Can I add backlighting to this instrument?

[daisizhou]

Hallo everyone

This is a meter on a power analyzer.Since it has no backlight, sometimes it is difficult to see the screen display clearly in a dark environment.

So I plan to use light-emitting diodes as a light source to add a backlight display to this screen,I don't know if my plan is feasible.
I haven't found any more information about this screen. If you can find it, I hope you can tell me. Thank you.

[Nominal Animal]

Backlight, or frontlight?

I suggest you remove the PCB and display module from the plastic enclosure.  The PCB forms tabs into the enclosure, and I don't see any screws, so a spudger or thin flat screwdriver will help.

You'll find out whether it is viable by looking at the actual display module between the PCB and the plastic enclosure.

[SiliconWizard]

If the LCD is purely reflective (which I suspect), there's no backlight possible that I know of. But that'll be "frontlight", as Nominal suggested.
Then you'll just have to place maybe one LED on each side of the panel, which will illuminate the front of the LCD. Just like (reflective) LCD watches do. No need to try and get access to the back of the LCD again, as I don't think you can "backlight" it anyway.

[daisizhou]

My plan is this, as shown in the figure below, there is a white LED on each side.
Light passing through clear glass.
The two LEDs require a voltage of about 3V. I hope to connect them directly to the rear PCB, but I am not sure whether it will affect the measurement, so I have not started.

[SiliconWizard]

Yes, that should be fine. You need to power them with a supply that is able to provide the required current obviously (don't forget a series resistor). As to noise otherwise, as long as you don't PWM them, it shouldn't not add much noise at all to the power rail.

Using small light diffusers on each side would give better frontlighting, but it'll work without that.

[SteveThackery]

My plan is this, as shown in the figure below, there is a white LED on each side.
Light passing through clear glass.
The two LEDs require a voltage of about 3V. I hope to connect them directly to the rear PCB, but I am not sure whether it will affect the measurement, so I have not started.

Why would that work? Surely the glass will work like a light pipe. There would seem to be no reason why the light would exit the glass and illuminate the face of the display.  You would need to abrade the surface of the glass to make it leak light out sideways.

[Nominal Animal]

Why would that work?

Because of the silvered foil on one side of the display.  It is not mirror-like like the total internal reflection surface would be, and it is diffusive enough to make it work.

How do I know?  Because several of my 1" - 3.5" TFT display modules do backlighting that way, using two to six (side-emitting?) white LEDs, depending on the display module size.  It is not exactly the same, but I've also seen it used on DMMs with exactly this kind of reflective foil backing, too.

[coromonadalix]

is it a voltmeter ??   is there a led variant of this one ?

as other wrote     i don't think putting led behind the lcd will work,  and you need a light difuser too,   witch in turn   thicken the display

[squadchannel]

Thinklight?

[daisizhou]

I tried googling "UP-6000" or "D2868"。

I hope to find the schematic diagram of this instrument or more detailed information and data, but unfortunately I have not found anything.

[SteveThackery]

Why would that work?

Because of the silvered foil on one side of the display.  It is not mirror-like like the total internal reflection surface would be, and it is diffusive enough to make it work.

How do I know?  Because several of my 1" - 3.5" TFT display modules do backlighting that way, using two to six (side-emitting?) white LEDs, depending on the display module size.  It is not exactly the same, but I've also seen it used on DMMs with exactly this kind of reflective foil backing, too.

I'm sorry, I still can't visualise it.  The drawing by @daisizhou seems to be of a clear sheet of glass (not unlike a microscope slide, but smaller) placed on top of the display, with light sent into it from each side.  Obviously I've got that wrong because light will not exit the glass sheet to illuminate the display unless the glass is etched in some way to cause scatter.

So where is this silvered foil? And is it in physical contact with the glass? Because if it isn't it won't cause any light to leak from the glass.

I apologise - I've obviously got completely the wrong picture in my head. A diagram might be good.

[daisizhou]

Sorry, I have drawn a simple schematic diagram of the LCD display screen.

Usually, the front of the glass is for display, and the back is covered with a silver reflective film

[Nominal Animal]

I'm sorry, I still can't visualise it.  The drawing by @daisizhou seems to be of a clear sheet of glass (not unlike a microscope slide, but smaller) placed on top of the display, with light sent into it from each side.  Obviously I've got that wrong because light will not exit the glass sheet to illuminate the display unless the glass is etched in some way to cause scatter.

It will, if the foil is directly on the glass surface without any gap in between, because total internal reflection is from glass-air interface.  If the glass-foil interface has a higher refractive index, then the light will scatter and the backlight work.

See this Newhaven display module for an example.  You can see the exact same type of foil, with the LED diffuser on the right side.  It is a sector to ensure even lighting from a single LED.

[pienari]

My lab psu uses 2pcs same meters.
I have drilled 3mm holes both side of the plastic frame and installed green leds.

It works but not perfect solution.

[SteveThackery]

See this Newhaven display module for an example.  You can see the exact same type of foil, with the LED diffuser on the right side.  It is a sector to ensure even lighting from a single LED.

Hang on, that's a totally different thing, isn't it?  That appears to send the light through the LCD glass itself.  Of course if the back of the LCD glass is frosted, or touching the foil, light will be scattered, as we need.

But @daisizhou seemed to be talking about putting another layer of glass in front of the display, and sending light into either side of it.  That won't work well because the light will stay inside the glass unless the glass is etched (or even scratched or dirty).

So what is the proposed solution? Illuminating the LCD glass itself from the side? (That will work if the back of the glass is in contact with something like reflective foil.)  Or an extra layer of glass in front of the display?  (That won't work unless the glass is treated in some way so it leaks light out of the back face.)

[Nominal Animal]

But @daisizhou seemed to be talking about putting another layer of glass in front of the display, and sending light into either side of it.

No, that won't work; you'd need most of the light to be reflected on the display side, as otherwise you'd be washing out the display by the light scattered in front of the LCD.  I always understood them to intend to side illuminate the existing display module glass layer, no added new layers.  The other option is to have external LEDs outside the display module, in the plastic enclosure part, directed towards the display, away from the viewer.

[PCB.Wiz]

…I don't know if my plan is feasible.

You would need to remove from the housing to see how much room is available, and experiment with various light injection to see how feasible.
 It will never match a true backlit display, but you might manage ‘good enough to read’ in the dark.