on the same battery if the time is viewed a few times a day.

SparkFun’s BigTime Watch displays the time for two full seconds after the button is pressed. With my design, I decided instead to show the time for only as long as the button is pressed.  This allows the user to illuminate the display for only as long as needed to read the time.  This will help extend the battery life.

This is my third project to use Osh Park’s excellent PCB service. The board outline is as small as the outline of a regular watch, with slots routed out on either end to allow a strap to pass through. This is meant to be a wristwatch.

To allow a slim profile, I defined a cutout in the PCB to fit the bubble display.  I bent the display’s pins outward until they were horizontal, and trimmed the pins down to a reasonable length that would cover the solder pads but not go beyond them.

I included a footprint for an optional capacitor to serve as a backup power source.  There is enough charge in the capacitor to keep the clock running while changing the battery.

Underneath the battery, I defined a large pad to contact the negative terminal.  When soldering it together, I added a blob of solder and pushed it around the pad until the whole pad had solder, and then added enough to create a sort of “pillow” that was raised enough to press against the battery.

This design features a programming interface made of a 2-layer edge connector with 0.1 inch spacing, similar to an ISA card edge connector. I connect to it using a 6-pin card socket that I harvested from some computer circuit boards. I have used this programming interface design successfully on a couple of previous projects.

The Result

When the battery is first put in, the clock starts at 12:00.  To view the time, push the button. The display stays lit for as long as the button is held down.

To set the hours, hold down the button for 10 seconds, until the decimal point under the hours turns on.  The hours increment each time the button is pressed and released.  Next, to set the minutes, hold down the button for three seconds, and the decimal point will re-locate to under the minutes.  Like the hours, the minutes increment each time the button is pressed and released.  Also, the seconds are set to zero each time the minutes are incremented.  To exit set mode, hold the button down for three more seconds, and the display turns off.

Unfortunately, I found that the watch is susceptible to ESD.  So far, the microcontroller hasn’t been fried, only reset multiple times. A discharge usually happens when pulling a shirt sleeve over the watch, as happens when putting on or taking off a shirt, and results in the clock resetting itself to 12:00.

There is no ground plane in the PCB, so maybe that would be something I could improve in a later version.

Component	QTY	Source
100 Ohm Resistor	8	SMD 0805 Resistor and Capacitor Book (Adafruit)
10 K Resistor	2	SMD 0805 Resistor and Capacitor Book (Adafruit)
100 K Resistor	1	SMD 0805 Resistor and Capacitor Book (Adafruit)
0.1 µF Capacitor	2	SMD 0805 Resistor and Capacitor Book (Adafruit)
1 µF Capacitor (optional, approx. 3.3mm x 2.6mm)	1	salvaged from old circuit board
SMD Pushbutton	1	SparkFun
MSP430g2231 TSSOP-14	1	Element14
74VHC595 Serial-in Parallel-out shift register	1	Element14
Bubble Display – 7-segment (4-digit)	1	SparkFun
32.768 kHz SMD Watch Crystal	1	Element14
CR2032 3V Coin Cell	1	convenience store
SMD Battery Clip for CR2032	1	SparkFun

Designator	Component
R1	100 K Resistor
R2	10 K Resistor
R3-R10	100 Ohm Resistor
R11	10 K Resistor
C1, C2	0.1 µF Capacitor
C3	1 µF Capacitor (optional)
DISP	LED Bubble Display
H1	programming header (not a physical component)
IC1	MSP430g2231
IC2	74VHC595
SW1	SMD Pushbutton
X1	Crystal

This is open hardware. You can download an archive containing the source code, BOM, gEDA PCB layout file, Gerbers, and netlist.