Display Selection
 
The display is the most important part of the whole project -- Not only because it is the face of the watch, but because it also defines the size and power requirements of the project.
 
When it comes to watch displays, there are three basic types available:
 
  1.  LCDs (“Liquid Crystal Display”) are the type typically found in modern electronic watches. A small electric field causes the crystals to rotate, which causes the display to act as a shutter. In one state, room light is reflected back; in the other state, the light is absorbed. Since no actual light is generated, these displays are very power efficient. This strength is also their main drawback -- with no ambient light, you cannot see them. This problem is inelegantly solved with a backlight. Backlights are inefficient because they must be bright enough to light the whole display. The crystals selectively block some areas and that wastes energy. Even if you want to light just a tiny area, you must run the energy-hungry backlight at full power.
 
  1.  LED (“Light Emitting Diode”) watches were the first types of electronic digital watches, popular in the 70’s. They use much more power than an LCD, but less than an LCD backlight. These watches require a button press to see the time, even in the daytime. Each point of light requires an individual semiconductor crystal and a tiny gold wire, making it expensive to manufacture LED screens. Even when this is done (an example), the resolution is typically very low.
 
  1.  OLED displays are a modern version of LEDs. The “O” stands for organic, and it’s a compound that glows when electricity passes through it. Kindof like an electrified pickle, but different chemistry. Since the organic layer can be screen printed between transparent wires, it is a lot easier to manufacture than traditional LEDs. Since this is a new technology, there are still a few problems being worked out. The biggest one is that blue OLEDs (necessary for a full-color display) will wear out over time. I’ll use yellow, which is rated for 40,000 hours. As far as I know, there are no commercial OLED watches yet - just research projects like this one.
 
A fourth option that would be awesome if I could do it would be a purely mechanical version, like pong mechanik. I’ll stick to something simpler, though, for this first time.
 
My goal is to have a watch that looks cool, is visible at night, and doesn’t require a button to illuminate. I think the backlight button is inelegant not only from a technical aspect, but also from an aesthetic aspect. (Well, that and I’m not sure I can machine a button into the case well.)
 
You’d think that this is impossible, but I’ve got three tricks up my sleeve:
 
  1.  The neatest thing about LED watches (LED and OLED) is that the amount of power they consume is related to how many pixels are lit. Rather than a full-face display, I will be lighting up only 1-2% of the screen at any given time. So, my watch should take less power than a backlit one.
 
  1.  I’ll use a rechargeable battery. Rather than putting enough energy to power the watch for a year, I’ll only put enough in for a day or two and count on the user to recharge the battery nightly. I charge my cell phone every night; this won’t be much different.
 
  1.  Lastly, I’ll use a modern Lithium-Ion Polymer battery. This is the same type used in cutting-edge electronics, and has much more capacity than rechargeables from the 70’s.
 
 
Tuesday, August 1, 2006