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I needed a new telephone set for my lab. Preferring a corded
telephone, I purchased a BT Decor 2200 from Argos for about £25.
It has nice big keys, decent sound quality, and a good feel
about it. Unfortunately though, the LCD is really crap. It
doesn't have much contrast (despite the adjustment), and has
quite a poor viewing angle.
I decided that adding a backlight myself would improve things.
This page explains how to build a simple LCD backlight, and can
be applied to most LCDs, not just this telephone's LCD.
Also, check out the backlight improvements I did on my Mastech
MS8218 Multimeter.
Finished Item:
Step 1: Prepare the LCD
LCDs are visible because they normally reflect light. To
achieve this, there is a mirrored surface on the back of the
LCD. This will probably be a sticky film. This must first be
peeled off to make the LCD transparent. Then, once the LCD is
transparent, our backlight can shine through it. Be careful to
only peel off the reflective film, and not the polariser film.
This photo shows the LCD in its original state, with the
silvery film backing. This particular one isn't very reflective,
and gives a parallax error.
Step 2: Start Making the Backlight; Cut some acrylic.
First, we must cut out a piece of acrylic, about the same size
as the LCD, and cut a little slot for an LED to fit in. I used
only one LED here, but you can use as many as you like, in
whichever position you like. The acrylic I used is diffused on
one side, so as to diffuse the light. the rest of the acrylic is
clear, to allow the light to travel through it. I glued a piece
of shiny paper on the clear side of the acrylic to reflect the
light forwards (see photo in step 3).
I added a small piece of foil to one end to reflect some of the
light back. This will make it slightly more efficient.
Step 3: Wire up and insert an LED
The slot I cut was about 3mm, just big enough for a 3mm LED.
You may bend the leads appropriately, and solder on some wires.
The LED you choose will depend on what colour you desire, how
bright you want it, and what kind of power supply you have. This
is running all the time from AA batteries, therefore, I don't
want my LED draining the battery too much. I chose a 'Superbright'
Green LED, which was purchased from Spiratronics for about
24p. I chose a series resistor so that it draws a tiny 200
μA. This will hardly affect the battery life, as the telephone
itself only draws about 2mA at most when in use. This particular
LED can be run at up to about 30mA, and at 30mA, it is BRIGHT.
In comparison, a regular 'indicator' LED will use 15--20mA and
be fairly dim.
This photo shows the LED in the slot. It also shows the piece
of paper glued on to the acrylic.
Step 4: Test the newly made backlight panel
I connected the newly made backlight panel up to a power supply
to test it. The whole panel is lit up well. This is far from
perfect; the lighting isn't even, and there is a big hotspot
where the LED is, but it will do absolutely fine for this
application. If you need the backlight to be more even, adding
an extra LED at the other side will help. Also, do be careful
when gluing things. In the photo, you can see the glue used to
hold the reflective paper on.
Step 5: Install
The panel is slipped underneath the LCD.
Finished!
The LCD now has a nice green background, the diffused
background means there is no parallax error, and yet it is
sufficiently reflective that it is visible in bright day light.
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