A Compact Home-Made Raspberry Pi Tablet

June 28, 2017

Although a software developer by day and tech hobbyist by night, I’ve never been into buying every new gadget that comes out. I got my first smartphone in 2015, and have so far resisted getting onto the tablet bandwagon, perhaps because I’m often at or near a computer anyway, rendering a tablet redundant.

I recently got to try an Android tablet for a few days but found it too limiting due to the closed nature of most of the software available for it. Around the same time, I discovered the Raspberry Pi 3 and its matching 7″ touchscreen, and realized I could build a nice Linux tablet completely to my liking based on those, so that is what I set out to do.

Mine certainly isn’t the first Pi-based tablet, as Googling “Raspberry Pi Tablet” will reveal, but I believe it to be the smallest and lightest so far:

• CPU: Raspberry Pi 3B with 1.2GHz quad core ARMv8 and 1Gb RAM
• Storage: Lexar 32Gb micro-SD card
• Display: 7″ 800×480 multi-touch capacitive touchscreen
• Audio: On-board with 25mm speaker, and built-in USB with mic and earphone jacks
• Ports: One external full-sized USB port, micro-USB port for charging
• Operating System: Raspbian Jessie
• Dimensions: 195 × 112 × 17.6 mm (7 11/16 × 4 3/8 × 11/16″)
• Weight: 484 g (17.1 oz)
• Battery: 6200mAh LiPoly, giving 4 to 12 hours battery life (6 hours watching YouTube)

To build such a compact device, I had to heavily modify the two major components and get creative about how to arrange them. This also necessitated a custom enclosure.

In contrast to most of my projects, I did not plan everything out in advance. I very much, “made it up as I went along”, and this article is as much about the journey as it is about the result. If you want to build a tablet like mine, consider this article a rough guide, as there are no complete plans. The photos are approximately in chronological order, although I did do some jumping back and forth between sub-tasks during the actual construction.

Components

The components used in this project are:

• Raspberry Pi model 3B with a 32Gb SD card
• Official Raspberry Pi 7″ touchscreen
• Adafruit Powerboost 1000C charger and 3.7-to-5V converter
• DS3231 temperature compensated real time clock
• USB audio adapter
• PAM8302-based mono class D amplifier
• 25mm 8Ω speaker
• Battery monitor and shutdown control circuit
• 6200mAh Lithium-polymer battery
• SPDT slide switch and 3 small pushbutton switches
• USB and micro-USB ports

Trimming the Fat

The Raspberry Pi is very small, but not small enough to squeeze into such a tiny package. The combined thickness of the Pi and display when assembled as recommended is about 40mm (1 9/16″). I reduced this considerably by mounting the Pi beside the display controller instead of on top of it, and by removing a few unneeded large components from both the Pi and controller.

Raspberry Pi

The tablet would not make use of the wired Ethernet port nor the on- board USB ports, and since these both use a lot of space, I removed them.

Although the tablet makes extensive use of the GPIO pins, everything is hard-wired, so the space-consuming header was unnecessary.

Display

The display is designed for the Raspberry Pi to be stacked on top of the controller board, an arrangement that is about 40mm thick. Since I would be mounting the Pi adjacent to the controller, the tall standoffs could go. The USB power output port is also useless (it doesn’t even provide 5V when the display is powered from 5V), so I unsoldered it too.

The mounting tabs would interfere with the Pi on one side of the display controller and the battery on the other side, so I removed them. I first cut through each one at the centre with diagonal cutters, then bent it up slightly to cut the centre sections off of the sloped sections. The latter were then folded down flush with the metal plate.

The Case

With both the Pi and display controller thinned down, I experimented with various standoffs to see how close to the back of the display I could mount the Pi (still partially overlapping the controller). I eventually arrived at 16mm (5/8″) as the amount of interior space I would need to fit everything comfortably.

I decided that the overall width and height should be just a tiny bit bigger than the screen itself, just enough to protect the glass edges from impacts.

The case consists of three parts. The front is the touchscreen itself, the back is a piece of 0.8mm fiberglass (unclad circuit board material), and the two are separated by a 16mm thick frame.

To construct the frame, I began by building a jig on a piece of very flat plywood left over from another project.

I built the frame inside the jig from solid maple, cut into strips 16mm wide and 2.8mm thick (there is nothing special about the thickness; I just happened to have thin maple of this thickness on hand).

The corners were rounded to about an 8.5mm radius. To do this precisely and consistently, I built another jig to hold the frame, first for rough cutting on the bandsaw, and then sanding to shape with a sanding drum. I designed the jig to pivot around the centre of curvature.

The completed frame was remarkably sturdy, despite being held together only by small triangular blocks and epoxy in the corners (the stuff is strong enough to glue airplanes together). No additional bracing was needed, since the display and case back would provide the necessary structure to prevent the frame from flexing.

The case needs holes cut in it for the power switch, buttons, and various ports, but I deferred this until I knew exactly where all these parts would end up.

Attaching the Pi to the Display

I mounted the Raspeberry Pi face down on three standoffs cut from 5mm (3/32″) square maple blocks, each 8mm (5/16″) long. After fashioning the blocks, they were screwed to the board with M2.5x4mm screws, and then epoxied to the metal display back.

Because of the unusual mounting position of the Raspberry Pi relative to the display controller, I had to slightly trim the ribbon cable at the Pi end (so it could bend closer to the board), and fold it as shown below to connect it to the controller.

Positioning the Display in the Frame

To keep the display in the correct position in relation to the frame, I fashioned a series of spacer blocks to fit between the metal display back and the wooden frame. These were then epoxied to both the metal and the back of the glass, thus also preventing any display creep that some users of the touchscreen have experienced. Some of these spacers also double as mounting surfaces for other components.

All the Other Bits

Once the location of the two major components was cast in stone (or epoxy), I began to play around with the design and/or placement of the other parts.

Power Supply

My tablet is powered by a lithium-ion-polymer battery (whose size I did not choose until I was sure how much space would be left), feeding an Adafruit PowerBoost 1000C. This handy little board contains both a 3.7V-to-5V boost converter that can power the Pi and display, and a lithium ion charge controller. When an external power supply is plugged in, it provides power to both the charger and the system at the same time.

I made a few small modification to the PowerBoost 1000C. As shown in the photo above, I cut the trace to 200kΩ pull-up resistor R13 for compatibility with my shutdown control circuit (described later). This and the other changes are detailed in the modified schematic below:

Due to the confined space with minimal airflow in which the PowerBoost 1000C would be installed, I decided to attach heatsinks to the charging and voltage conversion chips. I made these from thin aluminum sheet, filed the bottom surface flat, and fastened them to the chips with a mixture of epoxy and heatsink compound.

Function Buttons

I wanted to have a few (three seemed about right) physical buttons on the tablet to access operations that might sometimes be awkward to perform using the touchscreen, such as bringing the on-screen keyboard to the foreground or maximizing a window. The switches for these buttons were salvaged from a non-working mouse I pulled out of the hardware recycling bin at work.

To make the actual buttons, I cut the legs off of several old (1970s?) transistors I had in my junk drawer, epoxied them to cardboard from the back of a pad of paper, and then used a hole punch to create nice round buttons with a flange at the bottom.

USB Audio and Realtime Clock

The Pi’s built-in audio is not very good (approximately 11 bits resolution, with poor high frequency response), so I added an aftermarket USB audio adapter for use with earbuds or external speakers. The USB audio also allows for use of a microphone.

I removed the adapter from its case and replaced the USB plug with wires that would be soldered directly to the USB pads on the Raspberry Pi board. I also unsoldered and resoldered the two jacks, which were surface mounted despite the fact that both the jacks and the board were configured for through-hole mounting (which is more secure for something that undergoes repeated physical stress).

The Pi does not have an on-board real-time clock (RTC, really the only thing that it’s missing), so I purchased an external RTC module on eBay. The one I received uses a DS3231 RTC chip, which is identical to the (more common?) DS1307, but features a temperature-compensated crystal oscillator for better long term accuracy. The module is designed to plug directly into the Pi’s GPIO pins, but since I removed the pins, I also removed the socket from the module and replaced it with wires.

Shutdown Controller and Battery Monitor

The PowerBoost 1000C has an enable input, EN, which if pulled low, will turn off the 5V output and put the entire device into a very low power mode. Unfortunately, doing this will not allow the Raspberry Pi to shut down properly. Requiring the user, even though it’s just me, to perform a proper shutdown seemed error prone.

The PowerBoost also has a low-battery warning output (LBO) which can be used to signal the Pi that the battery is almost dead. However, I wanted more precise battery information so that I could display a state-of-charge indication just like an off-the-shelf tablet would have.

To address both of these problems, I designed a small circuit containing both a shutdown controller and a battery voltage monitor.

The shutdown circuit consists of DPDT power switch S1, R8, R9, and C1. When the power switch is turned on, C1 quickly charges through S1A and R8, bringing the EN output high and turning on the PowerBoost 1000C after about 0.1 seconds. When S1A is opened (i.e. S1 off), C1 slowly discharges through R9, and after about 20 seconds, reaches a low enough voltage to turn the PowerBoost off. This time delay circuit is the reason why pull-up resistor R13 had to be disconnected on the PowerBoost. With it present, C1 can only discharge to about 4.2V.

When S1 is switched off, S1B will also immediately pull GPIO pin 40 (physical pin number) low through D2, signalling the Pi that power loss is imminent. A daemon monitoring this pin can then initiate a shutdown.

The circuit consisting of Z1A and Z1B, the two voltage comparators of an LM393 dual comparator IC, comprises the battery voltage monitor. Z1B, R1 through R5, and C2 form a triangle wave oscillator oscillating at approximately 100Hz, with low and high peaks at 1.87V and 3.33V respectively. This triangle wave is then compared against a fraction (0.689) of the battery voltage by Z1A, producing a rectangular wave at its output with duty cycle varying from 0% when the battery is at or below 2.72V, to 100% when the battery is at or above 4.83V. This is fed to GPIO pin 38, where the duty cycle is monitored by the daemon.

The PowerBoost’s low battery output (LBO) is connected through D1 to GPIO pin 36 to warn the Pi, via the daemon, that a critical battery situation has been reached and that it should drop everything and shut down immediately.

As has become my custom for one-of-a-kind analog circuits, I constructed this on a small piece of stripboard. In the photo above right, there are two electrolytic capacitors, 10µF each and wired in parallel to give 20µF. I would have used a single 22µF capacitor if I’d had a small enough one on hand.

For those wanting to reproduce this board, here is the stripboard layout:

Completing the Case

I began by attaching all the display-mounted parts that would require matching holes in the case frame.

Making Holes in the Frame

The next step was to make all the necessary holes. I laid out all the remaining parts needing holes, and carefully measured and marked their locations on the outside of the frame. At the same time, I also marked the locations for the screws that would hold the frame to the display.

Round holes were made using successively larger drill bits so as not to splinter the wood. I made the rectangular holes by drilling a series of round holes, chiseling out most of the remaining material, and finishing the shape with small files and sanding blocks.

The Back Cover

I made the back cover out of a piece of 0.8mm (1/32″) unclad fiberglass circuit board material. I traced around the display directly onto the material and then cut it to size. The straight edges were cut by repeatedly scoring and then snapping the board, and I then rounded the corners by hand, cutting them roughly with tin snips and then sanding them to shape.

The cover is held in place by eight M2.5x4mm screws, countersunk into the cover as far as possible.

Finishing the Frame

I debated with myself for a long time how I wanted to finish the frame. I considered a natural wood finish, glossy black, and metallic silver. Small defects in the wood that occurred while drilling the holes and were repaired with filler ruled out the natural finish, and I finally settled on the slightly retro look of silver.

Component Installation and Wiring

At this point, all the parts were completed. All that remained was to put everything together. The two USB ports and the button switch assembly were already attached to the display, but the PowerBoost 1000C, shutdown controller and battery monitor, USB audio adapter, power switch, real time clock, and battery all still needed to be installed.

I forgot to take a picture specifically of the PowerBoost 1000C installation. It sits on a small platform in the top-left corner, held in place by one screw, and a small peg to prevent it from rotating. It is visible in several of the pictures below.

I also neglected to photograph the power switch, which is visible in the wiring photos further below. The switch is a simple DPDT slide switch, with a rather long handle, from my parts collection. Two 3mm thick blocks were epoxied to its mounting ears. These block then take M2.5x4mm screws from the outside of the frame.

After all of the above components were in place, I began wiring them together, since later components would make access difficult. I then installed the Raspberry Pi and completed most of the remaining wiring.

Instead of a fancy Fritzing diagram as seems to be popular for Raspberry Pi and other projects, I hand drew and then followed the wiring diagram below. The parts are obviously not to scale, but are drawn in their approximate relative locations.

More Work on the Case Back

Once all the components (except the battery) were in place, I could mark where I planned to install the speaker and amplifier, drill holes for the speaker and for cooling, and then finish the outside of the case back.

Battery Installation

I had ordered the battery from a seller on eBay as soon as I knew how much space I would have, but after three months, it still had not arrived. I contacted the seller, and he shipped a replacement by a faster method, which arrived in just over a week.

Internal Amplifier and Speaker

Although the primary audio output of the tablet is via the USB audio adapter, I wanted to have a built-in speaker for applications like an alarm clock, or watching a YouTube video with a friend. I used a tiny and very thin 2.5W (overkill) class D amplifier, and an equally small 8Ω speaker.

Since there is only one speaker and a mono amplifier, I mixed the left and right audio channels together using a pair of 220Ω resistors soldered to the Pi’s audio jack test points, and fed the resulting signal into the amp. Click on the image to the right to see a close-up of the connections to the Raspberry Pi board.

To avoid damaging the speaker, which can’t handle even close to 2.5W, I turned the amplifier’s physical volume control to zero, played a maximally loud recording at full volume, and then turned the amp up to just below the point where the speaker started to sound unhappy. This ensures that nothing I play through the speaker in the future can damage it, even if I watch Star Trek: Into Darkness on my tablet.

All Done!

After screwing the back cover on, the tablet was done!

Software

My tablet runs Raspbian Jessie, which although a great Linux distro, needs a few additions to be a viable OS for a tablet. I added several pieces of software, some open source, and some that I wrote myself (also open source). I started development of the software early in the project so that I had something to do while waiting for various parts to arrive (I powered the tablet from an external 6600mAh lithium ion battery for a few months).

xvkbd

A tablet needs an on-screen keyboard (although I also use it with a Logitech K380 Bluetooth keyboard for tasks requiring a lot of typing). I tried a few alternatives, including matchbox, but found them not to my liking. I eventually settled on xvkbd, customized via the ~/.Xresources file.

twofing

Raspbian treats the touchscreen the same way as a one-button mouse, so common gestures like dragging to scroll or long pressing for a right- click do not work (except in Chromium, which supports these directly). The excellent twofing utility solves all these issues by implementing two-finger gestures for right clicking, horizontal and vertical scrolling, and zooming.

Pi Tablet Daemon

In the construction notes, I’ve made reference to a daemon which monitors various bits (literally) of the hardware. The daemon, named pitabd, is written in C and is launched from /etc/rc.local (at some point I may take the time to configure it properly as a service). It performs the following functions, once per millisecond unless otherwise noted:

• checks for a shutdown signal from the power switch and initiates a proper shutdown.
• monitors the three buttons and performs the corresponding actions:
• bring keyboard (short press) or dashboard (long press) to front
• increase brightness by 1/8 (short press) or to maximum (long press)
• toggle foreground application between normal and maximized (short press) or full screen (long press)
• monitors commands from the dashboard (every 5 seconds):
• enable/disable screen dimming on idle when on battery power
• enable/disable USB and Ethernet ports
• enable/disable Wi-Fi and Bluetooth
• power monitoring:
• status of PowerBoost 1000C charging and charge-completed indicators
• battery voltage
• estimate of energy remaining
• information is written to a tiny RAM disk for display by dashboard
• monitors X11 idle time (at varying intervals depending on need):
• dims display to half of selected brightness after 2 minutes of inactivity
• turns off backlight completely after 5 minutes
• monitors PowerBoost 1000C LBO and performs an immediate shutdown if triggered.

The daemon makes use of the following libraries and utilities:

• bcm2835 – low level GPIO library used to monitor buttons, voltage, etc.
• libxss and xscreensaver – allows the daemon to monitor user idle time
• wmctrl – command line utility used by the daemon to resize windows

I’ve made the source code for the Pi Tablet Daemon available under the GNU General Public License at github.com/svorkoetter/PiTabDaemon.

Pi Tablet Dashboard

The dashboard is a small graphical utility that is launched from ~/.config/lxsession/LXDE-pi/autostart to display the current state of the system, and to allow the user to adjust some settings. It is written using FreePascal and Lazarus (an open source Delphi-like RAD environment). The dashboard performs the following functions:

• displays system status:
• battery voltage
• estimated energy remaining
• CPU/GPU temperature
• lets user adjust power saving settings:
• dim display when idle on battery power
• enable external USB and audio (and the unused Ethernet port)
• enable Wi-Fi and Bluetooth
• displays concise information in its taskbar icon:
• energy remaining both as a gauge and a number
• gauge blinks at 2Hz when energy is below 5%
• blinking lightning bolt at 1Hz indicates charging is in progress
• solid lightning bolt indicates charging is complete

I’ve made the source code for the Pi Tablet Dashboard available under the GNU General Public License at github.com/svorkoetter/PiTabDashboard.

Conclusion

Well, it’s been a long build and a long article. I spent about three months of evenings building this tablet, slowed down from time to time by having to wait for parts to arrive. It has pulled together several of my interests, including electronics, software development, wood working, and just plain tinkering.

The end result is a tablet that is somewhat comparable to commercial offerings in many ways, but is a full blown portable Linux system of about the same power as my travel laptop (and will likely replace said laptop).

This project wouldn’t have been possible without that great little computer and display from the Raspberry Pi foundation, as well as the many add-on bits produced by third parties. And of course, it would have gone nowhere without the huge amount of free software out there, including but not limited to Raspbian (GNU Linux) itself, FreePascal and Lazarus, xvkbd, twofing, and countless other packages that make Linux complete. Without these, I would have been reinventing a lot of wheels.

Addendum February 2018: Adding Better Bluetooth

After successfully using this tablet as a laptop (netbook) replacement I really missed having a mouse. The touchscreen is fine for browsing and e-mail, but tasks like text and photo editing are better done with the higher precision of a mouse.

I ordered a Logitech T630 ultra-thin touch mouse, but found that when I tried to use it together with the keyboard, both the mouse and the keyboard behaved erratically (jerky mouse motions and lost or repeated keystrokes).

A bit of Googling revealed that this is a known problem with the Pi 3’s built-in Bluetooth, and the only solution is to use an external adapter. I decided to try an old Cirago Bluetooth 2.1 adapter I had lying around, and was pleasantly surprised to find that it worked with Raspbian.

Of course, I didn’t want a Bluetooth adapter sticking out the side of my tablet and tying up the only USB port. I followed the the same approach I used with the USB audio adapter, stripping it of it’s casing and wiring it directly to one of the unused ports on the Pi 3 board.

In order to use the external adapter, it was first necessary to disable the built-in one by adding the following to the end of the /boot/config.txt file:

dtoverlay=pi3-disable-bt

After rebooting, Raspbian automatically recognized the new adapter and was happy to pair with the keyboard and mouse, both of which functioned flawlessly. Although it’s too soon to say definitively, it seems that the built-in WiFi is also working better with the built-in Bluetooth disabled.

The adapter I used has long been discontinued, but the same technique should be applicable to any tiny Bluetooth adapter that is known to work with the Raspberry Pi.

Addendum November 2018: Real Time Clock Battery

A few weeks ago, I went to use my tablet after it had been idle for a while, only to find that the clock was showing the wrong time and date after booting. I opened up the tablet to find that the 3V lithium cell in the real time clock module was reading about 0.2V after only about a year of use. I had bought a total of three of these modules at the time, so I went and checked the others and found that their cells were also dead.

I placed an order for replacement cells from a seller on eBay, but while waiting for those to arrive, did some research on the DS3231 chip used in the clock module. It turns out that the allowable range of backup battery voltage is quite wide, so I decided to power it directly from the tablet’s main 3.7V lithium-ion battery instead of a dedicated cell.

To do this, I spliced an additional thin red wire to the connection between the positive terminal of the main battery and the tablet. Then, after removing the clock’s lithium cell, I connected this to where the positive terminal of the cell had been connected. While doing this, I had to be extremely careful to not allow the temporarily exposed positive connection to come into contact with any circuitry or metal parts in the tablet, as it could easily fry something (since the negative terminal was still connected to everything).

                    

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