A little while ago, I got an used Asus Transformer Prime (TF201) with the keyboard dock from a friend. I really like it. It's lightweight, after rooting and several software modifications, fast enough for what I want to do and the battery in the tablet and the second one in the keyboard dock keeps it running for hours. It's the perfect device to take with you on the go or to the datacenter (in combination with an USB ethernet module).
But after visiting MetaRheinMain ChaosDays and their dark hackcenter, I really wanted a keyboard backlight for it since I'm used to it from my Dell Latitude.
So I sat down and thought about what I could do to accomplish this.
- Clip some LEDs to the top of the screen
- LED strip beneath the keyboard
- Electroluminescent wire / panels
But LEDs at the top would look silly and needed an external power source, LED strips beneath the keyboard would be too bulky. So finally, after opening the dock and taking some measurements, I settled for the EL panel.
Finding a panel
To cover the whole keyboard, you'll need an EL panel that's about 9,5cm * 25cm.
Since I couldn't find one on ebay, I bought 3 10cm * 10cm panels here.
Be careful, the shipping time shown at the top on ebay is wrong. Shipping to Germany took 3 weeks.
The panels are advertised as "white" but are more of a blueish color.
I cut the panels down to size (don't cut the leads off!), sealed the cuts with some clear tape, wired them to one of the three inverters included
and tested how good you could see the light through the keys
Powering the panel
Now I needed to find a power source inside the dock to power the inverter.
The Asus Transformer Prime has a 7.5V battery inside the dock and the USB power supply can provide 5V and 15V. So chances are, there is something like that somewhere inside the dock. After looking at the circuit and parts, I started measuring some pads against ground until I finally found this part in the middle:
What you see there is a PH7030L made by NXP. It has 3 source pins (in this case all 6V), 1 gate and the mounting base is drain, delivering 10V. So I settled on using this as VCC and the copper pads next to the hinge mount as ground.
Luckily, the inverter delivers a steady 3,3kHz even at lower powers. It starts working at about 1V and goes up to 15V, measuring 140V at its output at an input voltage of 10V.
Fitting the panels
There's not much space between the battery and the keyboard left. Not nearly enough to use the original wiring. So I got to work replacing the default wiring with something better.
The panels come with 2 metal soldering pads each that are easy to solder to. I used a 0,14mm² wire and stripped some of the isolation at the needed places and secured it with some isolation tape after soldering.
A quick check and the fit was still a bit tight at the SD Card reader.
To get the EL panel, cables and plug inside the case, I had to do some modifications. For the panel connector at the SD Card reader, I removed the reader and used a Dremel to cut away some of the support material underneath.
Next, I had to make room for the plug to connect the external inverter. I choose a spot near the charging port since that's where I would get power from. I took the Dremel and removed the plastic, cut 2 slots in the aluminium and bent it down.
I decided to use a small standard USB port for the inverter connection since that has enough spacing between the contacts and it is small enough to fit in there. Here's a picture of the soldered connections to VCC and ground. There's some paint on the solderpoint that you'll have to scratch off.
I put some isolation tape over it to make sure it doesn't short out with the metal backside of the keyboard. I placed the USB port in the slot I cut earlier, measured all 4 wires (2 for power, 2 for the panel) and soldered those on.
On the left you see the 2 wires to the panel, VCC after that and ground on the right. The casing and aluminium backside of the dock are ground too.
All the wires were put in place and the port secured with some black superglue and filler.
The plug for the inverter is a standard USB plug, all the wiring is isolated with some heatshrink tubing. To provide some strain-relief, I put a larger piece of heatshrink tubing over the plug and filled it with some silicone like glue that stays flexible.
Closing everything up
The flexcable for the keyboard is long enough to go under the EL panel, the one for the touchpad is not. But it doesn't block much light.
You may notice the piece of plastic missing from the housing on the lower left. That's for another project and I'll write about that once it's finished.
So finally everything is back together and working nicely.
The light is strong enough to be seen in a dimly lit room. I can't yet tell how much impact on runtime it has, but the setup draws about 10V at 150mA. You can also see the huge inverter in this picture. I plan on building a smaller one sometime.
If you've got any questions or ideas, you can contact me on Twitter @nullpointr or by mail at [email protected]