Month: March 2021

#0021: Repairing an LCD with missing segments

Otto

#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.

example of an LED display
LCD segment map
segment circuit animation

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.

#0020: Plastic welding techniques

Otto

#0020: Plastic welding techniques

Preamble

This is an introductory tutorial on welding plastics. The goal of this tutorial is to be a rather brief yet sufficient guide, that will allow the reader to be able to weld shut cracks and holes in plastic containers to the point of being water tight. It will cover welding technique, tooling, plastic types, PPE, and best practices. Everything an aspirant will need to effectively weld plastic containers.

Tools and materials

Tooling:

  • temperature controlled soldering iron or gun
  • (optional) fan

Materials:

  • appropriate donor plastic strips
  • (optional) duct tape or electrical tape

Personal Protective Equipment:

  • safety glasses
  • filter mask
  • thin rubber gloves

Core tool summary

As you can see the core tools and materials list is tiny. All you really need is a hot piece of metal to melt the plastic; and donor plastic material to flow into the various cracks and holes. This material is to reinforce and buttress the affected areas against any structural stress. That’s it.

Temperature controlled soldering iron

I specify a temperature controlled soldering iron (or gun) because you actually need relatively low temperatures to weld plastic; just enough to melt it, but not enough to burn the material. Most thermoregulated soldering irons would be too hot, because they are specced for melting solder. A material that generally has a higher melting point than many plastics. If the your iron is glowing red (even a little bit), then it’s probably a couple of hundred degrees (celsius) too hot.

Additionally, it would be beneficial if the soldering iron had a large thermal mass to enable it to maintain a stable temperature whilst it is in active use. I.e. actively transferring heat into the workpiece. A bit with a large surface area will also be useful, this is to effectively melt a good area of material at a time. Working with smaller bits makes the job more tedious. A smaller bit also concentrates the heat across a smaller surface area, which could cause heat spikes in the workpiece and consequently burn the local plastic. For the reasons above are why I chose to use a soldering gun with it’s widest tip rather than my usual general purpose soldering iron.

Plastic donor material

There’s not actually much to say about this. I like to make sure that the donor plastic is the same type of plastic as the item under repair is made of. This is to assist them in chemically bonding together more effectively. If you don’t want to purchase plastic donor strips like I have; you can alternatively just cut up similar items (i.e. items made from the same material) and use that. I have also seen many people use zip-ties as a donor plastic due to their convenient strip shape.

When selecting a donor plastic, look for a recycling symbol on the donor item (e.g. water bottle). Here it should have a few letters under it. These indicate what type of plastic the item is made from. Another thing to keep in mind when selecting a donor plastic is the food grade safety factor. To know whether or not a container is food safe, look for the knife and fork symbol. Specifically, when repairing a food grade container: do not assume that just because the donor plastic is the same type, as the food grade plastic container; that the donor is also food grade. Even if it initially is, the repair itself my change the chemical structure of the plastic to the point that it will now leech into any food (or drink) stored in it.

A good example is the container I repaired for this article. It was initially a food safe box, but now after the repair, even though it is now watertight again and I repaired it with the same type of plastic (PP); I would not use it for food, for fear of it leeching toxins into my foodstuffs.

common plastic types:

  • PET (Polyethylene Terephthalate) typically used in water bottles
  • PP (Polypropylene) general use plastic for containers
  • HDPE (High-density Polyethylene) typically used in milk bottles

Additional tools

Apart from the essentials, a fan will also help; both to blow any toxic plastic fumes away from you and to help cool the piece quickly as you work on it. Sometimes I find that as I work, an entire section of the container can suddenly soften to the point of almost liquefying, meaning that I will have to wait until it reconstitutes somewhat before work can continue. This however is an excellent state for moulding the pliable material to seal any cracks. Other than that, some duct tape may be useful to make the repair look more presentable and to give it a little more structural strength after a complete repair. Tape can also be used to hold the piece in place, as you weld the cracks.

Recommended PPE

As for PPE: I highly recommend wearing safety glasses and a filter mask. You really don’t want to risk globs of hot plastic flicking into your eyes accidentality. Especially since, if you are anything like me: chances are that your face is very close to the workpiece, in order to see every little detail. You also probably don’t want to suck up all those toxic fumes from any burning plastic either, so a cheap filter mask will help avoid that.

I would also recommend wearing a pair of thin rubber gloves. Although the plastic shouldn’t reach any serious temperatures before melting, a pair of gloves helps you comfortably shape any jellied plastic by hand; should you wish to do so.

Plastic welding techniques

I usually start by placing any loose container segments back into the gaps and holes that they broke out from. In a similar manner to welding metals, I then tack the loose segment into place. This is done by melting small spots across the circumferences of the segments; i.e. melting little bridges across the cracks. Do enough to keep each break out piece in place, or to stabilise a crack (in the case where there aren’t any breakout pieces, just cracks). For any holes where the original broken out piece has gone missing, you’ll have to use the sections of donor material to fill them in. I recommend melting the complete area into jelly, and shaping the mixed plastic mass into form. This is in order to properly blend the plastics into a watertight seal.

As an alternative to tack welds, some sticky tape can be used to hold a breakout piece in place while you weld. But this can get messy when you introduce heat near that tape. Depending on the type of tape you used, just some residual heat can cause the tape’s adhesive to turn into a sticky treacle that cakes the work area. The heat resistant kapton tape may be useful here, but it seems like a waste of resources using it for this application.

The general technique I employ in the actual repair, is by melting the donor strip onto the crack of the container. Then with the broadest side of the soldering gun’s tip, I scrap the extra material into the container’s crevasses. Like a plasterer covering a brick wall; pushing the plastic deep into it’s valleys. Then once one side is completed (exterior, interior), do the same for the other side of the fissure. That’s the short of it.

One thing to note however, this type of repair is not pretty. Although it can be very effective structurally. This is due to the repair involving melting and blending away the cracks. Then adding material to help remove any residual weak points. Weak points which tend to linger after a repair that doesn’t use any donor plastic.

The main reason for using donor plastic is because (in my opinion) plastic shrinks in the presence of heat. In other words, as you repair it with your soldering iron, the repaired plastic is actually smaller than what it was prior. This means that the repaired crack areas are actually thinner than they were originally. So consequently bolstering it with donor material is often necessary. Although admittedly it does make it look awful.

  • segment alignment
  • welding tacks
  • outer side done, inner side awaiting blending
  • inner side done
  • almost watertight after just initial fill and blend
  • container holds water well, despite drip leak

Demonstration of initial fix and seal

Closing thoughts

In summary plastic objects can be repaired should you wish to do so. It’ll require a soldering iron set to a low temperature and some donor plastic of the same type. Take care not to burn the plastic, and for goodness sake don’t breath the fumes in. Also remember that ideally any repaired food safe containers, should no longer be considered food safe. (However that could just be my general paranoia speaking.)

That’s all folks. Best of luck with your plastic welding adventures.

Thanks for reading.

#0019: Creating custom cables for test equipment

Otto

#0019: Creating custom cables for test equipment

Preamble

I wanted to write a little on test cables in general, more specifically about the actual quality of the cables available at the lower end of the consumer market. Highlighting the prevalence of these types of cables, including the products that they tend to accompany. I also intend to instruct briefly how the end user can test these cables and how they can create their own superior home-made versions.

Low-end retail cables for test equipment

In my opinion low quality cables are especially evident in instances where the cables come bundled in with many lower-end chinese consumer test devices, as opposed to purchased separately. This is especially true in cases where the product is unbranded. Think cheap test equipment such as: £10 multimeters, or £50 bench power-supplies. For example the probe cables that come with the DT-830 multimeter.

To clarify I am not talking about genuinely dangerous or illegal (according to british safety law) cables, just low quality ones. Things such as chinese power cables that do not comply with the british safety standards – which are genuinely hazardous to the user and not fit for purpose; are outside the scope of this article. Genuinely dangerous cables like that run the risk of being confiscated at customs. I wrote an article on such a cable; it was a non-compliant BS1363. Link below. No, I am specifically talking about cables such as the ones on multimeter probes, and on bench top power-supplies. Inoffensive dreck.

article hyperlink: #0004-dangerous-non-compliant-bs1363-plug

Example of low quality cables bundled with multimeter

What factors dictate a cables quality?

Many times low quality wires will have a noticeable resistance value across them. This is often due to the cost cutting measures of the manufacturers. These include constructing items with lax specifications (quality control), or by saving on materials used. For example by minimising the number of actual strands of the copper conductors present within the cable, or by opting for a cheaper material substitute such as aluminium.

These cables being built to such a restrictive price-point: is what makes them very flimsy, basically disposable in many cases. Many of these types of cables are in my opinion, are merely designed to just tick the “comes with accessories” box in a products’ marketing materials.

This unfortunately makes the cable virtually without genuine use outside of the specific one that it was manufactured for. Often not even that use-case for long, due to their general fragility. So if you have some of these low quality cables in use. It may benefit you to replace them as soon as convenient; as they may be hampering your devices’ ability to perform.

For example, cheaper multimeters such as the previously mentioned DT-830 or the XL830L, are generally rather accurate. Often staying within a less than 2% error deviation from each other. However any variable resistances across their probes’ cables may affect their readings. They may for example: cause a notable voltage drop when in use, or affect the threshold for a continuity test.

A good example of a low quality cable would be: a power-supply cable that I got with my chinese bench-top power-supply. An unbranded QW-MS305D. By the by, it was actually the same unit that came with the non-compliant BS1363 plug. Never-the-less, the power-supply’s cable in question was used to power devices under test with DC voltages; as such they contain a male banana jack on one end and a crocodile clip on the other.

Although it is a relatively cheap power-supply, it could still reliably output it’s rated 30 volts at 5 amps. The only issue with the out-of-box setup (minus the bad plug), was that the output cable heated up and often got soft when exposed to the higher amperages that the power-supply could output. Although it took sometime for this to happen, i.e. it needed continuous output over several minutes. I still judge the cable as unsuitable for purpose. One thing I found humorous after the fact is that the online seller that I bought from had this same accessory packaged with the 10 amp version of this power-supply as well (QW-MS3010D).

Example of low quality cables that came with the power-supply

  • 10 amp PSU

Using salvaged materials to create custom cables

As a response to the performance of the bought cable, I retired it. I removed the cable ends and attached them to a salvaged mains cable; and it has worked fine since. I really like using salvaged mains cables for these types of applications. Especially ones from UK safety certified devices; and especially ones from heating units. Such as electric room heaters, or toasters, electric grills, kettles, etcetera. Basically anything that uses electricity to generate resistive heat. This is because their cables are specced to allow large amounts of current to pass through them without heating up themselves.

For example: a typical mains oil heater, is rated for 1500 watts. This means that it’s cables need to safely pass that amount of power across them concurrently while the device is in use. When converted to volts and amperes; this means that these cables are able to handle 240 volts AC at 6.25 amps. This leaves me confident that it can safely handle the maximum 5 amps, 30 volts DC of my power-supply over long periods of time.

They do this by having very little resistance across the cable length. This is accomplished by actually putting copper in your copper cables. Although after looking at the example photos that I have, it appears that this particular cable is actually using aluminium strands as conductors.;) However the point stands; there are sufficient conductors within the cable, that the current can pass across it unhindered. I.e. The cable is of a big enough gauge, not to bottleneck the higher currents.

One thing to pay attention to when creating your own cables beyond the quality of the cable itself; is how you connect the various plugs you wish to use to it. This is because a poor connection here can impede the passage of electricity; and add resistance to the line. I suggest firm connections with as much conductive surface area touching as practical. It is also good form to do a resistance test across the entire thing once completed.

Another good source for quality salvaged cabling, is old or damaged ethernet cables. The twisted pairs within work very well in lower voltage DC applications; including carrying signal voltages (like binary data). The pair windings are configured to minimising interference for their carried digital signals after-all.

I like to use them to make home-made breadboard jumper cables. This can be done by just unwinding a length of cabling, cutting to size, then tinning the ends so that they can interface with the breadboard. Will it pretty? probably not. Will it be functional? 100%.

You could also use ethernet cables for replacing a cable between an external DC power-supply (or power-brick) and it’s paired device (e.g. Laptop). I tend to opt to wind together a pair of cables, if I wish to reliably carry current at higher DC voltages (e.g. 20VDC @ 3A for a laptop). However any higher than that and you’d be better served by using thicker gauge wires.

Home-made custom cable for power supply

Example of resistance test used to determine cable quality

Closing thoughts

That’s all really. I just wished to highlight that some low end cables are not good, and to encourage you to create your own superior cables using parts from common household devices.

Thank you for reading.