SIMS 247: Design of a Digital Wristwatch
(postscript)
Ka-Ping Yee, 15077367, pingster@cs
This design for a digital wristwatch is guided by two main principles. First, we aim to minimize modality of the interface. Rather than putting the watch into different modes for different facilities, we try to make all the facilities visible all the time. When modes are a required part of its operation, we make the mode visible and give it a physical representation whenever possible. Second, we consistently distinguish analog and digital input by choosing continuous (analog) input devices for continuous variables and discrete (digital) input devices for discrete variables.
The three facilities -- alarm, clock, and timer -- are each given their own separate, dedicated areas on the display. This reduces the modality of the display and avoids confusion. When the alarm is not set, the alarm area is blank; the presence of an alarm on the display directly indicates that the alarm is set and active. Similarly, if the timer is at zero and not running, the timer area is blank.
A lever on the bottom edge of the watch is dedicated to controlling whether the timer is running. If it is in the center position, the timer is stopped; if it is on the right, the timer counts up; if it is on the left, the timer counts down. Pushing in on the lever clears the timer to zero. This lets one start the timer running instantly at any time with a flick of the lever.
There is one modal input device: the circular ring around the face of the watch can be rotated to adjust a time value. It is modal in that it can be used to adjust the alarm, the clock, or the timer; a lever on the left edge of the watch selects which is to be controlled by the ring. Even though there is modality here for the sake of economy, it is physically realized in the position of the lever, which is visually and tangibly obvious.
The circular ring is appropriate since it represents a continuous variable, the time. By using a ring instead of discrete inputs such as buttons, we take advantage of motor control skills; it's easy to change the value quickly or slowly by directly manipulating the ring. The standard rotational input on an analog watch is a twirl knob on the right side; this has the disadvantage that it can only be conveniently used if the watch is on the left wrist. The rim of the watch face is more accessible, and turning it suggests the movement of analog clock hands. We avoid questions such as "which button increases and which button decreases?" -- it is clear that turning clockwise advances the time. Also, the ring is larger than a twirl knob, providing much finer control of its angular position. The ring clicks in small steps as it rotates; about 120 divisions in one revolution should be reasonable (for an outer rim with diameter 3.5 cm, this corresponds to about a 0.9 mm movement at the outer edge for each click, which is easily within the control of the fingers).
Personal experience and a cursory survey of digital watches show that their designs have largely converged on a common four-button interface: a button to change modes, a button to activate the setting or adjusting of something, and two buttons for custom use. When the "set" button has been pressed to put the watch into "set" mode, these two custom buttons usually increase and decrease the selected value, while the "mode" button selects the different values to be adjusted. In "timer" or "stopwatch" mode, the first custom button usually starts and stops the timer, and the second clears it.
Here are some comparisons of common operations on this digital watch and on the standard four-button watch. Let us assume that the four modes of the standard watch are the clock, alarm, stopwatch, and countdown timer in that order. The aim is to make these common operations simple and obvious enough to master just from a glance at the watch.
| Operation | Standard Interface | Ring-Lever Interface |
|---|---|---|
| Set the clock time. | SET, PLUS or MINUS several times to adjust hours, MODE, PLUS or MINUS several times to adjust minutes, SET. | Left lever to CLOCK position. Rotate ring to set time. Lever back to NORMAL position. |
| Set the alarm. | MODE, SET, PLUS or MINUS several times to adjust hours, MODE, PLUS or MINUS several times to adjust minutes, SET. MODE to get back to the clock again. | Left lever to ALARM position. Rotate ring to set time. Lever back to NORMAL position. |
| Measure a time interval. | MODE, MODE, START. Then STOP. MODE to get back to the clock again. | Bottom lever to the right. Bottom lever back to center. |
| Count off five minutes. | MODE, MODE, MODE. SET, PLUS, PLUS, PLUS, PLUS, PLUS, SET. START. MODE to get back to the clock again. | Left lever to TIMER position. Rotate ring to set five minutes. Bottom lever to the left. |
Note: it was assumed that we were required to constrain ourselves specifically to digital watches worn on the wrist. If we are permitted to suggest other kinds of technology, there are lots of other interesting possibilities (for example, a heads-up display of the time in the corner of one's eyeglasses).