Building a Pi-based ARM laptop

Mark II

The key insight in the improved design is that a gender-changing adapter from HDMI plug to micro-HDMI socket is now easily sourced. Additionally, with the removal of the plastic finish in the dock hinge designed to match the smartphone's rounded contours  [5], it becomes possible to interface the Pi directly to the lapdock (Figure 6).

Figure 6: Removing the dock's phone contour plate.

Shaving the side of the connector with a bench grinder to ensure full length insertion is necessary with some rounded adapters. A tightly coupled adapter can support the Pi in place with no other structural assistance  – this design frees the Pi from the need to be attached to the screen's back or to sit on the table at the end of a cable.

Because it sits on the docking hinge, the board won't affect screen movement when attached in this way (Figure 7), and it will follow the lapdock without needing to be lifted off a table independently of the keyboard and screen.

Figure 7: The Raspberry Pi solidly coupled to the lapdock by means of a short HDMI adapter.

The second component of the revised design is a micro-USB splitter cable designed to allow the simultaneous charging of two devices while delivering a data connection to only one of them. You will need a micro-USB socket to USB-A plug adapter on the keyboard side and, to complete the design, a micro-USB left-angle adapter on the lapdock side, because there is not sufficient room between the hinge and the Raspberry Pi to fit the splitter cable.

These four parts can be sourced for less than US$  5 each from many eBay sellers; selected US, Hong Kong, or China-based suppliers often ensure free US shipping as well. The cost of shipping is quite important for the project's budget, because you're looking at six different orders at least (four cables/adapters, lapdock, Rasp Pi). However, I was still able to indulge in design details further and added a Raspberry Pi sticker to my order for the Pi itself on Adafruit's site, to adjust the device's natural Motorola look.

The finished device, in this no-solder, no 3D-printing, no cable-splicing edition looks pretty good, and it can withstand real-world use (Figure 8). I call it the "LEGO version" for its use of premade components.

Figure 8: The finished Mark II design.

Software Configuration

In the latest Raspbian [6] image as of this writing (2013-12-20 Wheezy), almost everything works out of the box. Exceptions that need some tweaking include sound; editing the /boot/config.txt file, standing in for what would be a BIOS configuration, allows you to force the SoC (system on a chip) into the DMT mode the lapdock utilizes. If you uncomment the option


by removing any leading hash (#) symbols, the speakers will come to life.

You can easily test this by playing any wave file (.WAV) with aplay. The default choice is for DVI mode, which does not support audio, so forcing the DMT selection at bootup is required. Similarly, if HDMI detection proves unreliable and you need to force this selection over composite, set hdmi_force_hotplug=1 in the config.txt file; I did not find this to be necessary.

The Raspberry Pi USB-A port might not provide enough power for some WiFi modules, in which case the WiFi module may be plugged into one of the lapdock's USB-A ports instead. It otherwise configured out of the box as wlan0 and was easily set up with the WiFi Config tool (icon on desktop).

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