The suprisingly complex technology behind Ravensburger’s King Arthur board game

In the early-mid-2000s, the German company Ravensburger put two games on the market, that combined classical board games with interactive electronic components: King Arthur and Die Insel (“The Island”). I played both of them when I was young and in a sudden nostalgia attack I got a copy of King Arthur pretty cheap off eBay. The game isn’t considered very deep, but there is one thing about it, that still fascinated the engineer in me after all these years: How does the “Touch-and-Play” technology behind this game work?

Ravensburger's King-Arthur Board Game (2003)

So, in this post, I’m reverse engineering the technology behind these games a bit. But first, let me explain how they work from a player’s perspective: The board contains a big plastic element in the shape of Merlin’s stone (King Arthur) or (somewhat) of a volcano (Die Insel), housing the electronics and a speaker. On the board itself you have several location fields for the player figures and a few fields with actions (e.g., interact, fight) located to the side. In both games, you have four figures in four colors available, one for each of the 1-4 players. Besides this, the games contain other typical board game components, such as cards and tokens.

King-Arthur's board: "Merlin's Stone" housing the electronic in the back; two player figures standing on some location fields. For interaction, the player has to touch the knight's head on a field and an action field on the bottom of the board.

To interact with the game, a player puts his figure on the relevant field on the board, and touches the head of the figure simultaneously with one of the action fields. This triggers some (actually stateful) game logic and some audio output, which is the main feedback channel from the game to the player.

Incorrect Theories

And this is actually the interesting part of it, because in touching the figure and the action, the game needs to determine three parameters:

• Which action was taken.
• The location field the figure is on.
• Which figure (color) is acting.

Sounds simple at first, but when you think about it, it is actually not that easy. If it was only two parameters, everything would be simple: Your body is conducting, you close an electrical circuit between the field and the action and the game knows what to do. And indeed, some kind of electrical circuit is closed, as you can clearly see electrical traces on the board leading towards the fields and actions. But how does the game get the third parameter?

Traces on the game's board

Let’s begin with some initial (incorrect) theories I had on how this might have been implemented:

• Resistance measurement: The figures could have a specific resistor built-in, and the game measures the resistance between a location field and the action field. The issue here is that the body resistance of a player is pretty high (~8 MΩ between my arms), and it varies a lot (depending on your arm length, skin moisture, …). After measuring the “resistance” of the figures (~2 MΩ) it is safe to say: This isn’t how it is done.
• Capacitive measurement: Placing a capacitor in a figure and measuring the impedance between two fields? This has the same issues as above and was not used either.

Ok, why not ask Ravensburger directly? I sent them my question via their website, and to my outstanding amazement, I actually got an answer by the engineer that supervised the project back then - super cool! 😎👍. Unfortunately, he didn’t really know the details either, as the technology was done by an external company - but still very kind of him to answer!

So, let’s take a look at what’s inside Merlin’s stone!

Disassembly-Time

Opening the plastic stone is quite simple, it is just held by 12 Torx screws. Inside we can see the PCB and a bracket with an elastomeric connector to connect the PCB to the traces on the game. On the other side of the PCB we can find a switch, two seven segment displays, a touch knob (used to register players to the game), an uninteresting dip component (I think it was a shift register) and a daughter board with a blobbed IC. Behind the PCB we can find the speaker as expected.

Disassembling "Merlin's Stone"

Ok, this doesn’t give us a direct hint on how things work, but at least we can find the name of the company that created this technology on the PCB: Innovision. We’ll come back to this later. Maybe the figures themselves give us more insight?

Contrary to opening the “Stone”, the figures are actually not designed to be opened, but are plastic parts glued together. As I want to keep the game intact, let’s first check what we can find out from the outside. The figures contain a conductive rubbery pad at the bottom, and the head of the figure is made of some kind of conductive plastic.

Sigh, I guess I’ll have to open a figure. Fortunately, I got a second set of figures for ~5€ off eBay, so let’s sacrifice the blue knight - for science!

The inside of a player figure. There is nothing inside the head, just a solderable metal rod screwed inside.

We find another small PCB by Innovision inside and the curious thing about it is, that this PCB contains a small antenna. Taking a second look at the “Merlin’s Stone” PCB, we can see an antenna on there as well. And that’s actually part of the answer to the question: Each figure sends the location and action information along with the color code via near-field radio. Looking up old versions of Innovision R&D’s website (they apparently have been acquired or so lately), this makes sense—the company is specialized on RFID and other near-field radio.

Player figure PCB with another blobbed IC and a small antenna.

So upon touching the figure and an action field on the board, the player closes the circuit and powers the IC inside the figure, which then sends information to the main unit. Still, the question remains: How does the game know which action and location the player is on? Let’s grab the oscilloscope and investigate the fields on the board!

Measuring the signal on an action field. There is special conductive black ink on the board.

The capture of the oscilloscope reveals: “Merlin’s Stone” modulates a 20 Hz signal on all fields that encode the field ID. This signal differs from field to field and the action fields have a significantly different waveform than the location fields.

Signals measured on a location field (blue) and on an action field (yellow). The two images are captures of two different action fields. It can be seen that one bit in the yellow trace differs (red arrow).

And this is the final solution to the mystery:

• An individual signal is modulated to each field.
• Touching the figure and the action field closes the circuit and the figure’s IC is powered through the player.
• The IC in the figure perceives the composition of the field and action signal and can thus determine the location and action chosen.
• The IC in the figure sends the combination of color, location and action via near-field communication to the main unit, which triggers the game logic.

An individual signal is modulated on each field. The composition of the signals gives a unique identification for the field and action. The PCB is powered by the signal as well.

By connecting the Action field and the figure, the circuit is closed, and the figure sends the color, field and action to the game.

Conclusion

One could go further from here and decompose the RF communication or decode the signal for the individual fields and actions, but I’m satisfied for now. Overall, I’m quite impressed how many smart ideas are below the surface of this at first glance simple game. Now my mind can rest, and I can finally start to actually play the game.