#0021: Repairing an LCD with missing segments
Preamble
This is a quick guide to repairing a specific fault found on undamaged low information monochrome numerical LCDs. Such the ones present within calculators. As such it will not go into detail about the functioning of LCDs in general, types of LCDs available, or any other information outside of the scope of simply repairing the missing displayed segments fault.
What is an LCD?
An LCD or Liquid Crystal Display, is a type of flat panel display. At its most basic an LCD operates by using the properties of liquid crystals coupled with polarisers. Polarisers are a type of optical filter that only allow light waves to funnel through them in a particular orientation. In other words, they remove light scatter; only allowing it through in a uniform manner. This, coupled with the liquid crystals’ property of altering their physical orientation when in the presence of an electric current; means that the narrow beams of light that make it into the crystal solution can either be allowed to pass through, or blocked, depending on the orientation of the crystals within the solution.
The specific type of LCDs we are dealing with are low information monochrome single line seven-segment displays. These types of simple LCDs are typically used in devices that predominantly output numbers. But may also display static symbols, such as the “E” in calculators for numbers that are too large to display without index notations. These types of LCDs are most commonly associated with pocket calculators. However they have been used with such devices as: alarm clocks, multimeters, solar charge controllers, battery monitors, household mains electricity meters; and I have even seen them used as a display on an electronic keypad lock.
I think this type of LCDs popularity is mostly due to it’s relative simplicity and low operating costs. It follows the KISS design philosophy. Keep It Simple Stupid. If a device would not notably benefit from a more complicated display, if all that it displays are simple numbers and basic symbols; then there is little reason to incur the (production and operating) costs of increasing design sophistication beyond this type of display.
As for the mechanics of how liquid crystals work, I like to (keyword) imagine a matrix of magnetic rods. At rest, the viewer can only see them from the top, and considering their microscopic size, this renders them essentially invisible. Whereas when a current is passed through them, the entire matrix of rod shaped crystals reorientate themselves to reveal the entire length of each of the rods. This greater surface area against the polarised light from the viewing angle, makes them appear opaque. There’s more to it than that, but that’s the general mental model I use to conceptualise the process. Although strictly speaking it isn’t accurate.
So how does a basic monochrome seven-segment LCD actually display information?
An LCD of this type is mapped with discrete segments. These primarily consist of seven dashes arranged in a number ‘8’ pattern. These are the core seven segments that are used to display numbers. Additionally LCDs have segments in the shape of static symbols; such as a period on calculator, or a colon on an digital clock.
Every discrete segment is given a set of electric probes. These probes are designed to allow a current to pass across the segment’s liquid crystals. This is how the liquid crystals within each individual segments are switched on and off. It operates in an analogous way to seven-segment LED displays. I.e. they both require an a electric current to be passed across each individual segment in order to activate it. Additionally, this electric current is controlled by a display controller IC (Integrated Circuit). Which translates any numerical values into display data (active segment and inactive segment map) that it uses to power it’s display accordingly.
Missing segments fault (on undamaged LCDs)
First of all, I specify that the LCD is undamaged because if the LCD you are attempting to repair is damaged, (e.g. has a crack across it); then chances are this fault is not the major contributor to your LCD’s malfunctions. That being said, missing segments on undamaged LCDs are likely caused by a break in the particular missing segment’s individual electric circuit supplying it.
Earlier I likened LCD seven-segment displays to their LED counterparts. This is because both require an individual IC controlled circuit that connects with each discrete display segment. Its just that with LEDs, its a lot easier for people to understand what is happening, when some of a displayed number’s LED segments fail to turn on. The failure to activate can be intuited due to a break in it’s branch of the circuit.
In this case the break in the segment’s circuit is usually caused between the LCD module and the underlying PCB; which hosts all device circuitry, including the display controller. This break usually occurs within or around the bridging material between the LCD and PCB. Namely the elastomeric connector (trade name: “Zebra strip”). This black and pink rubber like material is a soft electric conductive material that conducts electric signals across the naked pads of the PCB to the LCD and vice versa. It does this by having many tiny channels (or layers) of conductors and insulators, that alternate across it’s black strip. This black strip is then sandwiched with a pink insulator that runs the length of the outer sides of the elastomeric strip. This configuration allows the elastomeric strip to act like a large assortment of miniscule wires that electrically connect together whatever pads or traces that they touch at either of their ends.
The circuit break in this missing segments fault could be caused by two main things. Firstly, it could be due to an electric insulator getting in between the elastomeric strip and the exposed pads of the LCD and PCB. This could come in the form of a build up dust or grim, or even oxidation of the exposed PCB pads. To repair this, just clean all the pads and the elastomeric strip itself. I recommend using isopropyl alcohol and a cotton ear bud or cue tip. Just saturate the bud with the alcohol and scrub until it’s clean. Then reassemble the device and test.
Alternatively, this fault could also be caused by a separation between the elastomeric connector and it’s adjoining contacts. I.e. it has lifted off or away from the pads that is it supposed to be pressed against. This is usually caused by vibration. Typically, there will be a method of mechanically tightening or pressing the elastomeric connector against it’s pads. Such as a screw or adhesive, which with time and vibration (and maybe a little heat) can become undone enough that it allows the connector enough space to move away from it’s pads. To repair this, just re-tighten the screw or re-secure the elastomeric connector with tape if needs be.
The fault in the case with the example unit; I believe was caused by both a build up of dust between the contacts and the elastomeric connector, and also the connector physically separating from the LCD. The separation was caused by a loosening of the self tapping screws which held in place a bracing bar. This bracing bar applied pressure to the assembly consisting of the PCB, strip, and LCD, which sandwiched them together and kept them in place. That got loose, the strip moved slightly, and then dust got into the gaps that it created. After a good cleaning and tightening, it now works flawlessly.
Closing thoughts
When it comes right down to it, this is as simple as a repair really gets. No replacement of parts; just a basic disassembly and cleaning. It is essentially maintenance.
In my opinion, this type of repair is especially good for an aspiring repair tech with no confidence. The tools needed are basic, there are no additional parts (i.e. expenses) required, and the device being worked on it likely inexpensive; so there should be little in the way of consequences of failure. Such as fear of damaging a device, which may put people off from just ‘aving a go. Essentially the repair has little in the way of friction that may prevent a person from trying. It a good confidence builder.
Lastly, I just wanted to raise awareness encase you ever come across this type of fault in the future. It is easily repairable; and hopefully you’d be more inclined to at least give it a go, rather than discard the unit and purchase a new one as is usually the case for these types of cheaper mass market products.
Th-th-th-th-that’s all folks!
Thanks for reading.