Duet
Duet is a pair of handheld devices that enable two people to collaborate on a drawing, or compete in games like Tic-Tac-Toe and Four-in-a-Row.
At the heart of each device sits an Adafruit Feather M0 with an RFM69HCW Radio, which is attached to a 2.4" 320x240 TFT Touchscreen FeatherWing, and powered by a Lithium Ion Polymer battery, all encapsulated in 3D-Printed case. Creating Duet was an interesting exploration of the intersection of Electronics, 3D Design/Printing, and Software.
-
-
System Indicators
Like many battery-powered devices, it is important to indicate the battery level so the user knows when they should charge the device.
Similarly, a device that communicates wirelessly should indicate the signal level so the user knows when the devices are getting too far apart.
An area in the top-right of the display was reserved for a battery meter, which would periodically update based on battery voltage measurements, and a signal meter which would update after each packet was received with the RSSI.
Each reading is added to a circular buffer from which the average is calculated and used to update the meters, this prevents the meters jumping around too much and appearing glitchy.
It was here that the first problem was encountered - measuring the battery voltage. While the Feather M0 can read the battery voltage from analog pin #7, and Adafruit kindly provided sample code to read and calibrate the value, it turned out that analog pin #7 and digital pin #9, which is the chip select wire for the display, are the same physical pin!
As if the folks at Adafruit had foreseen this exact scenario, the TFT FeatherWing was designed with jumpers on the chip select wires so the display could be reassigned by cutting the trace connecting it to digital pin #9 and soldering on a short wire to connect it to digital pin #11. However this meant that the tiny 105mAh LiPo battery that used to live between the headers connecting the Feather to the FeatherWing would no longer fit and would need to move next to the headers. Though this turned out to be a blessing in disguise because both the display and radio are relatively power-hungry and the new location has more space so to maximum running time the battery could be upgraded to the larger capacity of 260mAh.
-
Adding Games
The next step was to add some games! Tic-Tac-Toe and Four-in-a-Row were chosen due to their ubiquity and simplicity.
The gameplay is quite simple; players take turns touching the screen to make a move which is applied locally to their board and then transmitted to the other device to be applied to that board. Each device will then determine if the game is over, either because a player has won or because there are no more legal moves left.
Since the top-right corner of the display already held the battery and signal meters, it made sense to make a toolbar along the top edge from which the user could select and see the current mode; either Draw, Tic-Tac-Toe, Four-in-a-Row, or System Info (including Player #, Battery Voltage, RSSI, Radio Temperature, Free RAM).
The unreliable communication method that was initially chosen so the drawing was smooth was not sufficient for turn-based games in which each device needs to maintain consistent board state. Both the RadioHead and LowPowerLab library include a reliable communication method in which the receiver acknowledges receipt of each packet and the sender retransmits packets that are not acknowledged within a time limit.
-
Four-in-a-Row
-
Designing the Case
First the electronics were measured and modeled in OpenSCAD so the case could fit them well. Extra cutouts were added for the visible area of the display, the USB port, the reset button, and On/Off slider switch.
Next, the Minkowski and hull operations were combined to create a thin shell that could encompass the electronics. Then mounts were added to hold the PCBs, nuts, and bolts.
Initially insets we created so buttons can be accessed, however this was suboptimal - while the reset button was easy to press, the on/off switch was small and difficult to slide.
Since all players would use the on/off switch but only some players would need to use the reset button, a cap was designed to fit onto the on/off switch and enable it to be gripped and slid more easily, and a hole was added so the reset button could be pressed with a paperclip.
-
Assembly
The case is printed in Polymaker Polylite PLA in four pieces; Front, Center, Back, and the Cap for the On/Off slider switch. It could be printed in three, with the Center and Back combined, but more filament would be wasted on supports.
-
Plug Feather into FeatherWing, then plug the Battery into Feather, and tuck excess wires between the Feather and FeatherWing.
-
Push the Cap onto the On/Off slider switch.
-
Put electronics into the Center/Back case, put front panel on, and tighten the nuts and bolts.
The total package weights 55 grams.
-
-
3D Design/Printing is an iterative process...
-
-
-
-
An early prototype 🪛
-
