Fiat Lux: retrofitting LED backlight to unlit LCD displays - Page 1
Forum section: electronics / projects

author: Lomax
author_rank: Frequent Contributor
timestamp: March 07, 2016, 06:25:00 pm
content: Nothing beats the warm fuzzy feeling of being surrounded by an array of glowing displays, indicators and read-outs, at least for a night-crawling techie like myself. But wait, there's a problem: some of my T&M equipment have unlit displays, not only missing an opportunity for additional bling, but making them difficult to read in anything but a brightly lit room. Take for example my bench PSU, which sits on a shelf above the workbench, at slightly above (seated) head-height. With only my desk light lit it's impossible to read the displays, and I have to raise the light to where it gets into my eyes as well, which quickly gets annoying. So a while back when I spotted these cheapoLED backlight modules on Adafruit, and noting that the dimensions were perfect for the LCDs on my PSU, I decided to try adding backlight to them.Some initial experimentation revealed that although there appeared to be space for such LED modules behind the LCDs the reflective film on the back of them was completely opaque, so the first step would be to remove this. That's actually a pretty scary operation, as the glass substrate of the displays is quite fragile, and damaging just one of them would effectively render the PSU unusable, unless a replacement LCD module can be found. It is also important thatonlythe reflective film is removed, and not the rear polariser as well (LCDs require both front and rear polarisers set at different angles to be readable - remove either one and you will no longer see any characters). After hesitating for a while, this weekend I finally plucked up the courage to have a go.Of course the LCD modules have to be removed from the PCB in order to get at the film on the back, which was quite simple to do with a desoldering iron. While doable, it would be a chore to do all those through-plated pins with a manual pump and wick - if I had to do this I'd first use wick on the component side, to remove as much solder as possible, followed by the sucker on the reverse.Removing the LCDs reveal aTC7106RCPLdriver/ADC chip behind each one (the same type is used for both the voltage and current controls).To get the reflective backing off, I first used a loose scalpel blade to very carefully lift one corner of the film, checking to make sure the purple-ish-grey polariser remained attached to the LCD. I then heated the module up slightly with a hot-air pencil set to 80°C, to soften the adhesive, before starting to pull the film off using forceps. You really need forceps, or needle nose pliers with serrated jaws, to get sufficient grip on the film; tweezers will probably not do. I tried to reduce stress on the glass as much as possible by pulling close to paralell with the surface, and going veeeery slowly. Still, the stress was enough to make the LCD segments darken and flicker as I went along, which gave me a bit of a scare at first, but it's normal for an LCD to behave like this.Pulling the film off left the adhesive on the polariser, and at first I did a half-hearted attempt at removing this with alcohol, but realising this would be a major chore, and seeing that it looked smooth and perfectly reusable, I decided to just stick the diffusion layer straight onto the old adhesive. I don't think you really need any adhesive here, and I'm not sure what kind I would use if I wanted to add any.

author: Lomax
author_rank: Frequent Contributor
timestamp: March 07, 2016, 06:27:32 pm
content: The backlight modules from Adafruit consist of three layers: on the back is a thick white plastic film, which is relatively opaque - this reflects some light forward that would otherwise exit through the back. In the middle is a just under 3mm thick piece of clear acrylic, with a pattern of white dots etched on the back. It is these dots which enable a fairly uniform light from the single white LED; if you look closely you can see that they grow larger as you get further from the LED, thus increasing refraction as the intensity falls off.While the perfect width and thickness, the LED modules were a bit too long for the displays in my case, and I had to trim off 20mm from each with a fine saw. I clamped all four modules together in a vice, between two pieces of wood, and cut through them all in one go - with the films still attached. This worked well, and they were easily cut, but despite clamping them firmly (on the part to be cut off!) they still managed to slip a little, and the cut ended up slightly off from right angle (really made me long forthat Proxxon table sawI've been dreaming about) - but hey, good enough! I then sanded the cut edge with very fine sanding paper until flat and smooth, before transferring a strip of the reflective film that covers the sides from the cut off pieces.I'm not sure how much this contributes to light evenness, but it will at least stop light spilling out of the cut side. Below you can see what the module looks like when lit and with the topmost layer removed. This layer is a thinner and more translucent film than the one on the back, and serves as a diffuser (so you do not see the dot pattern behind the display). Neither the diffuser or the rear film is attached to the acrylic in the middle, other than right where the LED sits - and they are both easily removed.I removed the diffuser on mine so that I could fit this more easily to the sticky surface on the back of the LCDs. It is simple to identify the diffusion film of the LED module; it has a clear protective film on it, which needs to be removed before attaching it to the LCD.In order to avoid any air bubbles being trapped behind the diffuser, I only attached one side, and used a q-tip to rub it onto the adhesive, going gradually from one side to the other. After checking (and sometimes adjusting) the position I then placed the LCD face down on a firm surface, so that I could press the diffuser more firmly onto the adhesive using a finger and a micro-fibre tissue, taking care not to catch on any of the long legs in the process. Again I used the hot-air pencil to heat it up just a little bit beforehand, to activate the adhesive. Holding the panel up against a light confirms there are no air bubbles (or dust!) trapped.It was clear from earlier testing that although the thicker film on the back of the LED modules didn't let as much light through as the front, they still chucked out a surprising amount of light, in pretty much every direction. Since the PSU case has many vents (it's fanless, which I quite like) I decided to seal the sides and back with black electrical tape, to reduce the light-spill.Finally, it's all ready for reassembly. I put some double-sided adhesive foam pads on the TC7106s, and did a dry-fit to make sure everything would fit correctly, before cutting the legs on the LEDs to the right length. It fitted so well in fact that I opted not to peel the top of the adhesive foam - the LED modules are nicely held in place without it, and makes any future disassembly much easier. I tacked the corners on each LCD with a bit of solder and then connected up the LEDs. The controller in the PSU has a symmetrical power supply of +/- 5V, and the simplest solution was to grab both ends of this and hook the LEDs across in series with a resistor (the brown and orange wires are just temporarily tucked out of the way in the photo - they are actually for the LEDs).Once I was happy with everything, I soldered the remaining pins on the LCDs, and reassembled the PSU.

author: Lomax
author_rank: Frequent Contributor
timestamp: March 07, 2016, 06:34:06 pm
content: I'm always a bit nervous when switching any of my hacks on for the first time - will it (like too many have) go up in smoke and have me kicking myself for not leaving things alone, or will it defy the odds and work, perhaps even really really well? Ta-da!Ok, ok, so I've only done one side so far, but hey, look at that! Moar pretty lights! And a vast improvement in low-light readability - without any notable reduction in daylight readability too, as the three photos below show (only the right hand LCD has the backlight installed here - the left hand one is still original). First with my desk light shining onto the displays, and the backlight switched off, then with the backlight on under the same lighting, finally with the backlight on and my desk light in its normal position. Result!Here's a picture with no other light but bright daylight from the window. Contrast is actuallybetterin real life, not sure why the camera fails to do it justice.I will be doing the other side next weekend, and then - if I'm feeling really brave - I plan to give my TTi bench multimeter the same treatment, using a "large" backlight panel of the same type (which happens to be a perfect fit). One major benefit to using LEDs over EL type backlight is that it runs off low-voltage DC, while EL film needs HV HF which may interfere with sensitive electronics. Having seen what having one switched on in the same room as my scope does, I wouldnevereven consider putting an EL inverter in my bench multimeter!

author: SeanB
author_rank: Super Contributor
timestamp: March 07, 2016, 07:27:45 pm
content: Very nice mod, and a good looking result as well.

author: Lomax
author_rank: Frequent Contributor
timestamp: March 07, 2016, 08:04:29 pm
content: Many thanks! I thought the results were good enough - and the procedure simple enough - that others might be interested in doing the same thing. I've also felt for a long time that I really ought to give something back for all the excellent tutorials and guides I've benefited from over the years

author: edavid
author_rank: Super Contributor
timestamp: March 07, 2016, 09:22:40 pm
content: Nice work... but could you fix the Adafruit link?

author: Lomax
author_rank: Frequent Contributor
timestamp: March 07, 2016, 09:31:52 pm
content: Thank you, and thanks for pointing out the missing link (now fixed) - I originally lost half of this post due toa forum mishap, and the rewrite was bashed out in a frustrated rush.

author: cvanc
author_rank: Frequent Contributor
timestamp: March 07, 2016, 09:46:44 pm
content: Classy mod and excellent documentation - thanks for posting this!

author: RobertoLG
author_rank: Frequent Contributor
timestamp: March 07, 2016, 10:40:04 pm
content: nice joband thank you for sharing all details

author: wblock
author_rank: Regular Contributor
timestamp: March 07, 2016, 11:11:14 pm
content: Thank you for (re)posting this. Excellent photos and interesting observations.

author: jeroen74
author_rank: Frequent Contributor
timestamp: March 07, 2016, 11:19:20 pm
content: I think good old yellow/green would have looked better, white is a bit of anachronism

author: Lomax
author_rank: Frequent Contributor
timestamp: March 07, 2016, 11:48:58 pm
content: Thanks everyone, I'm happy to hear you like the guide!Quote from: jeroen74 on March 07, 2016, 11:19:20 pmYou know what, I agree completely. Yellow, or green, would have lookeda lotbetter - but I only had four of the panels and didn't want to risk one on trying to change the LED. Theyareonly glued in, andshouldbe possible to remove, but maybe not without damaging the acrylic light-guide. I also only have (by choice) low emittance LEDs in my bins; as indicators these look much better than the high-brightness type which always end up being much too bright. I'm going to buy a couple more panels though (I have a hand-held multimeter I want to try this hack on as well), and I'll get some high-brightness LEDs at the same time to experiment with! Another option might be to stick a colour gel filter behind the diffuser - there's certainly enough brightness for this, and I might be able to do that without having to desolder the LCDs yet againEdit: I just want to add that I do not haveanyblue LEDs at all in my bins. Ihatethe damn things. Nothing looks cheaper and uglier than blue LEDs, except perhaps chromed plastic. The combination is horrifying and ought to be a punishable offence. White LEDs are technically blue LEDs, but with a yellow fluorescing material added. I fondly remember the first blue LEDs that came on the market, oh, I dunno, thirty years ago? These used a different dielectric composition to today's cheapo ones and had a gorgeous rich deep blue colour, nothing like the eye-piercing optical shriek emitted by those fitted to every single modern gadget. They also cost something like £10 a pop, in old money, which probably explains why they disappeared.

author: edavid
author_rank: Super Contributor
timestamp: March 08, 2016, 12:31:54 am
content: Quote from: Lomax on March 07, 2016, 11:48:58 pmSilicon carbide. In addition to being expensive, they were also inefficient and unreliableI'm not sure why they would have a different appearance from modern blue LEDs, since as far as I know the peak wavelength was the same (470nm).Maybe it was just that they were dim point sources, so they had a different look.

author: John Heath
author_rank: Frequent Contributor
timestamp: March 08, 2016, 12:39:16 am
content: Quote from: wblock on March 07, 2016, 11:11:14 pmI would add that it put a smile on my face to see this amount of effort put into raising the bar a bit. Nicely done sir.

author: Lomax
author_rank: Frequent Contributor
timestamp: March 08, 2016, 01:18:52 am
content: Quote from: John Heath on March 08, 2016, 12:39:16 amThank you very much! I always say "if something's worth doing, it's worth doing well"I also found, after struggling with a conventional tripod, that using a microphone stand with an extendable boom, and a small ball-head on the end (no puns pls!), allows me to position my camera so that it always gets a good close-up of what I'm doing. And the voice activation that I used to think was a silly gimmick is brilliant for capturing pictures of steps that require both hands; I just have to pause what I'm doing and hold it up for the camera and say "cheese" to capture a picture of that step. This does make me feel a bit silly, but saves a yuuuge amount of time.

author: Lomax
author_rank: Frequent Contributor
timestamp: March 08, 2016, 01:34:23 am
content: Quote from: edavid on March 08, 2016, 12:31:54 amI stand corrected! Maybe you're right that it was the lower intensity, or just the novelty of them being, well,blue. I also seem to recall that they had blue coloured plastic potting, which may have helped. I am not a fan of glass clear potting on coloured LEDs. What can I say, there area lotof things I don't like!

author: nidlaX
author_rank: Frequent Contributor
timestamp: March 08, 2016, 02:47:17 am
content: A thorough write-up with excellent photos!

author: AF6LJ
author_rank: Unknown
timestamp: March 08, 2016, 03:03:13 am
content: I love it, looks great and a well executed mod.

author: cowana
author_rank: Frequent Contributor
timestamp: March 08, 2016, 01:56:34 pm
content: Great writeup - one of those projects that's made really satisfying by paying upmost attention to detail!

author: Cubdriver
author_rank: Unknown
timestamp: March 09, 2016, 06:27:59 am
content: Very nicely done! I remember reading elsewhere about the forum gorking and eating your previous post. It's incredibly frustrating when something like that happens! (Been there, done that, feel like putting my fist through the computer each and every time.) I'm glad that you decided to take the time and retype the whole saga. You got great photos and did a very clear write up.You've gotta love a tripod for getting good photos. This is the first I've heard of a voice controlled camera - that's sweet! I'm usually fumbling about for the cable release to trip mine when taking pics at the bench.-Pat

author: Lomax
author_rank: Frequent Contributor
timestamp: March 09, 2016, 02:46:22 pm
content: Thanks everyone - your kind words make me very happy!Quote from: Cubdriver on March 09, 2016, 06:27:59 amThank youIt did feel like I had wasted a lot of time unless I retyped it - a choice between spending 3hrs with zero result or 5hrs with a result that should only have taken 3hrs... Frustrating, but not really a difficult choice.Quote from: Cubdriver on March 09, 2016, 06:27:59 amRegular camera tripods are great in a lot of situations, but pretty useless when trying to take pictures of tiny parts on a workbench - you cannot put the tripod close enough, and it ends up being massively in the way. If your camera is light enough, a microphone stand with a boom arm is much more versatile, and allows you to place the stand out of the way while still reaching every part of the work surface. The threading at the end is a standard 3/8" which will accept amini ball headfor complete freedom.The camera is a strange smartphone/camera hybrid,Samsung Galaxy S4 Zoom. It's not very good at either function, but I only paid £140 for it (brand new from Argos outlet!), and thanks to running a smartphone OS (Android) it does have this voice activation feature. I suspect many (all?) cameras that run Android will have this!

author: Cubdriver
author_rank: Unknown
timestamp: March 09, 2016, 04:40:39 pm
content: There are some newer tripods available that have a removable center post that can be put in a pivoting bracket that will let them extend out horizontally - the one I have might be a Benro, can't recall exactly off hand. Very helpful for overcoming the limits of a conventional tripod. An old school option for serious flexibility in camera positioning in a tripod is a Benbo, though until you get the hang of using it setting it up is like wrestling with a drunken octopus.The mic stand and boom sounds like a great way to mount a lightweight camera for sure. I'll have to try that with my light camera. Thanks for the idea!-Pat

author: edavid
author_rank: Super Contributor
timestamp: March 18, 2016, 03:07:48 pm
content: Quote from: edavid on March 08, 2016, 12:31:54 amI found some SiC blue LEDs from 1997, and they do have a slightly different color - a little more of a pale blue. I guess the dominant wavelength is a little shorter. I wasn't able to get a good photo of the difference.

author: Macbeth
author_rank: Super Contributor
timestamp: March 18, 2016, 06:17:05 pm
content: Great job! I attempted to add LEDs to my pimped VC99 multimeter (I already did the serial/bluetooth). I even had a similar plastic light guide like yours that I salvaged from an old broken meter but found the opaque backing on the VC99 LCD a show stopper. I didn't dare try peeling it off, for fear of ruining my $25 Vichyso gave up on that mod.I think I'll have to have another go now you have bravely pioneered the way!

author: tooki
author_rank: Super Contributor
timestamp: March 18, 2016, 06:33:04 pm
content: Quote from: Macbeth on March 18, 2016, 06:17:05 pmI did a similar mod to a cheapie Mastech, maybe this will be helpful to you:https://www.eevblog.com/forum/testgear/lcd-backlight-mod-on-cheapie-mastech-my64-dmm/

author: SeanB
author_rank: Super Contributor
timestamp: March 18, 2016, 06:38:37 pm
content: Robrenz on the forum made a very nice camera positioner, all machined out of material in his workshop.

author: Macbeth
author_rank: Super Contributor
timestamp: March 18, 2016, 07:22:46 pm
content: Quote from: tooki on March 18, 2016, 06:33:04 pmNice! You didn't state how you removed the backing film (did you heat it up first?). Also did you finally do the backlight on the Keithley?When I attempted it, I noticed on the donor meter that came with a backlight diffuser it had a metallised translucent film on the back of its LCD. I attempted peeling this off as a trial and got so far before the LCD glass delaminated and all the liquid crystals started leaking out. I could slide the 2 glass panels against each other with the liquid crystal as a kind of lubricant which was interesting. But that destruction was enough for me to give up on trying to peel the opaque film from the back of the Vichy.

author: tooki
author_rank: Super Contributor
timestamp: March 19, 2016, 06:19:46 pm
content: I had no trouble peeling off just the reflective foil layer of the Mastech's LCD. It's just simple metalized plastic foil. Then I used Goo Gone to remove the adhesive residue, and IPA to remove the Goo Gone residue.Perhaps you need to use a scalpel blade to lift just the reflective layer of your LCD, so that you're not lifting the rear glass or polarizer.I haven't done the Keithley yet. I'm on the fence whether to hack the LCD, or to simply build a whole new display board for it (others have reverse engineered the signaling to the display board and written Arduino code to parse it!) so that the original remains unaltered.

author: Knuckx
author_rank: Newbie
timestamp: March 20, 2016, 11:20:29 pm
content: Quote from: Lomax on March 07, 2016, 06:25:00 pmI have a relation to your power supply, a Thurlby Thandar PL330DP. The quoted picture of your supply shows some metal/ceramic open trimpots (whch are Bourns 3359P/VA05H); these trimpots have a nasty habit of becoming open circuit, or even somewhat capacitive, especially when adjusted. I had to replace every single trimpot of this type in my PSU (with Bourns 3386G) after intermittent strange behavior developed, followed by permanent strange behavior after I attempted to adjust the calibration.I have to say that the result of this mod is very nice looking, however this mod is not applicable to my PSU, as it uses LED displays; but if it used LCD I would not dare try peeling the reflector off of the back of the LCD.BTW, if you need a service manual for your PSU, just give TTi a call or email; they tend to be very nice about providing service manuals and will not even ask for payment for older/discontinued models.

author: TonyD
author_rank: Contributor
timestamp: March 26, 2016, 12:21:12 am
content: Great mod."Goo Gone" now there's a product name that does actually what its says :-)

author: Lomax
author_rank: Frequent Contributor
timestamp: April 02, 2016, 03:48:34 am
content: Quote from: Macbeth on March 18, 2016, 07:22:46 pmIt may be that some LCD panels are more sensitive to stress than others, I guess this largely depends on thickness of the glass plates and how they have been bonded together. It was pretty scary to peel the backing off on the first one - had the panel been damaged the PSU would be almost useless. It definitely helps if you can soften the glue with a low-heat hot air gun (hairdryer?), but don't go crazy as heat can also damage the LCD, and it may cause the glass plates to delaminate - which might also be a risk when using (some) solvents. I tried to pull the backing offalongthe surface of the glass, instead of at an angle, to reduce perpendicular stresses. This isprobablya good thing to do.Quote from: tooki on March 18, 2016, 06:33:04 pmNice work!Quote from: Knuckx on March 20, 2016, 11:20:29 pmAh yes, I've seen the LED versions as well. There are loads of different models, singles & triples too, some which look really awesome for a retro nerd like me. I wouldn't mind a TSX1820P for example, 20A could be used for a lot of fun, and it's got a lot of nice buttons, as well as a handy power meterAnd the PL330QMT monster is pretty impressive, with an extra 0-5V output for powering logic. Probably nearly identical to the one you've got. I dig the two handles on topI've become quite a fan of their gear, at least the slightly older stuff - I don't know anything about the quality of the newer white plastic stuff. So far I've got, in addition to the power supply in question, theTG1010 arbitrary waveform generator(waaay overkill for my needs, but got it at averygood price...) and the1705 dual measurement bench multimeter(also as abargain, and next in line for the backlight mod) - both of which I love dearly, and both of which I see are still being made (though now in white plastic cases). A massive bonus with Thurlby Thandar/TTi gear is that you can easily get hold of full user and service manuals as PDFs - the service manual for my PSU includes the full schematics with BOM and circuit description; enough information to build a copy if you wanted to, and above and beyond what most manufacturers provide. Add to that that they are (were?) made in the UK and they get a big thumbs up from me!

author: Cubdriver
author_rank: Unknown
timestamp: April 02, 2016, 06:10:38 am
content: Nice looking juice boxes.-Pat

author: tooki
author_rank: Super Contributor
timestamp: April 02, 2016, 11:25:18 am
content: Quote from: TonyD on March 26, 2016, 12:21:12 amSo true! Wasn't Goo Gone one of those rare products that was born as an infomercial product, but was so successful it was relaunched at retail?Quote from: Lomax on April 02, 2016, 03:48:34 amThanks!