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DMM: Add backlight on the LCD ? "Add your Ideas"

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Kiriakos-GR:
Well I would like to know how is possible  to add  back-light on a DMM that it was not planed to had any.

I would like to see suggestions about spare parts selection (LED type), and most of all, I like to see similar DIY projects,
and the results that you got from that.   

The main idea of adding some LED around the crystal of the LCD display sounds simple,
but for me that I love perfection, I am willing to follow only the best and worthy suggestion. 

The display that I am planing to add a back light its a classic 6.5Cm X 3Cm.

What I need most is to find suggestions about  the proper placement of the LED for best results.

david77:
As the back of the reflexive LCD is probably black or grey a real backlight seems impossible.
I'd probably try to place small SMD LED's (eg. 0805) around the edge of the LCD screen, maybe two on
the left and right side, or 3-4 on the top and bottom. I'd expect that to light the display area quite evenly.

I know this is the obvious way but how else could you do it? Is it possible to remove the reflective strip on the back
and place a LED backlight from a dotmatrix LCD screen there?

BravoV:
I believe its possible, but quite tricky though.  It depends on the LCD frame construction, noticed at few panels they have two small windows that have access directly at the glass, probably they were designed for the led back lightning. Not sure though, never try it my self, but this should be interesting to see the result if someone decided to put a led there for panel like this.

For example below at my cheap meter non backlight, certain lcd panels are made with small windows opening at both sides left & right that has access directly at the glass, watch the small rectangle opening at the black plastic frame pointed by the arrow.





Other angle view, you can see some of the light from the other end of the glass escaped at this side (dim shady green light) at that small opening.





One small possible issue that I could think of, will the light scatter evenly at the panel even using wide angle led ?





.

Psi:
Yeah, all the DMM lcds i've seen have a ~1-3mm perspex/acrylic block and side mounted leds like the above pictures. Normally with a mirror coating around the edges where there are no leds.

Try one or two High Flux Piranha (UFO) type chip led.
The 110-130deg version is really good for back lighting.
Very even light over all 130deg and they are quite powerful at full power (~50-70mA).

You could even try a RGB one and see what color mix works best. The LCD optics are probably more transparent at some wavelengths than others.
I suspect green may work the best, as some lcds are kinda greenish and green is brighter to the human eye for the same optical power.


One thing to check tho, before selecting an LED, is what voltage do you have available. If the meter only has 3v (two cells) then you may not have enough volts for some led colors.

nukie:
Check the datasheet of the main IC, sometimes if you are lucky it has output pin for backlight control.

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