UPDATE: We now have a much simpler version of this project using CircuitPython and the Adafruit Prop Maker RP2040 Feather.

The 20-sided die — or “d20” as it’s known to tabletop gamers — is an icon of geek culture. In this project we’ll 3D-print a jumbo d20 and outfit it with electronics…because that’s how we roll at Adafruit.

The talking d20 is fun and good for a laugh…though with electronics inside it won’t be 100% perfectly random or balanced…so it’s not suitable for all gaming situations and doesn’t replace your trusty dice bag. Think of it as a geek version of those “executive decision maker” toys.

The code and sounds are completely customizable. For example, you could load each of the 20 faces with the names of local lunch spots and use it to pick the day’s destination.

Parts from Adafruit:

• Adafruit Pro Trinket microcontroller, 5V/16 MHz
• Audio FX Mini Sound Board w/2 MB flash
• Mono 2.5W Audio Amplifier
• Triple-Axis Accelerometer
• LiPoly Backpack
• Lithium Polymer 150 mAh battery
• Breadboard-friendly SPDT slide switch
• 8 Ohm 1/4W thin plastic speaker
• 26 gauge silicone-coated stranded wire, various colors

Parts NOT from Adafruit:

• Magnets: eight (8) disc magnets measuring 1/4 inch diameter by 1/8 inch thick. We used type “D42” from K&J Magnetics.
• Glue: either some E6000 craft adhesive or 5-minute epoxy. DO NOT use JB Weld for this!
• Hardware: eight (8) #4-40 x 3/8" flat-head (countersunk) machine screws.

You will also need:

• 3D printer.
• Soldering iron and related paraphernalia.
• Patience and a willingness to improvise.
• Any ONE of the following:
• #4-40 tapping tool -OR-
• One (1) #4-40 self-tapping machine screw (any length) -OR-
• One (1) #4 wood screw (any length)
• Optional: Ogre-slaying knife, with a plus 9 against ogres.

There are a few other incidentals. Read through the whole guide before ordering anything so you know what steps and tools are involved, and whether this project is a good fit for your skills.

The 3D files for this project can be downloaded from Thingiverse:

There are just two parts to this model. They’re fairly small and will fit even on entry-level printers. No support is needed. For best results, print each half as a separate job…keeps the outside faces cleaner, no strings between halves.

After printing, you may need to do a lot of cleanup with files and sandpaper. Make sure each of the 20 faces can sit flat and doesn’t rock. There may also be “drool strings” on the insides that need to be scraped away.

If the screws don’t have a good “bite” into the plastic, that’s okay…we can add a little glue later to help reinforce these.

If you like, wet-sand the outsides so it’s extra smooth and good-looking. Rinse it off and set aside for a day to dry completely.

One side has a pair of nubs, the other has matching sockets, so you can see how the two halves align. We’ll add some magnets to hold these two halves together, while still allowing easy internal access for charging or reprogramming.

How do they work?

A few important tidbits about this next sequence:

• It’s not simply 1-2-3-done. Each step requires some evaluation and decision making, and must be spread out over time…it may take a couple days to complete!
• Not all magnets are created equal. Even among the rare-earth neodymium type there’s considerable variation in pull among brands.
• The glues we recommend using have a “geometric” cure…they may form a decent hold in the first few minutes, but may take hours (even overnight) to reach maximum strength. Given the magnets’ strength, patience is required to avoid pulling them out accidentally.

Read through all the steps before starting so you know what you’re up against.

Sourcing Magnets

The two halves of the D20 model are held together with 1/4" diameter neodymium disc magnets, 1/8" thick.

There are slots for up to four magnets on each half. Do not fill them all with magnets on the first go! The correct number will depend on the strength of the magnets you acquire. We learned this the hard way…

During development we used four magnets per half, with rare-earth magnets from a particular eBay seller. When that source dried up we needed a steady alternative. Ladyada recommended K&J Magnetics. Indeed, their “D42” magnet is just the right size for this project and can be ordered in any quantity (some other sources required batches of 20, 50, etc.).

Unknown to us at the time, it’s like this company’s neodymium magnets are laced with kryptonite or something. They’re the Altoids of magnets. With our usual total of eight in there (four per half), it’s nearly impossible to pry the D20 open!

Therefore: take this in stages. Start by installing just four magnets (two per half). If these came from K&J, that’s probably enough to hold the die closed. If from a lesser source, the die might fall apart when rolled, in which case you can add a third pair, test again, then possibly a fourth pair.

The magnets are inexpensive, so just order ten or a dozen in one go. You’ll have spares for any slip-ups, or they’ll be handy on the fridge or for other projects later.

Prep Work

Magnets have a specific polarity…they will only click together when facing the right way. If you install even a single magnet upside-down, it will now push the two halves of the D20 apart rather than holding it shut. This first step helps avoid mistakes…

Now you have a polarity indicator when installing the magnets. A “dot” face will always attach to a “blank” face. Never dot-to-dot or blank-to-blank.

Gluing Magnets

This is a painstaking process. Do not rush through it.

I recommend a strong adhesive for this…either 5-minute epoxy or E6000 craft glue. Do not use hot glue. But especially…

JB Weld contains metal particles…when magnets are involved, it will not stay where you put it. Great for other things, but not this project. DO NOT USE.

These photos all show E6000 glue, and that works fine…but having assembled this a few times now I think 5-minute epoxy holds a little better. It’s just time-consuming mixing a tiny fresh batch for each magnet…as you’ll see, it’s vital that they be done individually. E6000 still works well for some other steps later, so you may want both around anyway.

You might do okay with cyanoacrylate “super” glue. Personally, I seem to be cursed when it comes to that stuff…the only things I can get it to hold together are fingers.

Before gluing any magnets in place, do a test-fit in each of the eight magnet sockets. The 3D printing may leave little bumps or artifacts in the sockets that prevent the magnets from being fully seated. Make sure they sit flat and do not rock. Clean out each socket with a file until the magnet cooperates.

When the glue or epoxy has gone tacky enough that the magnet won’t shift, walk away for at least 30 minutes. Make sure your glue is capped, go wash your hands, take a bike ride or watch some cat videos or something. Do not try to install the other magnets yet. Keep them far away (they have a habit of rolling across the table).

These glues have a “geometric” cure…they form a basic grip in a few minutes, but stronger holds take progressively longer. Even “5 minute” epoxy requires 24 hours to fully cure…five minutes is just the working time.

These magnets are so strong that they’ll tug on others in nearby sockets. We need to wait for the glue to set up a bit before adding additional magnets on the same side of the D20.

Second Half…

In a moment we’ll add some magnets to the second half. But let’s take a look at something first…

Also, on the first half we installed magnets with the dots facing up. On the second half we want the blank faces showing. No dots!

Wait

The glue on both halves must be completely dry before proceeding. Completely. That means you’ll resume tomorrow. Make sure the glue is capped, and go wash your hands thoroughly.

Do not allow the two halves anywhere near each other yet! Keep them well apart until tomorrow.

Test Fit

To open the die (to access the electronics later), there’s a certain way you can grip it and pry the two halves apart. I find it helpful to put thumbs on the “9.” and “6.” faces (the only two faces with orientation dots) and hinge it open like a clam.

If any magnets pull out of their sockets: you’ll have to re-glue them and try again tomorrow.

Roll the die around the table a bit. Do the two halves stay securely together, or do they come apart?

Die holds together well: you’re done with this phase and can skip ahead to the next page!

Die halves come apart, or don’t feel quite secure: this calls for additional magnets.

Unless you’re using weak magnets, add these one pair at a time. Each half receives one magnet (follow the same dot/blank face orientation) and keep the two halves separated overnight until the glue is fully dry, then test again. If it’s now solid enough with three magnets, you’re done, otherwise you can add the fourth pair.

Let’s install all the code and sounds on the boards before assembling the circuit. This makes it easier to handle basic troubleshooting (in case a part needs replacing) before everything’s carefully packed inside the model.

If this is your first time using the Adafruit Pro Trinket microcontroller, it’s recommended that you read through the Introducing Pro Trinket guide first, which in turn will direct you to the Adafruit Arduino IDE Setup guide. Get to where you can upload the basic “blink” sketch to the board…that’s a good indication that all the software parts are present and working.

There are three Arduino software libraries that the d20 sketch depends on. These are easily installed in recent versions of the Arduino IDE (1.6.5 or later). From the “Sketch” menu, select Include Library→Manage Libraries… the install the following:

• Adafruit MMA8451 Library
• Adafruit Unified Sensor
• Adafruit Soundboard library

The first two are needed for the accelerometer board that gives us the d20’s orientation, the third handles communication between the Pro Trinket and Audio FX Sound Board, to trigger specific sounds.

The software and audio files for the Talking D20 can then be downloaded from Github:

The Audio folder contains sound files to be installed on the Audio FX Mini Sound Board. This board plugs into your computer with a Micro USB cable…it should then appear as a small (2 megabyte) flash drive. There may be some sample files already on the drive…if so, those can be deleted…then copy the contents of the Audio folder here. Do not copy the Audio folder itself…copy the files inside…33 at the time of this writing.

The d20 folder contains the Arduino code for the Pro Trinket. Open this in the Arduino IDE, select “Pro Trinket 5V/16MHz (USB)” from the Boards menu and upload to the board (press the reset button to start the bootloader).

If you just want to use the standard code and don’t plan to make modifications, skip ahead to the next page.

Custom Sounds and Behaviors

The sound files are stored in Ogg Vorbis format (.ogg) to save space…it’s a format similar to MP3 but without patent restrictions. Few systems can play or record .ogg files “out of the box” — so if you’ll be making your own sounds, you’ll probably need to download a package such as Audacity to convert audio files to this format.

In addition to the 20 sound files corresponding to each face, there’s a startup greeting plus a few jabs or complements if you make a particularly bad or good roll. These are selected randomly (as is the announcement, “You roll…” etc.) so it’s less repetitive to play with.

If you want to make changes to the code for more sounds or different gags, toward the top of the sketch is this big table:

Because of the way Arduino handles strings in program memory, we’re doing something a bit unconventional here: all the filenames are smooshed together into a single large string, but we’re using code formatting shenanigans to make them appear distinct (there are quotes around each filename, but no commas in-between…the compiler joins these into a contiguous string).

Each filename must be padded with spaces if necessary to fill exactly 8 characters. Any more or less will throw the whole thing off!

Declaring these as an array of separate strings in program memory (PROGMEM) would be even uglier…string tables are one situation where Arduino’s PROGMEM syntax just gets nasty.

To play a sound file from this table, pass its index (starting from 0) to the play() function. For example, index #29 is the string "STARTUP " — corresponding to the file startup.ogg, the sound played when the d20 circuit is switched on:

If you insert or rearrange sounds, the indices will change, and you may need to edit several places in the code to accommodate the changed order.

The various boards used in this project have different mounting hole sizes and component clearances. To make our hardware all fit and to play nice with the 3D-printed parts, we have to take some slightly uncouth actions…

On the Pro Trinket, only the two corner holes should be tapped. The center mounting hole is very close to some vital signal traces!

After reaming these holes, clean away any copper rivulets that may be hanging on. You don’t want these dropping off later and causing electrical shorts.

Mounting circuit boards using countersunk screws (rather than round head) is not usually done, but affords us just a little extra clearance around neabrby components, making electrical shorts less likely.

The Plan

Here’s a wiring diagram you can refer to later…maybe even print this out. The physical arrangement of parts won’t look exactly like this…it’s mostly to make all the connections between parts clear:

Red and black lines are power and ground, respectively. Other colors represent various signals…you don’t actually need to color-code everything like this, but it helps prevent mistakes if you do have some different colors of wire available.

26-gauge silicone-coated stranded wire is the bee's knees for this stuff. Super flexy, and the insulation doesn’t shrink when soldered.

There’s not a huge amount of soldering involved, but the tight quarters make this project a challenge.

Additionally, since the die will be dropped and rolled around a lot, the solder connections need to be really solid. Cold solder joints — where solder beads up on surfaces and hasn’t flowed smoothly between components — don’t handle physical stresses and will soon fail. The Adafruit Guide to Excellent Soldering shows what good (and bad) solder joints look like and how to avoid common pitfalls.

Here’s the wiring diagram again for reference. This is laid out to make the connections clear, not as an indication of wire lengths:

When installed in the die, some of the boards are stacked: the amplifier sits underneath the sound board, accelerometer under the Pro Trinket.

We can use fairly short wires between stacked boards, longer wires for any connections across…everything will be trimmed to an exact length later. Some wires will be soldered top-to-bottom, others bottom-to-top.

There should be no loose wires at this point. Every connection should go from one place to another. Double-check all the connections against the wiring diagram before moving forward!

Troubleshooting

Success!

Okay, let’s go through one more test and then plan to seal this up…

Now temporarily wrap some masking tape or some paper around the LiPoly backpack and the switch (to prevent shorts during this test). Wrap them separately, not together.

Turn the switch on and wait for the “Greetings, adventurer!” announcement. Then carefully fold the speaker, battery and other parts inside while closing the other half atop the shell.

Try rolling it a few times. Whenever it comes to rest, it should correctly announce the number on top. It needs to drop at least an inch or so to be triggered…a totally sideways-roll won’t do it.

When everything’s working to your liking and you have a handle on opening and closing the die, take a good look through the interior for ideas to make it more durable. Some dabs of E6000, epoxy or hot glue in strategic places can help ensure that parts or boards don’t vibrate loose or wires don’t get snagged.

Allow any funky chemical glues to dry for at least 24 hours before playing.

As was mentioned in the introduction, the talking d20 is a “for fun” thing…the balance isn’t perfect and it will probably show a slight statistical preference for certain numbers…I’d keep it away from tournaments or OMG SERIOUS BUSINESS gaming sessions. It’s perfectly fine and entertaining for “farting around” games though.

It’s always announced the correct number during testing, but there’s always some possibility of a software bug or calibration mistake or something. So before using this in your own games, lay out the ground rules for situations if the top face and voice were to not match: who has the final word?

I’d suggest using the top face as the final word. Occasionally it won’t detect being rolled and won’t make an announcement, in which case you’d read it…so the “top face” method is consistent across different situations.