Do you wanna build a tablet?

Project, result, summary

Mainly inspired by Michael K. Castor awesome PiPad project, I built a homemade tablet. It is powered by a Raspberry Pi (model B), connected to a 10 inch touch screen by ChalkElec. The main technical difficulties are: building an enclosure strong enough to hold the board, yet light enough that it is still portable, and sending power to the electronic boards (it takes lots of soldering).

I have since then updated the tablet into a Mark 2, with a different architecture (PixelQi screen, Raspberry Pi 1B+, new enclosure).

The final product looks like this:


Top view	Side view

Inside

What does it look like inside?


Things are packed tightly	Which part is which?

How do you do it?

You have to solve four different problems:

Making the hardware work (that's the easy bit)
Carry electric power to the different boards (trickier)
Making an enclosure (trickier)
Making sure everything fits inside the box

The hardware

The electronic hardware itself is the easiest part: connect the screen to the video converter, plug the video converter into the HDMI port of the Raspberry Pi, connect the input from the touchscreen to a USB port, and you're set.

Now, the real problem is to send electricity to the different parts.

Carrying electricity

To power the tablet, you need an external battery. And you need to transport electricity from the battery to a power button, and from the power button to the different boards: the Pi itself, the video converter and the USB hub.


To take electricity from the battery, we use a USB plug and the USB specification (red is +5V, black is ground).	Same for powering the board, except it's a micro-USB plug (get your best magnifying glass)	To power the video converter, I had to solder directly onto the board, under it. (By the way, I made a mistake, and the wires should be reversed. In this picture, red is ground, black is +5V).

Once you're done, you connect all these to your power button:

(to power the USB hub, I just cut its original cable) Now, all should be working together, and you can check by booting the Pi.

The enclosure

There are several methods to build the enclosure: Michael K. Castor used Birch Plywood with a CNC machine. Harrison Holbrook used a 3D printer. I only had access to a laser cutter, so I used that.

I cut three layers in a plate of 8mm of plexiglas, giving a total height of 2.4 cm (roughly an inch):

The hole at the bottom left is for the light sensor from the screen. It is slightly larger in diameter for the bottom two layers, because the sensor is larger at the bottom. All layers are available as Adobe Illustrator files.


First, I glued the bottom two layers together.	Then, I added the boards. The beams in the bottom layer make sure everything stays in place without having to use scotch.	You have to drill a few holes to let things out.

I changed a few things with Michael design, because I wanted to reduce the amount of soldering (not my area of expertise). So the audio out (blue square) goes directly out of the enclosure. That means I had to remove the RCA out (yellow square next to it, visible in earlier pictures) because you can't have two holes next to each other. I also decided I didn't need the GPIO out (one less hole to drill).

Once you've done all that, you place the three layers together, and you drill your biggest hole yet: the 16mm hole for the power button. That's the scary part. Finally, I added rare-earth magnets all around the enclosure to stick the top layer (with the screen glued to it) to the bottom of the case.

Making sure everything fits

Even with careful design, 3D CAD software and (in my case) a cardboard mockup, things are a bit tight inside the enclosure. To make things worse, the touch screen reacts strangely to pressure on its back, so anything above the height limit has to go.


A cardboard mockup is not as glamorous as CAD software, but it can give you a better feeling of where things should go, and it lets you experiment.	A before-after comparison of everything I had to cut. Before: everything is still in place.	After: a scorched-earth tactics. But now it fits.

If I had to do it again?

I retrospect, if I was to restart the project again, I would:

Replace, as much as possible, the right angles in the plexiglas cuts with rounded angles. All these right angles are introducing extra constraints in the plexiglas, and making it more likely to crack.
Use a smaller power button. Or rather, on that does not require such a large hole in the enclosure. Not sure which one, though.
Keep the plexiglas (having a semi-transparent enclosure is kind of cool). But it isn't fully transparent in the end: simply because you have to glue, drill... you have to sand it, even slightly. The laser cutter also introduces imperfections that have to be sanded as well.
Use a more accurate drilling machine.
Use a different material for the bottom. I used a 0.2 mm carbon fiber plate. It works, but it feels a bit too thin.
Try to find shorter cables. In the end, all these cables take a lot of room inside the box. But I couldn't find cables in 10-15 cm lengths.
Probably use a different board. The Pi doesn't have wifi nor bluetooth included, which means you must use 2 USB dongles, which means you must have a USB hub, which adds more cables. Also, the range of a small wifi dongle is not too good. And a more powerful processor would be nice.
Rotate the screen so the USB plug for the touchscreen doesn't touch the board.

Full costs

How much did the whole project cost?

Component	Price	Port	Shop/comment
Raspberry Pi + NOOBS	37.50 €	8.95 €	Kubii
10 inch LCD Screen + touchscreen	107.00 €	27.70 €	Chalk-Elec. Works, straight out of the box.
Plexiglas plate, 8mm wide, 0.5 square meter	41.00 €		Home improvement store. Half the plate was enough.
External battery	31.99 €		Amazon. Any battery will do, so long as it outputs 5V.
USB Hub	39.69 €		Amazon. At least 4 ports, must be powered.
Micro-USB to Mini-USB converter	5.89 €		Amazon. To power the battery.
USB Wireless dongle	11.59 €		Amazon.
Heat sink coolers for Raspberry Pi	14.99 €	2.02 €	It seemed a good idea to cool the processor.
USB Bluetooth dongle	9.00 €		Supermarket. Cheap ones at 3 € didn't work.
Bluetooth keyboard	24.00 €		Supermarket
USB plugs (1 standard, 1 micro-USB)	2.45 €	3.49 €	Conrad
1 USB cable	13.95 €	3.49 €	Conrad (to make the external USB plug, for accessories)
20 rare-earth magnets (4mm diameter, 2mm thickness)	4.00 €	12.00 €	Magnets4you (to stick together the two halves)
Switch button	6.50 €	6.10 €	Audiophonics (on/off button. smaller would be better)
Carbon fiber plate, 315 x 245 x 0.2 mm	24.50 €	4.90 €	RC Tecnic (bottom of the case; thicker would be better)
Total	374.05 €	68.65 €

These are just the parts. I'm not counting the tools (soldering gun) and the glue... It's certainly not cost-competitive: for that amount of money, you could have a brand tablet. But it's definitely interesting to build your own tablet.