A Raspberry Pi Zero in a Handy Tech Active Star 40 Braille Display

TL;DR: I put a $5 Raspberry Pi Zero, a Bluetooth USB dongle, and the required adapter cable into my new Handy Tech Active Star 40 braille display. An internal USB port provides the power. This has transformed my braille display into an ARM-based, monitorless, Linux laptop that has a keyboard and a braille display. It can be charged/powered via USB so it can also be run from a power bank or a solar charger, thus potentially being able to run for days, rather than just hours, without needing a standard wall-jack.

[picture: a Raspberry Pi Zero embedded within an Active Star 40] [picture: a braille display with a keyboard on top and a Raspberry Pi Zero inside]

Some Background on Braille Display Form Factors

Braille displays come in various sizes. There are models tailored for desktop use (with 60 cells or more), models tailored for portable use with a laptop (usually with 40 cells), and, nowadays, there are even models tailored for on-the-go use with a smartphone or similar (with something like 14 or 18 cells).

Back in the old days, braille displays were rather massive. A 40-cell braille display was typically about the size of a 13" laptop. In modern times, manufacturers have managed to reduce the size of the internals such that a 40-cell display can be placed in front of a laptop or keyboard instead of placing the laptop on top of the braille display.

While this is a nice achievement, I personally haven't found it to be very convenient because you now have to place two physically separate devices on your lap. It's OK if you have a real desk, but, at least in my opinion, if you try to use your laptop as its name suggests, it's actually inconvenient to use a small form factor, 40-cell display.

For this reason, I've been waiting for a long-promised new model in the Handy Tech Star series. In 2002, they released the Handy Tech Braille Star 40, which is a 40-cell braille display with enough space to put a laptop directly on top of it. To accommodate larger laptop models, they even built in a little platform at the back that can be pulled out to effectively enlarge the top surface. Handy Tech has now released a new model, the Active Star 40, that has essentially the same layout but modernized internals.

[picture: a plain Active Star 40]

You can still pull out the little platform to increase the space that can be used to put something on top.

[picture: an Active Star 40 with extended platform and a laptop on top]

But, most conveniently, they've designed in an empty compartment, roughly the size of a modern smartphone, beneath the platform. The original idea was to actually put a smartphone inside, but this has turned out (at least to me) to not be very feasible. Fortunately, they thought about the need for electricity and added a Micro USB cable terminating within the newly created, empty compartment.

My first idea was to put a conventional Raspberry Pi inside. When I received the braille display, however, we immediately noticed that a standard-sized rpi is roughly 3mm too high to fit into the empty compartment.

Fortunately, though, a co-worker noticed that the Raspberry Pi Zero was available for order. The Raspberry Pi Zero is a lot thinner, and fits perfectly inside (actually, I think there's enough space for two, or even three, of them). So we ordered one, along with some accessories like a 64GB SDHC card, a Bluetooth dongle, and a Micro USB adapter cable. The hardware arrived a few days later, and was immediately bootstrapped with the assistance of very helpful friends. It works like a charm!

Technical Details

The backside of the Handy Tech Active Star 40 features two USB host ports that can be used to connect devices such as a keyboard. A small form-factor, USB keyboard with a magnetic clip-on is included. When a USB keyboard is connected, and when the display is used via Bluetooth, the braille display firmware additionally offers the Bluetooth HID profile, and key press/release events received via the USB port are passed through to it.

I use the Bluetooth dongle for all my communication needs. Most importantly, BRLTTY is used as a console screen reader. It talks to the braille display via Bluetooth (more precisely, via an RFCOMM channel).

The keyboard connects through to Linux via the Bluetooth HID profile.

Now, all that is left is network connectivity. To keep the energy consumption as low as possible, I decided to go for Bluetooth PAN. It appears that the tethering mode of my mobile phone works (albeit with a quirk), so I can actually access the internet as long as I have cell phone reception. Additionally, I configured a Bluetooth PAN access point on my desktop machines at home and at work, so I can easily (and somewhat more reliably) get IP connectivity for the rpi when I'm near one of these machines. I plan to configure a classic Raspberry Pi as a mobile Bluetooth access point. It would essentially function as a Bluetooth to ethernet adapter, and should allow me to have network connectivity in places where I don't want to use my phone.

BlueZ 5 and PAN

It was a bit challenging to figure out how to actually configure Bluetooth PAN with BlueZ 5. I found the bt-pan python script (see below) to be the only way so far to configure PAN without a GUI.

It handles both ends of a PAN network, configuring a server and a client. Once instructed to do so (via D-Bus) in client mode, BlueZ will create a new network device - bnep0 - once a connection to a server has been established. Typically, DHCP is used to assign IP addresses for these interfaces. In server mode, BlueZ needs to know the name of a bridge device to which it can add a slave device for each incoming client connection. Configuring an address for the bridge device, as well as running a DHCP server + IP Masquerading on the bridge, is usually all you need to do.

A Bluetooth PAN Access Point with Systemd

I'm using systemd-networkd to configure the bridge device.

/etc/systemd/network/pan.netdev:

[NetDev]
Name=pan
Kind=bridge
ForwardDelaySec=0

/etc/systemd/network/pan.network:

[Match]
Name=pan

[Network]
Address=0.0.0.0/24
DHCPServer=yes
IPMasquerade=yes

Now, BlueZ needs to be told to configure a NAP profile. To my surprise, there seems to be no way to do this with stock BlueZ 5.36 utilities. Please correct me if I'm wrong.

Luckily, I found a very nice blog post, as well as an accommodating Python script that performs the required D-Bus calls.

For convenience, I use a Systemd service to invoke the script and to ensure that its dependencies are met.

/etc/systemd/system/pan.service:

[Unit]
Description=Bluetooth Personal Area Network
After=bluetooth.service systemd-networkd.service
Requires=systemd-networkd.service
PartOf=bluetooth.service

[Service]
Type=notify
ExecStart=/usr/local/sbin/pan

[Install]
WantedBy=bluetooth.target

/usr/local/sbin/pan:

#!/bin/sh
# Ugly hack to work around #787480
iptables -F
iptables -t nat -F
iptables -t mangle -F
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

exec /usr/local/sbin/bt-pan --systemd --debug server pan

This last file wouldn't be necessary if IPMasquerade= were supported in Debian right now (see #787480).

After the obligatory systemctl daemon-reload and systemctl restart systemd-networkd, you can start your Bluetooth Personal Area Network with systemctl start pan.

Bluetooth PAN Client with Systemd

Configuring the client is also quite easy to do with Systemd.

/etc/systemd/network/pan-client.network:

[Match]
Name=bnep*

[Network]
DHCP=yes

/etc/systemd/system/pan@.service:

[Unit]
Description=Bluetooth Personal Area Network client

[Service]
Type=notify
ExecStart=/usr/local/sbin/bt-pan --debug --systemd client %I --wait

Now, after the usual configuration reloading, you should be able to connect to a specific Bluetooth access point with:

systemctl start pan@00:11:22:33:44:55

Pairing via the Command Line

Of course, the server and client-side service configuration require a pre-existing pairing between the server and each of its clients.

On the server, start bluetoothctl and issue the following commands:

power on
agent on
default-agent
scan on
scan off
pair XX:XX:XX:XX:XX:XX
trust XX:XX:XX:XX:XX:XX

Once you've set scan mode to on, wait a few seconds until you see the device you're looking for scroll by. Note its device address, and use it for the pair and (optional) trust commands.

On the client, the sequence is essentially the same except that you don't need to issue the trust command. The server needs to trust a client in order to accept NAP profile connections from it without waiting for manual confirmation by the user.

I'm actually not sure if this is the optimal sequence of commands. It might be enough to just pair the client with the server and issue the trust command on the server, but I haven't tried this yet.

Enabling Use of the Bluetooth HID Profile

Essentially the same as above also needs to be done in order to use the Bluetooth HID profile of the Active Star 40 on Linux. However, instead of agent on, you need to issue the command agent KeyboardOnly. This explicitly tells bluetoothctl that you're specifically looking for a HID profile.

Configuring Bluetooth via the Command Line Feels Vague

While I'm very happy that I actually managed to set all of this up, I must admit that the command-line interface to BlueZ feels a bit incomplete and confusing. I initially thought that agents were only for PIN code entry. Now that I've discovered that "agent KeyboardOnly" is used to enable the HID profile, I'm not sure anymore. I'm surprised that I needed to grab a script from a random git repository in order to be able to set up PAN. I remember, with earlier version of BlueZ, that there was a tool called pand that you could use to do all of this from the command-line. I don't seem to see anything like that for BlueZ 5 anymore. Maybe I'm missing something obvious?

Performance

The data rate is roughly 120kB/s, which I consider acceptable for such a low power solution. The 1GHz ARM CPU actually feels sufficiently fast for a console/text-mode person like me. I'll rarely be using much more than ssh and emacs on it anyway.

Console fonts and screen dimensions

The default dimensions of the framebuffer on the Raspberry Pi Zero are a bit unexpectedly strange. fbset reports that the screen dimension is 656x416 pixels (of course, no monitor connected). With a typical console font of 8x16, I got 82 columns and 26 lines.

With a 40 cell braille display, the 82 columns are very inconvenient. Additionally, as a braille user, I would like to be able to view Unicode braille characters in addition to the normal charset on the console. Fortunately, Linux supports 512 glyphs, while most console fonts do only provide 256. console-setup can load and combine two 256-glyph fonts at once. So I added the following to /etc/default/console-setup to make the text console a lot more friendly to braille users:

SCREEN_WIDTH=80
SCREEN_HEIGHT=25
FONT="Lat15-Terminus16.psf.gz brl-16x8.psf"

Note

You need console-braille installed for brl-16x8.psf to be available.

Further Projects

There's a 3.5mm audio jack inside the braille display as well. Unfortunately, there are no converters from Mini-HDMI to 3.5mm audio that I know of. It would be very nice to be able to use the sound card that is already built into the Raspberry Pi Zero, but, unfortunately, this doesn't seem possible at the moment. Alternatively, I'm looking at using a Micro USB OTG hub and an additional USB audio adapter to get sound from the Raspberry Pi Zero to the braille display's speakers. Unfortunately, the two USB audio adapters I've tried so far have run hot for some unknown reason. So I have to find some other chipset to see if the problem goes away.

A little nuisance, currently, is that you need to manually power off the Raspberry, wait a few seconds, and then power down the braille display. Turning the braille display off cuts power delivery via the internal USB port. If this is accidentally done too soon then the Raspberry Pi Zero is shut down ungracefully (which is probably not the best way to do it). We're looking into connecting a small, buffering battery to the GPIO pins of the rpi, and into notifying the rpi when external power has dropped. A graceful, software-initiated shutdown can then be performed. You can think of it as being like a mini UPS for Micro USB.

The image

If you are a happy owner of a Handy Tech Active Star 40 and would like to do something similar, I am happy to share my current (Raspbian Stretch based) image. In fact, if there is enough interest by other blind users, we might even consider putting a kit together that makes it as easy as possible for you to get started. Let me know if this could be of interest to you.

Thanks

Thanks to Dave Mielke for reviewing the text of this posting.

Thanks to Simon Kainz for making the photos for this article.

And I owe a big thank you to my co-workers at Graz University of Technology who have helped me a lot to bootstrap really quickly into the rpi world.

P.S.

My first tweet about this topic is just five days ago, and apart from the soundcard not working yet, I feel like the project is already almost complete! By the way, I am editing the final version of this blog posting from my newly created monitorless ARM-based Linux laptop via an ssh connection to my home machine.

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