Custom LCD Design: Part 1 - LCD characteristics

To develop a custom LCD, you first need to have some important information, such as:

• Operating temperature
• User position
• Indoor/outdoor use
• Circuit operating voltage
• Number of segments
• Cost limitations
• Driver IC

But it is also important to understand how the LCD operates and what options the manufacturer offers. Once you have chosen the type, you need to submit a mechanical drawing of the LCD with all the symbols and information on the LCD's operation.

LCD Types

LCDs can be classified by: the type of image (negative or positive); the mode of operation (reflective, transmissive and transflective); the type of liquid crystal (TN, STN, etc.); the type of drive (static and multiplexed); the number of bias voltages; and the type of connection.

Image Types

In positive LCDs, the image is formed on a light background, that is, the symbols and segments block the passage of light. The opposite occurs in negative LCDs, where the background is dark and the symbols are clear, as they either allow or do not allow light to pass through.

Operating modes

There are 3 operating modes for an LCD:

reflective: the light source is external and there is a reflective film on the back. It offers high contrast and a wide viewing angle. It is not compatible with backlight and is not usually used in negative LCDs. It is great for use in the sun, but it can be used in low-light environments.

transmissive: it does not have a reflective film and all the light comes from the backlight. The use of the backlight is mandatory. It is great for use in low-light environments, but due to the backlight it is terrible in the sun.

transflective: it is a combination of the previous ones, so it has the characteristics of both. It can operate in both low-light and sunlight environments, as long as the backlight can be controlled. 

Each type has its own applications, depending of end use:

Types of liquid crystal

The LCD is produced with a polarizing film on the back of the glass and another polarizing film on the front of the glass. One of them polarizes horizontally and the other vertically. This way, the light cannot “escape”, creating a dark region. For the light to pass through the two polarizers, it needs to be reoriented, creating a clear region.

At idle state, the liquid crystal filaments are misaligned (twisted), allowing them to change the polarization of the light, thus creating a clear region. To create the dark region, it is necessary to apply an alternating voltage, which produces the alignment of the filaments and consequently creates the dark region.

Types:

– TN (twisted nematic): produces a twist of up to 90° in the liquid crystal. Low contrast and very low cost.

– HTN (high TN): increases the twist up to 110°. Contrast slightly better than TN and low cost.

– STN (super twisted nematic): twist between 210 and 255°. Good contrast and moderate cost.

– FSTN (film compensated STN): same twist as STN. Very good contrast and high cost.

– EBTN (enhanced black TN) or VA (vertical alignment TN): here the filaments are completely horizontal. Great contrast and high cost. Here the rest state is symbol on (unlike the others), which is why it is more common to find negative LCDs.

Please follow this topic for more information about the custom LCD design process.

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