LED Backlight of LG LP133WH2 (TL)(M2) Laptop LCD Panel

I’m pulling apart some retired laptop LCD panels. For the latest panel, I decided to work on the polarizer film first and I was encouraged by those results. I’ll probably try the polarizer first for future panels. But before I move on to the next panel, I want to get a closer look at the LED backlight from this panel I pulled from a retired Dell laptop. The label says it is a LG Display LP133WH2 (TL)(M2) module. A quick internet search says its pixel resolution is 1366×768, which is pretty low by today’s standards and not worth the effort to bring back online as a computer display.

Like many previous modules, it had tape all around. Unlike some previous modules, there are several different types of tape involved.

Peeling back the tape, I could see the backlight connector in the center. The previous few panels had them to the side. I’m not sure what design tradeoffs are involved in the different placements.

The chip footprint closest to the backlight connector is unpopulated. This is usually a sign there’s another version of the device with enhanced features, but I’m not sure how that works for a display module like this. Whatever it may be, the absent chip is certainly not the backlight LED controller.

The other chip on this side of the circuit board is labeled LG SW0641A. I’m amused that my not-helpful search results included a LG clothes washer with that model number. I’m not sure what this is, but it is definitely not a clothes washer. It is probably the main display controller that talks to the rest of the laptop.

Flipping the panel over, high density data connectors for the LCD array are visible as well as two chips.

Searching for information on a SiW SW5024, I came across vendors willing to sell them but not much else.

But that doesn’t matter, because a search for ADD 5201 written on the other chip resulted in a pointer to a “High Efficiency, Eight-String White LED Driver for LCD Backlight Applications” by Analog Devices. Jackpot!

While the chip can drive up to eight strings, it appears we only have four on this panel. I see a VOUT_LED test point that fans out to four conductors on this connector. And I also see test points corresponding to four strings. FB1 is to the left, below VOUT_LED. FB2, FB3, and FB4 are to the right. If it follows convention of other panels, VOUT_LED would be the current source and FB1 through FB4 are sinks for each of four parallel strings of LEDs.

Probing those points with my LED tester confirmed the hypothesis, and highlighted another difference on this panel. Previous panels with parallel strings of LEDs would interleave them across the bottom. With an interleaved design a single failed string would still leave most of the display illuminated. But in this panel, each of these four strings are assigned a quarter of the panel area. So if one string failed, one quarter of the display would be darkened and difficult to read. My guess is this method is easier (and cheaper) to wire as a tradeoff for fault tolerance.

Toshiba LTD133EWDD Backlight

Examining the integrated control board on a Toshiba LTD133EWDD panel I had pulled out of a Dell XPS M1330 laptop, I found no information on communicating with its integrated LED driver chip so I’m going ahead with the backup plan of seeing if I could drive the LEDs directly. [NOTE: This was written before Randy commented with a link to the datasheet.] First order of business was to remove the LCD pixel array in front of the backlight. A marvel of miniaturization in its day, now I am no longer interested in its 1280×800 pixel resolution.

A thin strip of black tape around all four edges held the glass sheets in the frame. I admire how it precisely mated up against the edge of the polarizer film. It was either applied by a machine or hands of great skill.

Once the tape was removed, the glass LCD array was held only by the high density data connectors. Before I peeled them off, I noticed one item of interest: I see alignment marks that I don’t recall seeing on previous LCD arrays I had peeled off in this way.

While peeling off the high density pixel data connectors, I was reminded that glass is fragile and easily crack when abused.

Once the LCD pixel array was removed, I returned to examining the LED backlight connection. I see an eight-conductor connector, but only seven wires in the flexible cable. One of which is thicker than the rest. Based on the experience so far, my first guess are six LED strings in parallel sharing a common current supply but with six individual current sinks.

Flipping the circuit board over, I found several sets of six pads on the board. The circular pads look like they were designed for pogo pins on a test rig. The rectangular pads look like they are provisions for decoupling capacitors that were never installed. We could see the top row of six are all connected, consistent with a common supply. The bottom six each correspond to the six current sinks on the connector.

One interesting novelty was that while the leftmost four connectors are strictly in order, the fifth and six conductors were swapped. So if we count the small vias connecting to the connector on the other side in left-to-right order as 1,2,3,4,5,6. The rectangular pads are in the order of 1,2,3,4,6,5. Perhaps this was merely done to make PCB routing easier, but I’m curious if there’s a more profound reason.

Another interesting item of note is the surface mounted switch immediately to the right and below these pads. This switch would not have been user-accessible in the laptop. Even a servicing technician would have to peel off a plastic protective layer before this switch could be flipped. What does it do, and why is it important enough to take the hit of manufacturing parts cost and complexity? I can smell a story here but not enough interest to chase it down.

In my past LED backlight salvage operations, my next step would be to solder some wires to these points and determine the electrical properties of this string. But now I am armed with a dedicated LED backlight tester, and I could put it to work and probe these points directly. There are indeed six parallel strings of 10 LEDs per string. They each drop approximately 32V at 20mA. This is information I could have determined without the dedicated tester, but having the right tool made the job quick and easy.

Once I had these backlight details identified, I could store it away for future project. But it also had some auxiliary items I should take care of first, like a nice metal frame.

Dell XPS M1330 LED Backlight

My detour into laundry machine repair pushed back my LED backlight adventures for a bit, but I’m back on the topic now armed with my new dedicated backlight tester. The next backlight I shall attempt to salvage came from a Dell XPS M1330. This particular Dell product line offered an optional NVIDIA GPU packed into its lightweight chassis. Some engineering tradeoffs had to be made and history has deemed those tradeoffs to be poor as these laptops had a short life expectancy. In the absence of an official story from Dell, the internet consensus is that heat management was insufficient and these laptops cooked themselves after a few years. I was given one such failed unit which I tore down some years ago. I kept its screen and the laptop’s metal lid in case I wanted a rigid metal framework to go with the screen.

The display module itself was a Toshiba LTD133EWDD which had a native resolution of 1280×800 pixels. Not terribly interesting in today’s 1080p world. Certainly not enough motivation for me to buy an adapter to turn it into an external monitor, and hence a good candidate for backlight extraction.

Unlike the previous LCD modules I’ve taken apart, this one doesn’t cover its integrated control board in opaque black tape. Clear plastic is used instead, and I could immediately pick out the characteristic connection to the rest of the display. At the bottom are two of those high density data connections for the LCD pixel array, and towards the right is an 8-conductor connector for the LED backlight. The IC in closest proximity is my candidate for LED backlight controller.

Despite being clear plastic, it was still a little difficult to read the fine print on that chip. But after the plastic was removed I could clearly read “TOKO 61224 A33X” which failed to return any relevant results in a web search. [UPDATE: Randy has better search Kung Fu than I do, and found a datasheet.] Absent documentation I’m not optimistic I could drive the chip as I could a Texas Instruments TPS 61187. So I’ll probably end up trying to power the LEDs in the backlight directly.

Backlight LED Tester

I have successfully salvaged LED backlight diffuser assemblies from three different LCD screens. It gave me the confidence to attempt pulling the backlight out of other LCD screens in my pile of less-used and broken electronics, in the expectation that diffuse white light sources will be more useful than low resolution displays. But before I start merrily tear more panels apart, I wanted to address one particular pain point: deciphering mystery LED strings.

Only one of the three examples so far gave me an easy time, where the LED backlight power planes were clearly marked with + and -. The other two had multi-conductor cables that required a little decoding to find a common anode and cathodes for individual strings, and sometimes a few conductors remained mysterious. I had been doing this work by spending a lot of time probing with a multimeter, then soldering wires to test points, and cautiously putting power on those lines with a bench power supply. This has worked so far, but I knew there was room to make this process faster for future panels.

Enter the dedicated LED backlight tester.

I wanted something that could put current-limited power over a set of probes. This would let me probe LED strings directly in a single-step versus my current multi-step workflow of multimeter, then soldered wires, then bench power supply. I considered buying a set of probes that I can connect directly to my bench power supply, but a quick search for dedicated LED testers found them quite affordable and I made the jump to try one.

Looking over several options on Amazon, I decided to try a SID LED KT4H(*) because it advertised a few extra features I thought might be useful enough to worth the extra cost. It has household AC input so I don’t have to worry about a separate power supply or batteries. Separate numerical displays for voltage and amperage allows me to read both metrics simultaneously. Simple dials for current and voltage limits make the user interface simpler than designs that use infuriating combinations of unintuitive button presses. There’s a switch to toggle between two current limits: the one set by the dial, and 1mA for testing purposes. This is much better than turning the current limit knob back and forth. And finally, it can also test the other side of the system: whether the device’s constant current supply is putting out any power at all.

The package also included a convenient carrying case, in the form of a generic First Aid Kit zippered fabric bag. Not the fanciest branding, it gave me a chuckle, but it should be quite sufficient.

The probes had sharp tips more than precise enough to hit the kind of test points I had been probing with my meter.

For a quick familiarization run, I used this tester on a single 5mm through-hole LED and saw it light up dimly in the 1mA test mode and brightly with current limit set at 20mA. Then I moved on to illuminate the recently-liberated backlight from an AU Optronics B101EAN01.5. A nice feature is that the current limit ramps up gradually: there is a slow (2-4 seconds) ramp-up as the device seeks the correct voltage level to deliver 20mA. In comparison to my bench power supply which will snap to a voltage almost instantly. I’m optimistic the slower ramp-up will prove valuable.

Before I could put this tester to work, though, life threw me a curveball and I had a broken clothes dryer to fix. The LED backlights will have to wait a bit.


(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

The Great Backlight Liberation Begins

There’s a small market in LCD panel controller boards. When we salvage a panel from a retired laptop, we can enter its model number into eBay. If the panel is used in a high-volume laptop (for example, one of Dell’s consumer Inspiron laptops. Or most Apple MacBooks) then someone is likely selling a driver board that accepts HDMI input and translates it into the signal to control a panel’s integrated electronics.

I had salvaged the panel from one of my old Dell Inspiron laptops and, buying one of these adapter boards for $50, converted it to an external monitor. Eventually it became the onboard screen for Luggable PC Mark I. (Mark II used a commercially available monitor, and stories on both are available here.) A few months after that adventure, I received another retired Inspiron laptop and used its screen for the Portable External Monitor project.

I have several more LCD panels salvaged from retired laptops, but I don’t need very many external monitors. Furthermore, resolutions on these panels were lower than 1920×1080 limiting their utility. It’s hard to justify spending $50 for a circuit board to hack-up converted laptop screens when a Full HD 1920×1080 desktop monitor can be had for about $100.

I had thought it might be interesting to build my own LCD interface boards, but there are several obstacles. One is the requirement to build connectors to carry high-speed raw pixel information, which is tricky to do correctly given the high bandwidth translating to low tolerance for sloppy work. The LVDS (low voltage differential signaling) system doesn’t connect directly to typical microcontroller output pins, but translator chips are available and some task-specific microcontrollers have integrated LVDS output.

But the biggest hurdle against building my own boards is documentation. The protocol carrying that high-speed raw pixel information is a mystery. Counter to popular electronics convention, datasheets for panels aren’t distributed freely online. Many of them are proprietary and difficult to get, others lie behind paywalls. It makes sense to pay if I’m designing a device using millions of panels, but it doesn’t make sense if I’m fiddling with a single panel.

So those salvaged LCD panels have sat in my workshop, gathering dust waiting for a new purpose in life. Now that I’ve successfully extracted and illuminated the backlight module from three different LCD panels, I believe I have found that new purpose. Utility of extra monitors quickly pass a point of diminishing returns, but gentle diffuse white light sources are far more useful in wider variety of settings. No need to buy $50 driver board makes this a lower cost project, combined with higher utility of diffuse white light sources means the great backlight liberation begins.

AU Optronics B101EAN01.5 Backlight Power

I’ve pulled the LED backlight illumination panel out of an AU Optronics B101EAN01.5 LCD panel, which was in turn salvaged from an Acer Aspire Switch 10 (SW5-012) tablet/laptop convertible. I want to see if I can get it to light up. Using my multimeter I found test points corresponding to all six control lines on the backlight, and soldered wires to all of them. The next task is to determine what these wires are.

The other end of those wires were crimped and assembled into a six-pin connector with 0.1″ spacing. I first arranged them in whatever happened to be convenient, but then I changed my mind and rearranged them to be in the same order as that on the backlight cable. From top to bottom: Power, ground, FB1, FB2, FB3, and FB4.

This gave me something suitable for breadboard exploration. I have two hypothesis about what the FB connectors are. Since power and ground were already identified, I thought maybe these are control lines (gates) for MOSFETs in line with each string, which implies I could turn on a LED string by pulling its signal high. But if I look at precedence set by the LG LP133WF2(SP)(A1) panel I took apart earlier, these could be four current sinks for four LED strings.

To test both concepts simultaneously, my breadboard exploration wired one string to a pull-up resistor in case FB is a MOSFET gate, and another string to a pull-down resistor in case it is LED current sink.

I started seeing a dim glow when I turned the power supply up to 17V. To determine which hypothesis was correct, I removed the pull-down resistor and it went dark. So FB1 through FB4 are current sinks for four strings. As a double-check, I calculated voltage drop across the pull-down resistor and calculated the current flow to be 1.4mA. This is far too high for a MOSFET gate but completely consistent with current-limiting resistor for a dimly lit LED.

Hooking up a current-limiting resistor to each of FB1 through FB4, the backlight has dim but usable illumination starting at about a 15.6V drop across an individual LED string. Whenever I find a project for this light, I will need to either solder more permanent current-limiting resistors, or find an intelligent LED controller with a more efficient current-limiting control scheme.

There is one remaining mystery: If the VOUT wire is voltage source, and FB1 through FB4 are current sinks, why is there a line connected to the ground plane on the control circuit board? It feels like there’s another aspect of this backlight I have yet to discover. Or possibly destroyed by clumsy overvoltage on my part. Either way, it doesn’t seem to be critical for illuminating this backlight, so I’ll leave that mystery for another day.

AU Optronics B101EAN01.5 Backlight Wiring

I have a broken Acer Aspire Switch 10 (SW5-012) that I have taken apart. Among the pieces I salvaged was the screen, an AU Optronics B101EAN01.5 whose 1280×800 resolution is not terribly interesting in this era when even cell phones have higher resolution displays. So I decided the most interesting thing to do is to liberate its LED backlight for potential future projects.

The backlight connector has six visible conductors. Two conductors are wider than the rest, which imply power and ground to me. There is a test point labeled VOUT adjacent to this connector, and my meter confirms it corresponds to the topmost wide conductor. The meter also confirmed the second wide conductor has continuity to the ground plane of this circuit board, so power and ground confirmed.

What does that mean for the four remaining thin conductors? Looking around the backlight control IC, I looked for a likely group of four test points and found FB1, FB2, FB3 and FB4. Meter confirms they correspond to the remaining four conductors on the backlight cable. “FB” probably doesn’t mean Facebook in this context, but I’m not sure what it would represent. I’m just glad they were numbered.

As for the backlight control IC itself, the large AUO letters say it is something AU Optronics produced for internal consumption. The earlier LG panel project found a TI TPS61187 chip with publicly available documentation, but here I found no documentation for an AUO L10716 controller. Since the chip is so tiny it’s pretty probable I’ve misread the numbers, but no search hits on the variations I could think of either: LI0718, L10216, etc. If I had found a test point labeled PWM I would be tempted to see if I can get it running with an Arduino PWM signal, but I saw test points labeled SCL and SDA telling me this is an I2C peripheral and my skill level today isn’t good enough to reverse engineer it without official documentation of its I2C protocol.

So instead of trying to interface with the existing backlight control chip as I did on the LG backlight, here I will interface with the backlight LEDs directly. I found test points corresponding to all six wires on the backlight connector and soldered wires to all of them. Then I used hot glue to help hold them down and relieve strain, as I don’t want to lift a pad again!

With the wires securely attached, I need to figure out what they actually do.

Acer Aspire Switch 10 (SW5-012) Backlight Removal

While I took apart the base unit of this dead Acer Aspire Switch 10 (SW5-012) tablet/laptop convertible, the main display had been left under the punishing direct heat of southern California summer sun. I don’t like fighting glue, but heat at least helps reduce their tenacious grip. I pulled out my prying tools from iFixit and plunged into the seam between gray and black plastic surrounds holding its AU Optronics B101EAN01.5 screen in place.

The double-sided adhesive foam tape was thickest around the left and right sides, gripping tightly enough that I broke the frame on both sides trying to peel them off. There were slightly less of it across the top, and surprisingly little across the bottom.

I had hoped the LCD module would pop free once the touchscreen digitizer glass had been freed from its frame, similar to how an Amazon Fire tablet was put together. But no such luck, there appears to be more adhesive involved.

Once I pushed a pick into the gap between the digitizer glass and the LCD polarizer, I realized the bad news: they have been glued together across the entire visible front surface of the screen. It’s going to take a lot of effort to separate them and I don’t see how it could possibly be worth the effort.

My objective here is the LED backlight, so just as I did for the Chromebook cracked screen and the Amazon Fire screen, I peeled back the black tape holding the LED backlight to the LCD. Starting with the bottom section to expose the integrated driver board.

I am starting to recognize the signs of a LED backlight power connection: a few connectors separate from the high-density connectors used for controlling LCD pixel data. An inductor and a diode for voltage boost conversion, and an IC controlling it all.

The black tape holding this display module together is much more difficult to remove than those encountered on previous screen backlight salvage projects. The glossy substrate is weaker than the adhesive, causing it to easily stretch and break. Now that I’ve identified the portion I cared about for my project, I pulled out a blade and cut the rest of the tape allowing me to open up this display module.

Once the backlight folded away from the LCD pixel array, I can see I’ve already cracked at least one LCD glass layer in my effort to pry it from the front digitizer glass. I’m not even going to try to salvage the polarizer filter from this one, so my blade continued its work cutting all pixel data lines to free the backlight for further examination.

Amazon Fire SR043KL Backlight Layers

I took apart an Amazon Fire tablet (SR043KL) retired by cracked touch digitizer glass, seeking to salvage its display backlight and I was successful. I am fascinated by the optical behavior of modern LED backlights, even those used in products with a low price target like this tablet. After fussing with light diffusers for my Glow Flow project, I have a great deal of appreciation and respect for how evenly these backlights distributed their LED light.

I had a lot of time invested in the earlier LG laptop backlight project and was timid about fully exploring all its backlight layers, fearing that I would break something. Now that I have a smaller backlight with lower stakes, I’m going to take the layers apart and see how they act and interact with each other.

At first glance the layers for this backlight are arranged slightly differently from the LG laptop backlight. I’m too new into this field to guess what tradeoffs are involved. What I do know is that the bottom-most layer on the Fire backlight appears to be non-removable. When acting alone, I could see a dotted pattern almost like dithering.

Above this layer is a sheet of smooth matte translucent white that I would have expected to be the top layer, but here it is.

When in place, it blended the dotted pattern together into something smoother. I think this looks great as-is, but we have two more layers to make it even better.

The third layer looks wild, with the optical characteristics I associate with Fresnel lenses and lenticular lenses, but this pattern looks different and I wished I knew the right name for it so I could read more about it.

When installed, it imparted a bit of pattern along with a rainbow-like sheen.

The fourth and final layer also has that optical property, but dialed back a bit. It also has a matte top finish similar to the second layer.

When in place, we have our backlight, providing an impressively even illumination across the entire area with all light provided by a row of LEDs on just one edge.

Speaking of those LEDs, I count eighteen of them. Given that they start illuminating at around fifteen Volts, my guess is that we’re looking at three parallel strings of six LEDs each. I don’t have anything to accurately clamp current at 3*20=60mA (my bench power supply current limit is only guaranteed to be +/- 10mA) but I estimate that would be somewhere near eighteen volts which makes this barely over one watt at maximum brightness. Pretty neat!

I’m setting this aside for later use. Emily Velasco has said she has a project idea that might make use of a small backlight, so it might go to her instead. If it does, I’m sure we’ll get something really weird and cool out of it because that’s what Emily builds. But in case this backlight isn’t what she needs, I can salvage others so we have alternatives.

Amazon Fire SR043KL Display Disassembly

I have taken apart an Amazon Fire tablet (SR043KL) and retrieved the prize I sought: an intact display assembly under the cracked digitizer glass. Though presence or absence of cracks in the LCD wouldn’t have mattered for my project anyway. My objective is actually the backlight behind it.

Just like the LG laptop display I disassembled earlier, this display module is held on all edges by thin precision black tape. Peeling back the tape, I had hoped to find a LED backlight driver as I did on the laptop display, but not this time. There are a few small passive components here, but the backlight driver must be on the mainboard hidden under one of those metal shields.

Lacking an easily accessible LED driver, the next objective is to hunt for the backlight LED circuit itself. I expected them to be the largest traces relative to the other components, and I see two exposed contacts already labelled with + and -. Hmm… could it be that easy? I could do a quick test: since these two points were already exposed, soldering some wires to them were straightforward.

In order to see if the LEDs glow, I peeled back more of the tape. Slowly increasing the voltage, I started seeing a glow at around 15V. Wow, it’s really was that easy.

I have no idea how to drive this LCD array, and I have no intention to learn. My objective for today is the LED backlight. So after I peeled away all black tape around the perimeter, I sliced the high density LCD pixel data ribbon in order to separate the two parts.

There isn’t much more to be said about the LCD array. I was able to peel off the polarizer film, this time without cracking any glass, but using acetone to clean off the adhesive once again caused the film to disintegrate. That’s two strikes against acetone, I’ll have to try something else next time.

I have to put more thought into polarizer film recovery, but that’s only a mild distraction from my fascination with the backlight and its sheets of optical magic.

Laptop Backlight Is Now Webcam Light

I have successfully salvaged the backlight module of a LG LP133WF2(SP)(A1) laptop LCD display, which meant in addition to all the lessons I learned along the way, I now have a rectangular ~15″ diagonal LED panel that can emit diffuse white light. What do I do with this light? I looked around at places in my life where I felt I had a lighting challenge, and the most relevant issue in these pandemic times is my webcam for video calls.

Right now my primary workstation is in a room with decent sunshine during the day but only a dim overhead light at night. Resulting in grainy video as the camera struggles to capture limited light, and the position of the light also cast some unfortunate shadows. There is a far stronger light in the room, but it is set up to illuminate my workbench behind me. If I forget to turn that light off during a video call, I can immediately tell there’s a problem because I turn into a silhouette on camera. What I need is a light behind the webcam, which is something I can easily buy. There’s an entire product category for this usually in the form of a ring that surrounds the camera. What I have on hand is a rectangle and not a ring but I still want to try it. To test this idea I’ll need a way to mount the panel on top of my computer monitor.

Since this is supposed to be a quick test, I didn’t want to go full out with CAD and 3D printing. I pulled some cardboard boxes out of the paper recycle bin and happily started cutting with my Canary cutter. It was a highly iterative trial and error process and after a few hours I had a cardboard contraption that held the panel above my monitor, sitting on its top edge.

This top edge mechanism was the trickiest part of the design as it needed to be strong enough to hold the weight of the entire assembly. This assembly was heavier than I had originally planned because I didn’t foresee the very obvious fact the panel would make the assembly top heavy unless I added a counterweight (in the form of a large lithium polymer battery pack) sitting below the lip in order to drop the center of gravity. This is too much weight for just cardboard to hold against, so I had to pull in some plastic to help. But still no 3D printing: I cut up an used-up Starbucks gift card into an inverted U shape to give me the necessary strength at this key junction on top of the monitor.

This is definitely not the final design. I want to move the panel lower and further away from my face so it is directly behind the camera instead of above it. I chose the current panel height because I needed to be able to reach the brightness adjustment knob mounted in the lower left. After I put this box together, I realized I could have easily rotated the panel 180 degrees so the knob is in the top right corner instead of the lower left, allowing me to sink the bottom edge below the camera. Alternatively, I could have the brightness PWM adjustment module as an external module mounted elsewhere.

So that is the first and most significant change I want to make for the next iteration, but I’ll use this cardboard first draft for a little while longer and see what other issues I might want to address. In the meantime I proceed to the next backlight exercise with an Amazon Fire tablet.

Installing Arduino Circuit, Round 2

I have a small circuit board to generate a PWM signal that tells the TPS61187 LED driver chip how bright to illuminate the LED of a backlight I salvaged from a cracked LG laptop LCD screen model LP133WF2(SP)(A1). It’s not the most compact thing I could have built, but it was simple and quick. Or at least it was supposed to be quick, because my first attempt at installing it destroyed a solder connection to the screen control board and I had to redo my soldering joints and secure them with hot glue in the hopes I wouldn’t destroy any more solder connections.

Now I’m installing the circuit board again, and I realized I forgot a very important detail: The location I wanted to mount this thing is on the metal frame of this backlight, because I didn’t want to block any light that might emit from the plastic back side of the panel. My circuit board had many soldered connections on the bottom. Putting soldered connections on a metal plate causes short circuits! Fortunately I realized this before destroying anything.

Adding to the bulk of this project, I placed a sheet of clear plastic packing tape as the first layer of insulation, followed by two layers of double-sided foam tape to raise it off the first layer. The foam tape wasn’t as secure as I had hoped, so I warmed up the hot glue gun again to squirt out some secure standoffs. Thanks to the first layer of clear packing tape, I’m semi-confident I can replace this with a different PWM generator if I decide to do so in the future. But for the moment I have completed all electrical work for this light panel that I can power off USB. A happy end result of a lot of very useful and valuable electronics lessons learned building this project. From reading datasheets and their schematics to figuring out what to do when things go wrong.

But the happy result does have one downside. When I have a failure, I can dispose of the pieces after thanking them for their valuable lessons. But when I have a success, I can’t just throw it out! So now I have a ~15″ diagonal rectangular LED light and I need to think of something useful to do with it.

Need Backup Plan For TPS61187 Interface

I had thought I was near the finish line for my backlight revival project, but then I tugged on one wire a little bit too hard and destroyed the circuit board test point I had soldered to.

This is bad, and it got worse. As I tried to gently unwind this LED_EN wire, it was not gentle enough and the soldered points for Vin and GND started unraveling as well. For those two wires I had soldered to either end of a (relatively) large surface mount decoupling capacitor bridging those two voltage planes, because the tops of these capacitors presented a metallic surface area for me to solder to. I had thought they were metal end pieces, but they were actually a thin layer of metal that I just learned would peel under stress. The good news was that I was able to melt the solder and remove those wires before they did any permanent damage, the bad news is that I’ll need another approach for these connections as well.

Since I’ve already proved to be clumsy with three out of four wires, I pulled out my hot glue gun to better secure my solder points starting with my PWM wire that still remains. I dropped a dollop of hot glue and, as I pulled my hot glue gun away, I felt the now-dreaded “pop” sensation in my fingertip holding the wire in place. I think I just lifted the PWM copper pad, too! Fortunately, my meter said I still had electrical continuity. So even if I did lift the pad, it is still connected, held in position by my drop of hot glue. But just in case I needed it, I probed around the board and managed to find a backup location for the PWM signal next to the main display control chip.

Backing up further, I found another test point to the left of the LED_EN test point I destroyed. It was labeled VLED and it was connected to the backlight supply voltage line. As fragile as these test points have proven to be, I think they’re still better than the end of a surface mount capacitor. I’ll use this one and hope I don’t rip this one out as well. Finding a replacement location to solder to the ground plane was easy, as the entire circuit board shared a common ground plane and I had many choices for ground. Including the metal housing of the now-unused data connector that formerly connected to the rest of the laptop. So I’m not worried about a ground connection, I have a plan C, D, E, F, etc. For now I found what looks like a test data bus will use the ground pad for that.

Which leaves me with the original problem: the LED EN pad that I’ve destroyed. I had no luck finding another test point, and while I expect it is connected to one of the pins on the main LG Display ANX2804 chip, I couldn’t find a good contact point for that either. Then I remembered the TPS61187 datasheet, where it said it was valid to connect EN to VDDIO which will cause the chip to be enabled whenever it receives power on Vin. From my notes probing the components around the chip, I knew there were some surface mount components adjacent to each other. They are tiny, but I was able to get a short length of wire to solder across those two components. Since I won’t be tugging on this wire, I should be OK here.

After I verified the VDDIO and EN pins are now connected, I realized there was another way: Since these two pins are adjacent to each other on the TPS61187 itself, a blob of solder can theoretically bridge those two pins right on the TPS61187. I’ll keep that in mind as a potential plan C if I should need it.

To minimize the chances I’ll need any of those backup-to-the-backup plans, I became very generous with my hot glue application. I sincerely hope I won’t have to take this apart again, because I don’t see how I can undo all this hot glue without destroying these solder points.

Fortunately I wouldn’t have to undo anything because my second attempt at system integration was a success.

Installing Arduino Circuit Caused Setback

I didn’t understand why I couldn’t pull USB power through the existing jack on my Arduino Nano, but I was willing to create a small circuit board to wire up VUSB directly as a workaround and move on. I originally soldered two 0.1″ headers next to each other for power and ground, but the first test run instantly pulled those wires out of the socket. So I wired up JST-XH connector for that beheaded USB cable instead. I wanted a connection more mechanically secure than the generic 0.1″ headers and towards that goal I used JST-XH 4-conductor connector. Even though I needed just two conductors, I wanted the wider connector for two reasons. (1) I hoped a wider connector will latch more securely, and (2) I was running low on 2- and 3- conductor connectors in my assortment box. (*)

Next to the power input connector is the potentiometer(*), now soldered and fixed to this perforated prototype board instead of dangling off somewhere via wires. I plan to mount this board on the sheet metal backing of the light, near the lower left corner so the knob for this potentiometer can be accessible.

Next we have the two rows used for seating an Arduino Nano. Even though I’m only using four pins, I soldered all the points on these two rows so this header will sit securely. I had originally thought I would run wires around the outside of these headers, but it turns out I could put all the wires, resistors, etc. in between these two rows so I did that. I doubt it makes much of a cosmetic difference.

And finally, the star of the show, my four-conductor connector to the wires I’ve soldered to various points on the LG LP133WF2(SP)(A1) LCD panel control circuit board. The connector is standard hobbyist stuff, relatively large and easy to work with for my projects. But the other end of the wires soldered to points on the control circuit board which were quite a bit smaller, so I had been concerned about the strength of my soldered joints. And when I lifted the connector to plug into my newly created perf board, I heard a “pop” and knew instantly that was bad news. I had destroyed the LED_EN connection. It was intended as a test point so it was small, but I had soldered to the tiny circle of copper and handling this circuit placed too much stress on this connection. The wire I added ripped off the copper pad, leaving non-conductive (and non-useful) bare circuit board material behind. This is not good. I need a backup plan.


(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

Arduino Nano Failed to Power Backlight via USB

It was fun to look at a revived LED backlight module, salvaged from a LG laptop display panel model LP133WF2(SP)(A1). It was controlled from a breadboard Arduino, and powered by my bench power supply. I’m still unsure what input voltage it was originally designed for, but it seemed to run well at 5V. When I turned brightness up to maximum, the bench power supply reported 1A of current draw. As a 5W LED light, it does feel approximately in the same ballpark as the 7W LED bulbs serving as 60W incandescent bulb replacements. But with the key and very valuable difference of the fact its light is evenly distributed across a much large area for a softer more diffuse light.

While I’m at it, I measured the electrical behavior of these LED strings. This is mostly for reference since the TPS61187 chip handles adjusting these voltage values to keep electricity flowing at the target current. When it sees a very minimal PWM signal, I measure the voltage drop from anode to ground to be roughly 15V and the panel is not visibly illuminated at this low level. When I turn the duty cycle up high enough to see just a little bit of visible illumination, the voltage differential has climbed to 24V. At max power, I measured about 28V. This was all generated by the onboard boost converter from a 5V input signal. In my experience white LEDs drop roughly 2.7-3V at full power, so these values are consistent with parallel LED strings of either nine or ten LEDs per string.,

Since this circuit seemed to run at 5V, I thought it would be fun to convert this to run on USB. The Arduino Nano was designed to run on 5V and already had a handy USB jack, and most portable USB power banks can supply 5V@1A or at least they claim to. When I hooked up the wires, it was able to illuminate up to a certain level. But beyond that level (roughly 1/4 to 1/3 brightness) the lights started flickering in a classic sign of power instability. Oops. What went wrong?

Whenever I see potential sign of power instability, my first reaction is always to perform the Big Honkin’ Capacitor test. Find the biggest capacitor I have handy, connect it across the power input terminals, and see if that solves the problem. In this case, the big capacitor failed to soothe the system.

Digging into schematics for an official Arduino Nano, I see that the VUSB line is not directly connected to the +5V output pin. There are a few components in the way, relating to power control and regulation. The Arduino Nano could be powered via its VIN pin. Following Arduino Uno barrel jack precedence, the input voltage is usually recommended to be 9V. When this happens, there’s a diode presumably to make sure that 9V does not feed back into the USB +V line. There are also several capacitors in parallel with VUSB but they should help rather than interfere with any instability.

Mystified as to why I couldn’t power the backlight via this Arduino Nano’s USB jack, I wanted to isolate the problem. See if the problem lies within the Arduino Nano or with my USB power bank. I took a USB cable and cut off its a damaged micro-B connector. Splaying out the wires, I found VUSB and GND wires, and I connected that to the Arduino Nano circuit. With this arrangement, my backlight module is happy all the way up to full brightness with no flickering problem.

Something about this particular (non-genuine) Arduino Nano module is causing interference, and I don’t understand why, but at least I have a workaround. That’s enough for me to ignore this weirdness today and proceed with my backlight project, even if there was a temporary setback.

A Closer Look at LED Backlight Panel

I’ve successfully interfaced with the existing TPS61187 driver chip on the circuit board of a LG laptop display panel LP133WF2(SP)(A1), and brought the backlight module back to life. Given all the new territory I had to explore to get this far, I was very excited by a successful initial test. After I was able to calm down, I settled down to take a closer look at its physical/optical behavior.

Since I tested it face-down, the easiest thing to look at first are the backsides of the LED strip. Most of it is hidden by the sheet metal frame from this side, but from earlier examination I knew there was even less to see from the front. Once illuminated, we can see the structure inside the light strip. The yellow flexible segment that connects to the green circuit board isn’t a separate piece like I thought earlier, it is actually all a single sheet of flexible circuit. All the LEDs are mounted on it, and they are located at the very bottom edges of the screen. I knew the lights themselves had to be very thin and well hidden right up against the bottom edge, but I couldn’t figure out where the wiring would go. Now we can see all electrical wiring runs above the LEDs, and when we look at it from the front we can see it as a thin strip of light gray along the bottom.

I had been worried that the illumination would be compromised because it is working without some of the friends it had earlier. The backside used to have a laptop lid that would have helped reflect and diffuse light. And the front used to be up against the LCD pixel array, which was backed by a mirror finish that would have also helped reflect light around.

I need not have worried. It was quite evenly illuminated and, as seen in the wire shadow picture above, there are no distinct spotlights marking location of individual LEDs.

I also wondered if the surprisingly complex four-layer diffuser required precise alignment with the LEDs in order to do their light distribution magic. They are no longer pressed by the LCD pixel array into a tight space, but happily they still worked quite well. While they worked visibly best at certain positions, the falloff is graceful. Not like aiming a laser at precision optics. Now I’m even more impressed by this stuff performing magic with light in ways I don’t understand.

But one thing I do understand is that they look thin and quite fragile. Designed to sit behind a LCD panel of multiple glass layers and without that, these layers of magical optical sheets flap around. I looked around and found a piece of 3mm clear acrylic that is nearly the perfect size and taped it to the metal backing chassis. The acrylic is far thicker than the LCD glass sandwich used to be, but it is also more rigid, so that’s a good tradeoff.

The final comparison I wanted to make before moving on is: how bright is the backlight alone compared to the full backlight plus LCD screen? I placed this backlight, turned brightness all the way up high, and set it side-by-side with the intact replacement screen still serving display duty in the Chromebook. I then turned on the Chromebook and increased its screen brightness to maximum setting.

I don’t have light level measurement instruments to obtain an objective number, but this picture makes it quite clear there is a dramatic difference in brightness. I knew some light would have been lost within the layers of a LCD panel, but it’s fun to see firsthand it’s far more than I had expected. This really drove home why alternate display technologies with self-illuminating pixels (OLED, micro-LED, etc.) can offer much brighter pictures than a backlit LCD could. My salvaged backlight is plenty bright running on just 5V, but running it on USB took more effort than expected.

Arduino Nano PWM Signal for TPS61187 LED Driver

Trying to revive the LED backlight from a LG Lp133WF2(SP)(A1) laptop display panel, I am focused on a TPS61187 LED driver chip on its integrated circuit board. After studying its datasheet, I soldered a few wires to key parts of the circuit and applied power, checking the circuit as I went. Nothing has gone obviously wrong yet, so the final step is to give that driver chip a PWM signal to dictate brightness.

This is where I am happy I’ve added Arduino to my toolbox, because I was able to whip up a controllable PWM signal generator in minutes. Putting an Arduino Nano onto a breadboard, I wired up a potentiometer to act as interactive input. 5V power and ground were shared with the panel, and one of the PWM-capable output pins was connected to the TPS61187 PWM input line via a 10 kΩ resistor as per datasheet. I found that my enable line already had a 1 kΩ resistor on board, so now I wired enable directly to the 5V line.

Since I wanted some confidence in my circuit before plugging the panel into the circuit, I also wired a test LED in parallel with the PWM signal line. I had originally thought I could use the LED already on board the Arduino, but that is hard-wired to pin 13 which is not one of the PWM-capable pins, so the external LED was necessary for me to run my PWM-generating test code, which thanks to the Arduino framework was as easy as:

int sensorPin = A0;    // select the input pin for the potentiometer
int ledPin = 3;        // select the pin for the LED
int sensorValue = 0;   // variable to store the value coming from the sensor

void setup() {
  // declare the ledPin as an OUTPUT:
  pinMode(ledPin, OUTPUT);
}

void loop() {
  // read potentiometer position
  sensorValue = analogRead(sensorPin);

  // map analogRead() range to analogWrite() range
  sensorValue = map(sensorValue, 0, 1023, 0, 255);

  analogWrite(ledPin, sensorValue);
}

My external test LED brightened and dimmed in response to potentiometer knob turns, so that looked good. My heart started racing as I connected the panel to my Arduino breadboard, which is then connected to my benchtop power supply. Even though I’m powering this system with 5V, I used a bench power supply instead of a USB port. Because I didn’t know how much the panel drew and didn’t want to risk damaging my computer. Also, by using a benchtop power supply I could monitor the current draw and also set a limit of 120mA (20mA spread across 6 strings) for the first test.

I powered up the system with the potentiometer set to minimum, then slowly started turning the knob clockwise…

It lit up! It lit up! Woohoo!

I was very excited at this success, jumping and running down the hallway. It was a wild few minutes before I could settle down and calmly take a closer look.

Soldering Wires to TPS61187 LED Driver

After passively studying its documentation, and passively studying how it is installed on an existing circuit board, it is now time for me to go active and start working on it. Whether I can get this backlight up and running is almost secondary at this point, it has already been a great electronics learning opportunity and I want to see how far I can get.

This next step tests my skill working with components far smaller than what I’m used to. The picture of these added wires spoke volumes: I used the finest spools of wire I had on hand, but 26 gauge wire looks absolutely gargantuan when soldered to this board. Due to their small size I assumed these surface mount components would not have the strength to handle external stresses. As a temporary measure I used a piece of tape to hold the wires in place, hopefully diverting all the little twists and tugs yet to come as I connect and disconnect these wires to power and signal sources.

Once the wires were in place, I had to make a very important decision: what voltage do I feed into the Vin wire? Earlier probing failed to find the values of resistors used in the boost converter feedback regulation circuit. If I had those resistance values I could hoped to calculate the expected input voltage range using the formula in the datasheet. The only other guideline I had was the requirement that input voltage must be lower than the voltage drop across individual LED strings so the boost converter (critical part of this circuit) can function.

Looking for hints elsewhere, I reviewed my earlier notes looking inside this machine. Its battery is labeled as a lithium-ion pack with a nominal voltage of 10.8V, implying three cells in series. This is within the valid input range for a TPS61187 chip and I thought it is possible they would wire the battery voltage straight through. This would avoid any conversion losses from a boost or a buck converter along the way.

Following my startup plan, I used my bench power supply to put 10.8V on Vin and was encouraged I didn’t see any sparks or smell any smoke. My probe saw 5V on VDDIO which I took to be a good sign and satisfies step 1. Moving on to step 2, I checked enable (EN) pin to find it is low, so nothing else on the board is raising it. I used a one kilo-Ohm (1 kΩ) resistor to pull EN high and it did, so either nothing else on the board is pulling it low or 1 kΩ is enough to overrule their signal. If something is fruitlessly trying to pull it low, the few milliamps involved here should not damage it. Or if I do damage it, hopefully it wouldn’t be anything I care about.

If I understand the datasheet correctly, once enabled the TPS61187 will put minimum voltage across the LED strings. I probed the voltage between LED common anode and ground, and found it was 6.7V, which is lower than the input voltage of 10.8V, exactly what the datasheet said not to do. Oh no! I quickly turned the input voltage down to 6V so it would be below the LED voltage, wondering if it was already too late. There were no obvious signs of damage so… whether I’ve just killed it or not, my next step is to put a PWM signal on the brightness control line.

Finding TPS61187 LED Driver Interfaces

I want to reuse the TPS61187 LED Driver IC already on board the display circuit board of a LG LP133WF2(SP)(A1) laptop LCD panel. Driving the backlight that used to shine from behind the now-cracked LCD screen. After studying the datasheet and probed around the circuit board to understand how the chip is configured, I am now probing to see where I can solder wires to interface with the chip.

There are two major concerns here. The first is that I’m learning to work with modern consumer electronics, with circuit boards populated by very small surface mount components. Most of the resistors and capacitors I probed earlier are barely larger than the tip of my soldering iron’s finest tip. The “normal” tip is comically large next to these things. If I continue with experiments like this I will need to buy my own hot air station and learn to use it well.

The second concern are the other components on this control board. If I supply voltage and ground to the TPS61187, the rest of the circuit will probably come alive in some way I don’t understand. I’m not worried about them draining a bit of battery, that’s the best case scenario. I’m more worried about them interfering with the backlight control signals for enable (EN) and brightness (PWM).

First target is to find a place to inject power. The datasheet told me where the power pins are on this chip, but it’s far too small for me to hand solder myself. I’m not saying it’s impossible to hand solder to them, I’m just saying it’s very difficult and unlikely to succeed with my current skill level. So I went poking around nearby components looking for a decoupling capacitor. Because this driver IC uses a boost converter circuit to raise the voltage to drive the backlight LEDs, I expect to find a sizable decoupling capacitor nearby to isolate the rest of this board from the electrical noise of boost conversion. I found one adjacent to the inductor. For a surface-mounted component, it is large and thankfully big enough for me to feel confident I could solder to its two ends for VIN and GND.

I found a resistor and capacitor nearby connected to the enable pin. But even though they are larger than the corresponding TPS61187 pin I couldn’t solder to them. I was able to connect to one side of the small capacitor, but an intended-to-be-gentle test tug ripped off the wire taking a corner chunk of the capacitor with it. Oops, not so gentle after all.

I had even less luck finding a PWM signal connection near the TPS61187. I could see its pin connection on the circuit board, but it instantly sank out of sight to one of the other layers of the board and I found no place nearby where it surfaced.

After some thought, I decided to look for these signals near the largest chip sitting on the other end of the board, labelled “LG Display ANX2804”. I reasoned that EN and PWM are probably controlled from this chip and perhaps I could find something. There was nothing obvious near the chip itself, but I struck gold on the backside of the board. Sitting effectively under the ANX2804 are several labelled and accessible test points, and I was happy to see “LED_EN” next to one pad and “PWM” next to another. (There’s also VLED visible further left I didn’t notice until later, which should be better than soldering to the VIN end of a surface mount decoupling capacitor.)

Continuity test confirmed these do connect all the way across this thin strip of circuit board to the TPS61187, I think we are in business! Time to do some soldering.

Probing TPS61187 LED Driver Configuration

I’ve read through the datasheet for a TI TPS61187 LED driver chip and I think I have a fair (if not perfect) grasp of how to use it. Specifically I want to use one I found on the integrated driver board of a LG laptop LCD panel I’ve taken apart, there to drive the backlight module I wanted to salvage as a LED light. Armed with the datasheet and a multimeter, I started poking at the driver board to understand how it uses the chip. Since most of the chip’s configuration are done via resistors connected to certain pins, I could use the ohmmeter to decipher configuration. I enlarged and printed out my picture of that area of the circuit board so I could scribble down notes as I went.

Here’s what I found on this board, listed in order of their corresponding datasheet sections:

7.3.1 Supply Voltage

For applications that are always-on, it is valid for the enable (EN) pin to be connected to the chip’s internal regulator output pin VDDIO. Since a laptop would want to put a screen to sleep, I did not expect EN to be tied to VDDIO and my meter confirmed it was not. Which means I’ll have to go hunting for my own connection to EN later.

7.3.2 Boost Regulator and Programmable Switch Frequency (FSCLT)

The internal boost converter can operate at a range of frequencies, giving the designer an option to tradeoff between efficiency, inductor component size, etc. I probed the selection resistor on this board to be 822 kΩ. Plugging this into datasheet formula I arrive at a switching frequency of 608 kHz. Table 1 lists a few recommended values, and 833 kΩ is one of the recommendations for 600 kHz. I suspect this was indeed the intent and this 822 kΩ resistor is pretty good at less than 2% off nominal value.

7.3.3 LED Current Sinks

This is arguably the core parameter of driving LEDs. I traced the circuit and found two resistors in series. ~20 kΩ and ~31 kΩ but they added up to about 58 kΩ so there’s obviously something else I missed. Nevertheless, plugging 58 kΩ into datasheet formula says it’ll sets the target at 20mA. Typical for driving LEDs.

7.4.2 Adjustable PWM Dimming Frequency and Mode Selection (R_FPWM/MODE)

There are two ways to control brightness of the backlight. Either they can be blinked directly by an external PWM signal, or they can be blinked with an internal signal generator. One advantage of using the internal signal is that the phase for each of six strings are offset, so they blink in turn instead of simultaneously, which I expect to give smoother dimming. Another advantage of separating the two signals is that the external PWM can run at a far slower frequency, even one that would otherwise cause visible flicker, but it wouldn’t matter. Because once its duty cycle is read, it is copied for use by the internal generator running at a far faster flicker-free rate.

Probing the configuration resistors proved this board uses the internal high speed PWM signal. The resistor is 3.9 kΩ which works out to about 46.6 kHz. This is not one of the Table 2 recommendations, in fact it is over twice the speed of the highest recommended value. at 9.09 kΩ / 20 kHz. Higher switching frequency usually mean smoother behavior but lower power efficiency, I wonder what design meeting decisions at LG led to this value. Though of course it’s possible I’ve misread the value somehow.

7.4.4 Overvoltage Clamp and Voltage Feedback (OVC/FB)

These resistors configure how the boost converter works, and there’s an ideal formula in the datasheet mapping input voltage to LED output voltage. I was able to measure Rdown as 20 kΩ, but Rupper did not converge. My ohmmeter’s initial reading was in the 370-400 kΩ range, but the value continued to increase as I kept the probes in place. Eventually it would read as off-scale high. I think this means there’s a capacitor in parallel?

Out of all the configurations I had hoped to read, this was the one I really wanted to get because it would inform me as to the best voltage level to feed into this system. With this ambiguous reading, I’m sadly no wiser.

But at least I have some idea of how this chip has been configured to run, so I could continue probing this circuit board looking for places where I can interface with this LED driving circuit.