Henry Ford once said that you can have any color car you want – as long as it’s black.
Times have changed, not only in the choice of automobiles, but in every facet of American life. We want it our way and we want it now. Heck, I wonder what percentage of coffee shops only serve black coffee?
The LCD industry is no different. There are several popular display technologies that fit every need including: TFT (Thin Film Transistor) that offers multi-color and larger sizes, OLED (organic light-emitting diode) which are thinner than TFTs, Custom Segment displays which are fully customizable, and Character LCDs.
Character LCDs are one of the original display technologies, and although they lack the sexiness of multicolor, high resolution displays (can we say that word in this article?), they still hold the position as one of the most popular technologies for new product designs.
Reasons include: low-power consumption, ease of programing, and . . . they rarely reach obsolescence and discontinuation. Here lies their one key benefit over TFTs and OLEDs: Character LCD modules have changed very little over the last few years. A 16x2 STN display designed in ten years ago, is still the same display purchased today. The only difference is the type/brand of controller/driver. In a lot of cases, the firmware written in the past works on the new LCD’s controller.
A bonus of these modules is that most are always in stock and prototype samples can be shipped to the design engineer the same day.
Another bonus is for OEM customers who order the same display unit for several years and don’t hear that famous, unwelcomed expression, ‘We discontinued that model and there is no direct replacement’, forcing engineering to drop their current design to find another module that will work and then invest the time, money, coffee, and aspirin to test the replacement unit.
Hence, if all your new product needs to display are letters, numbers, and punctuation marks, and you can accept a single color, then bingo! Character LCDs are your best choice.What they lack in color options, they make up for in available accessories that can be incorporated into the basic module.
OEM engineers have options and accessories
After choosing the correct size, color, voltage and backlight options of a standard character LCD, the engineer has other options available to them. Options that are necessary for their product and can save the company cost and time by incorporating them directly into the LCD. Hence, they purchase one LCD with all the necessary accessories so that their production department doesn’t need to buy these components separately.
Some of these accessories listed below include: Headers, Cables, Heaters, Touch panels, and even the type of interface you need.
Most COB (Chip on Board) character LCDs, as seen in the photo below, use a header to supply signal, commands, and power to the LCD and only cost pennies for the LCD supplier to purchase and attach to the module.
Headers are built to industry standard spacing, and most character LCDs accept 0.254mm or .1 inch spacing. It is possible to modify the display’s PCB to meet a unique spacing such as .5mm, .7mm, or even 2mm.
Headers come with varying levels of gold plating such as 15u, 30u and 50u. The higher the gold content, the more robust in harsh and humid environments.
Focus LCDs can include headers at no cost for most character LCD samples.
A cable is not a standard option on a character LCD since each customer requires a unique length. The NRE (Non-recurring engineering) or one-time tooling, to customize the cable eliminates the customer needing to add the cable at a later step in the production process.
The cable can connect LCD: logic voltage, signal, control commands, read/write and backlight power. In some cases, the LED power is a separate, discrete cable with black and red to designate positive and ground polarity.
Note: It is recommended to supply LED power independently of LCD logic power, even if their current and voltage requirements are identical.
Baby it’s cold outside is one of the most popular songs during the holiday season. I doubt the singers had LCDs in mind, but hey, cold weather operation is a major design consideration for design engineers.
If the display’s temperature drops below -20C to -30C, the fluid in the modules slow down, supply inaccurate information and, in many cases, the display freezes up.
Note: There is a difference between LCD operating temperature and storage temperature. Storage temperature is when the display is off and is, as a rule, about ten degrees colder than operating temperature.
If a display does become too cold, the fluid will expand and crack the glass.
Note: TN (Twisted Nematic) fluid is best for low temperature operations. For more information on the LCD fluid types.
Heaters are a popular accessory for character LCDs. They are necessary to keep the display in the game as temperatures drop and people retreat inside to their blazing fireplace with a mug of hot coffee.
Note: if the display is operating as temperatures decrease, in most cases, the module will continue to operate as it drops below its lower operating threshold. If, on the other hand, you power on the display while it is sitting in its cold environment, the display may not turn on.
Here are three possible LCD accessories to keep the display operating when even Santa Clause complains about the cold.
Heater film is thin, transparent, and applied to the top layer of glass. As with all heating solutions, it is only powered on when the temperature drops below a preset value.
-Even heat distribution
-Lower power draw than other heater options
-Higher piece price
-Possible NRE cost
Resistors are the less preferred option but hey, the solution does work well. Several ¼ watt resistors are added in series around the base of the Character LCD’s PCB (Printed Circuit Board) and can generate enough heat to keep the display's fluid liquid.
-No chance of discontinuance
-Available from multiple sources
-Higher power consumption
-May require a one-time NRE to add to the current PCB or add on a daughter PCB
This is the least effective method since LED’s generate very little heat, however if you need a backlight, this option may work if your display dips below the lower threshold temperature for short periods of time.
This is the lowest power option. Protecting the display within a case, or some environment that retains the heat and blocks wind, is the best option to keep your display operating. This is especially true if you are running on batteries and every mili-amp is precious.
Many products lack the space for several buttons, dials, and switches for the end user to interface with their product and are replacing most of the hardware with a touch panel or touch screen. Although these are not standard on character LCDs, they can be added.
There are two common types of touch panel: resistive and capacitive.
Resistive is the most popular for industrial applications that operate in a harsh environment and can be activated by any material, including gloves. It has the lowest cost options and a long life span.
Since the creation of smart phones, everyone wants a capacitive touch panel. Capacitive touch allows scrolling and zooming. They also perform poorly in harsh environments, are more expensive than RTP, and require more time to integrate.
The interface is how the LCD talks to your microprocessor. The standard option for character modules is 8-bit parallel. Most displays will operate on 4-bits if you are running low on I/O (In/Outs)
SPI (Serial Peripheral Interface) is growing in popularity since it saves I/O’s but slightly increase the unit cost. There is no tooling cost to add an SPI bus.
I2C is possible, but not common and would need to be special ordered.
As much as people don’t like to take the time to read instructions, it is critical that your LCD vendor supplies data sheets and source code. If this information is not readily available, then find another supplier, otherwise you will waste many hours trying to figure out how to turn this thing on.
Focus LCDs = LCDs made simple® where we offer same day shipping on most standard displays