Nov 072009

Phone7960_4452It’s a shame that the 7940/7960 doesn’t have a backlight. But there is a DIY-solution for this problem. You can modifiy the display of your Cisco IP-Phone relativley easy. You simply need a few inexpensive parts and can start to modify your phone.

You will need:

  • EL Foil 112x87mm (blue one looks very cool)
  • EL-Power Supply
  • 2 LEDs – 3mm – blue
  • some wires
  • and of course some tools for soldering and dis-/reassembling the phone

In Germany you can get the EL-parts at Conrad Electronic – but you should be able to get these stuff in all big electronic stores or even at ebay. Don’t worry if you couldn’t get the EL-Foil in the right dimensions – you can cut these foils with a pair of scissors. YES, the foil won’t be destroyed (but DON’T cut the connectors 😉 )

The powersupply for the EL-foil (the EL-converter) is a very big problem. These converters work with a frequence of about 400Hz to 900Hz for genereating the supply voltage of abt. 110 Volt. If you place this converter inside your phone (there is enough room for it inside) and even try to supply some power of your phone to the converter, you will hear a very annoying noise in your handset and even in the loudspeaker of your phone. This is absolutly inacceptable – so I’m currently searching for a good solution to power the EL-foil quietly.

If you have a good idea or even a ready solution – please let me know. It must be possible to build a cheap EL-converter with a working frequency outside the hearable soundspectrum. Someone told me that there are ICs from MAXIM which are working at about 1MHz – but I couldn’t find a suitable chip yet.

Ok, let’s begin with the Phone-mod.

Have a look at the following pictures for disassembling the phone:


Now let’s continue with the more tricky part:

DisplayUnglue_4472You need to remove the LCDisplay from the frame. It is glued with double-faced adhesive tape. Be VERY CAREFUL not to break the GlasDisplay! You can gently bend the frame very slowly away from the LCD. This works better if you get your display including the frame to a temperature of about 40 or 50 degrees.

Removing the reflective foil:

DisplayCloseup_4473DisplayRemoveReflective_4474Pay attention that you only remove the thin, reflective foil. There is another, thicker foil for polarization. If you remove this one you will not be able to see anything on your Display. At last use isopropanol to remove the glue from the polarization foil.

Place EL-Foil and Converter:

Backlight_4475Converter_4478Insert the EL-Foil between plastic frame and display. There is no need to glue this parts together – just be careful while reassembling the phone. If you decide to use the “noisy solution” here is a good position for placing the EL-converter. In this case it is highly reccomended that you power your converter from an external source outside the phone. This will keep the noises very low – and nearly unhearable. If you like a clean, blue optic change the 2 LEDs on the little PCB near the handset cradle. Connect all parts with the correct wires and reassemble the phone – and you are done.

Enjoy your 7940/7960 at night 😉


If you have a good solution for a silent EL-Converter, I would appreciate if you contact me or write a comment here. It doesn’t matter if it is a ready made converter or just an application note, design guide or partnumber of the right chip. Thank you.

 Posted by at 12:42