These long thin molecules also have a tendency to align parallel to an applied electrical field. This response and the optical properties of liquid crystals lead to their application in various electronic devices ranging from watches and calculators to computers and televisions.
Polarizing Filters and Polarized Light
As shown in Figure 1, polarizing filters are materials that allow only the passage of light waves whose electric fields are oscillating in a particular plane.
Figure 1. Initially unpolarized light passing through a polarizing filter becomes polarized. The polarized light will not be transmitted by a second polarizing filter turned at 90o to the axis of the first filter.
Unpolarized light passing through a polarizing filter becomes polarized. Polarized light directed onto a second polarizer, oriented 90o to the first polarizer, will not be transmitted. Many natural substances, including liquid crystal materials, have the ability to rotate the plane of the polarized light.
Liquid Crystal Display Watch
Liquid crystal displays (LCDs) have become quite common in watches due to the low electrical power demands of the LCD panel. This panel is composed of two polarizers that transmit light in perpendicular directions, a mirrored surface and a layer of liquid crystal material that is sandwiched between two electrically conducting glass plates. The liquid crystal material used is of the so-called twisted nematic type.
Figure 2. The liquid crystal molecules in all segments of the panel are precisely aligned in the absence of an applied voltage. Therefore, the entire panel appears silvery because light passes through both polarizers, reflects off the mirrored surface, and then passes back through both polarizers.
If the surfaces of the glass plates that will be in contact with the liquid crystal molecules are rubbed, the liquid crystal molecules orient in the direction of the rubbing. Figure 2 shows that the molecules have been oriented in the direction in which the adjacent polarizer transmits light, and the intervening molecules gradually rotate their relative orientation to accommodate the 90o change from one polarizer to the other. This gives a silvery appearance to the panel, Figure 2.
When a voltage is applied to a segment of the display, the precise alignment of the liquid crystal molecules is lost. This results in the polarized light from the first polarizer not being rotated by the required 90o to align with the second polarizer. The second polarizer blocks the passage of light and causes that segment of the panel to appear black, Figure 3.
Figure 4. Examples of 7-segment numeric and 14-segment alphanumeric displays.
Most of the displays in LCD watches are composed of several 7-segment sections. Each 7-segment section can display one number. The combination of these sections can display the date or time, Figure 4.
In addition to the enabling technology of the LCD, there are several other components that are critical to making the watch function properly. First of all, there has to be a timing mechanism (after all what is a watch that doesn’t keep time worth?). There has to be a controller to tell the display what time or date to display as well as set the time and date. There has to be a power source to drive the display, timing crystal, and controller. And finale, there has to be some means of connecting all of these components together.
This timing mechanism is a quartz crystal that provides a constant-frequency signal to the electronic circuit in the watch. When certain crystals, including quartz, are excited with an electric field, they vibrate at a particular frequency. This vibration can be sensed electronically to provide a means for observing the vibrational frequency of the crystal.
The integrated circuit (IC) or “chip” is programmed with the frequency of the crystal and “knows” how many cycles must pass in each second. Once that number of cycles has passed, it increments the second count by one. After 60 seconds it increments the minute readout. After 60 minutes it increments the hour readout. After 24 hours it increments the date readout, etc. The IC must tell the LCD what segments need to be turned on or off in order to display the correct digits in the LCD. The IC also stores the current date and time and has a means for setting them. This is usually done by connecting two or more “pins,” or leads, from the IC chip together. Since the IC chip is so small, the pins are connected to tabs on the side of the watch that are pressed to make the contact.
A 1.5 V alkaline batter is used to supply the power to the LCD, IC chip, and timing crystal.
Printed circuit board
In order to make all of the electrical connections, a printed circuit board is used that is placed in a specific position relative to the other components in the watch. The printed circuit board consists of several conductive paths made of metal on a plastic substrate. These paths connect the pins on the IC to the battery, timing crystal, and contact pads for setting the time and date. The connection to the LCD is a little different than the others since it requires more precise alignment of the conductive paths and the LCD in order to properly address each segment in the LCD. This is accomplished with a specially designed piece of rubber.
Conductive rubber laminate
The conductive rubber laminate is a layered structure of alternating layers of electronically conductive and electronically insulating material. This arrangement allows for the conduction of electricity in one direction, but not the other.