1 Glass Segment LCD Basics

A glass segment LCD typically has two glass plates joined together with a small gap in between the plates. This gap contains a liquid crystal fluid, which is transparent under normal conditions. However, if an electric field is applied through a voltage differential, the molecules arrange themselves to align with the field direction and a black segment is visible on a clear background. The potential difference is applied to the segment LCD through an alternating, and repeating signal through the segment (SEGy) and common (COMx) lines attached to the display. COMx and SEGy lines are driven in opposite polarity in order to create said voltage differential. The voltage applied between COMx and SEGy lines is typically alternated to limit DC offset levels, thus extending LCD lifetime. Consult the segment LCD’s manufacturer’s specifications for max DC offset allowed.

The optical contrast, or how dark a segment is compared to the background, is controlled by how large of a differential between the RMS voltage of an ON segment, and an OFF segment. Since a repeating signal is used, the frequency of the signal has a visual effect as well. If the frequency used is too slow, then one would perceive a “flickering” of the screen. Faster frequencies do not have this issue, but consume more power. Most segment LCDs have a typical driving frequency between 30 Hz to 200Hz, which is well within the range of a software based solution.