Backlight Hacking - BT Decor 2200 Telephone


Stephen Coates' Website - stevecoates.net

Created: 18th January 2014


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Backlight Hacking

BT Decor 2200 Telephone

Mastech MS8218 Multimeter



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:

Photograph of the completed telephone


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.

An LCD with reflective backing.


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).


A piece of cut acrylic


The slot for the LED


I added a small piece of foil to one end to reflect some of the light back. This will make it slightly more efficient.


A piece of foil added to the acrylic


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.

The backlight panel, with LED


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.

The backlight panel working


Step 5: Install

The panel is slipped underneath the LCD.

Backlight installed


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.

Backlight finished




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