Fluke 8012 backlight display hack - Page 1
Forum section: electronics / projects

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 23, 2019, 12:09:22 am
content: Just want to share the hack I come up with to add backlight to my fluke. May work in many other LCDs.The only thing you will need to solder is one LED. That is what I did.I removed the silver tape from behind the LCD.Dismantled one of those cheap ebay displays to get the acrylic diffuser and cut it to length.Carved fluke's lcd frame to accept the acrylic diffuser.Placed a 90 degree polarising film between the diffuser and the LCD.Now I just need to figure out where on the PCD I will hook my LED leads.

author: Electro Detective
author_rank: Super Contributor
timestamp: June 23, 2019, 12:53:29 am
content: Looking good mateSuggestion: if you haven't got one already, consider a Dremel rotary toolor one (or two..) of those cheapie rotary knockoff kits (Ozito at Bunnings or the Rockwell at Supercheap Auto/SCA)they make it a lot easier and precise to chop up plasticsI've got both, initially to try out.. and kept bothand sale prices so cheap it wasn't worth the petrol to drive backone for chopping, the other for sanding/smoothing or drilling/hacking,it saves time doing bit swaps (and finding them) = jobs get done fast, and you've got redundancyand grab an extra cheapie set or two of protective eye wear,don't trust anything that spins (aka tools/politicians/door to door floggers)fwiw I keep two sets stored with the rotary tools so there's no excuses or hunts for them

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 23, 2019, 07:55:32 am
content: Where would you guys recommend connecting the LED? It is rated 5v but does work with 3v.My initial idea would be to build a little voltage divider and harvest current from +VA (15v).

author: tooki
author_rank: Super Contributor
timestamp: June 23, 2019, 01:27:07 pm
content: The LED itself is almost certainly rated only for 3V. Normally, the current limiting resistor for 5V or 3.3V is on the display PCB itself. I wouldn't run it at 5V if you expect it to live any meaningful amount of time.I just checked one of my cheapie eBay LCDs, and it uses a 100 ohm resistor for 5V and draws exactly 20mA, which means it's a 3V LED.Originally I was gonna say that I'd run it off the -6.5V rail with an appropriate resistor (180 ohms assuming you want the usual 20mA). But after looking at the schematic, and admitting I don't totally understand the circuit, I'd leave it alone because it looks like it's connected to the voltage reference.So I'd use the +15V rail through a 5V regulator (properly configured with input/output caps) and a 180 ohm current limiting resistor. Probably overkill, but better than a simple resistor, since the ≠15V rails are unregulated. (Otherwise, you could use a 15V rail and around 600 ohms resistance, but it's a full quarter watt of dissipated heat, so either use a 1/2W resistor, or divide the resistance among a few resistors, e.g. two 1.2K 1/4W in parallel, or four 150 ohm 1/8W in series.)Using a voltage divider makes no sense; all it does is waste more power as heat.Also, you can put some tape (either aluminum foil or opaque white are ideal) on the cut edge of the backlight panel to prevent light leakage, both for improved efficiency and to not be lighting up the buttons.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 24, 2019, 06:03:46 am
content: I tried the resistor idea and implemented as per the sketch bellow.But the LED stays on all times, even when the power switch is off. Considering the power switch happens before the new components I am puzzled to understand what is going on.Any ideas?!