SurfCuit: Surface Mounted Circuits on 3D Prints
SSurfCuit: Surface Mounted Circuits on 3D Prints
Nobuyuki Umetani Ryan Schmidt
Autodesk Research { nobuyuki.umetani,ryan.schmidt } @autodesk.com ABSTRACT
We present, SurfCuit, a novel approach to design and con-struction of electric circuits on the surface of 3D prints. Oursurface mounting technique allows durable construction ofcircuits on the surface of 3D prints. SurfCuit does not requiretedious circuit casing design or expensive set-ups, thus wecan expedite the process of circuit construction for 3D mod-els. Our technique allows the user to construct complex cir-cuits for consumer-level desktop fused decomposition mod-eling (FDM) 3D printers. The key idea behind our techniqueis that FDM plastic forms a strong bond with metal when it ismelted. This observation enables construction of a robust cir-cuit traces using copper tape and soldering. We also presentan interactive tool to design such circuits on arbitrary 3D ge-ometry. We demonstrate the effectiveness of our approachthrough various actual construction examples.
Author Keywords
Creativity support tools; DIY; Fabrication; Rapidprototyping; 3D Printing; Electrics; Physical computing
ACM Classification Keywords
H.5.m. Information Interfaces and Presentation (e.g. HCI):Miscellaneous
INTRODUCTION
Recent advances in consumer 3D printing technology havemade it possible for end users to casually fabricate 3D plasticobjects. Many ’makers’ would like to add interactivity to theirprinted objects using sensors, lights, motors, and so on. How-ever, incorporating the necessary electric circuits into theseobjects has not become inherently easier with 3D printing.Electric circuits are typically designed in 2D, and mountedon planar geometries, such as printed circuit boards (PCBs)Inserting a flat circuit inside a 3D object requires extensivegeometry editing to create cavities, wire routing paths, andfixtures. This is generally beyond the reach of the novice orcasual maker.An alternative to is to use 3D circuitry, where 3D traces areembedded into the object volume or surface. However exist-ing CAD interfaces and fabrication techniques have not beendesigned with 3D circuits in mind. In this paper, we demon-strate both a design tool and fabrication technique to integrate
Figure 1. SurfCuit allows the user to design and fabricate surfacemounted circuits on 3D prints (left). An illumination circuit (top right)is mounted on the surface of Christmas tree shape (bottom right). the mechanical and electrical functions of simple objects. Ourapproach, which we call SurfCuit, allows the user to designand construct functional and durable electric circuits on thesurface of 3D prints (see Fig. 1).As a prototyping method, surface mounting has various ad-vantages over embedded circuitry. First, construction is mucheasier since the parts are accessible from outside – the userdoes not need to insert the electric parts during printing, ortry to fit components into tiny cavities. Second, it is easyto debug and repair surface-mounted circuits, while in manyembedded-circuit applications this is difficult or impossible.Finally, the circuit design task is greatly simplified – com-pared to three-dimensional arrangements of cavities, fixtures,and wire channels, surface layouts are intuitive and efficientto create.Our key challenges are (i) how to fabricate complex circuitson the surface of 3D prints and (ii) how to help the user per-form the circuit layout task directly on the 3D surface in acomputational design tool. Conductive inks do not readilyadhere to 3D print plastics and their resistance is too highfor many types of components. Instead, SurfCuit uses coppertape and tubes that are soldered together to achieve mechani-cally durable and highly conductive circuits. We leverage thefact that near soldering temperature, 3D-printed PLA plasticmelts to a sticky viscous fluid that bonds well to copper ma-terial. To further enhance the fabrication process, our designtool adds shallow channels and holes to the 3D model beforeprinting. These channels both help the user to re-create theirvirtual circuit traces in the physical world, and also help to1 a r X i v : . [ c s . G R ] J un rmly affix the copper tape and through-hole parts onto the3D print.The traces which connect components in an eletrical circuitmust be physically isolated, i.e. they cannot intersect with ea-chother. Because they exist in a (possibly curved) 2D space,with even a moderate number of traces it becomes very dif-ficult to create the circuit without carefully planning out thetraces ahead of time. Board layout and planning software likeEAGLE is an essential tool for planar circuit desgin. Howeverno existing circuit planning tool is applicable to arbitrary 3Dsurfaces. In addition, strips of copper tape cannot follow arbi-trary paths on 3D surfaces, as many paths introduce too muchtorsion into the tape, which will result in kinks or tears. Thestrips should be laid out along geodesics , and without com-putational guidance it is very difficult to ensure that the 3Dtraces have this property. SurfCuit offers an interactive de-sign tool that allows the user to easily adapt an existing planarcircuit schematic to a 3D surface.To demonstrate the capabilities of SurfCuit, we have designedand fabricated a variety of 3D objects with 3D circuitry. Someof these examples involve circuit complexity and levels ofvoltage/current that are well beyond what has been demon-strated in the literatuer. In addition, we also perform somedestructive testing to demonstrate the robustness of our fabri-cation process. We show various examples of how our systemfacilitates the user’s creation of functional 3D objects withelectric circuits. BACKGROUND AND RELATED WORKMolded Intergrated Devices
Various advanced manufacturing technologies support thefabrication of circuitry that conforms to curved surfaces.For example, the Optomec Aerosol Jet system can createmetal traces on simple 3D forms using laser metal deposi-tion techniques. Another technology, Molded InterconnectDevices (MID), makes it possible to install circuitry on plas-tic surfaces. MID enables functional integration of electriccircuit into small spaces, thus is often used for compact im-plementation of electronic products such as cell phones, cars,and advanced micro robots (see Figure 2). However, man-ufacturing such objects involves multi-axis laser engravingmachines and etching/plating baths only found in advancedindustrial facilities. Our SurfCuit system is inspired by thesetechniques, but our goal is to introduce MID-style fabricationin a more accessible context. In addition, currently there areno CAD tools for MID design. Our SurfCuit design tool is di-rectly applicable to these advanced manufacturing methods.
Figure 2. Example of molded interconnected devices. (Left) A roboticfinger tip sensor by CITEC, Bielefeld University. (Right) FEST BionicAnt Robot
Interactive 3D Prints
Various works in the human-computer interaction and fab-rication literature have addressed the topic of adding inter-activity to 3D prints. Techniques have been presented toconvert 3D prints into sensors that include cameras [20],acoustics [15] and light-guides [25]. Sato et al. [19] studiedfrequency-dependent impedance properties to detect config-urations of conductive 3D objects. Printput [7] and Capri-cate [23] use conductive filaments to convert the surface of3D prints into capacitive touch sensors. However, such con-ductive filament traces have very high resistance, making itdifficult to supply enough current to drive larger components(See Section Resistance Comparison).Fabricating 3D circuit traces inside ”tubes” passing throughthe interior of 3D prints was explored by Savage et. al. [21].This method is effective at hiding the circuitry, but also il-lustrates the challenge of repairing such devices. The ca-pabilities of the automatic routing algorithms also limit thecomplexity of the circuits that can be designed with this ap-proach. Hudson [9] studied the use of conductive threads forthe yarn-based soft 3D printing. The commercially-availableVoxel8 3D Printer [3] can embed circuits in a plastic print us-ing conductive inks. The liquid ink can be deposited on theouter surface of prints, but only in upward-facing regions.
Prototyping Flat Circuits
Solderless prototyping methods such as traditional bread-boards and LittleBits [5] are the fastest way to assemble acircuit, but these methods not suitable for permanent use.The resulting circuits are fragile, and also space consuming.Recently, advances in conductive ink have made it possibleto directly deposit circuit traces on flat sheets using con-sumer ink-jet printers [11]. ShrinkCuit [13] uses ShrinkyDinks TM as the substrate to enhance complexity and con-ductivity of conductive-ink-based circuits. Sketch In Circuit[16] uses copper tape for prototyping traces on paper. Cir-cuit Sticker [8] enables rapid prototyping of more complexcircuits by pasting ready-made circuit boards on the top oftraces. Ramakers et al. presented an interactive design sys-tem for circuits printed on paper [17]. The motivation be-hind these works is to enable creative circuit prototyping for“makers” and non-experts. We share this motivation, and ourgoal is to extend these ideas to arbitrary free-form 3D surfaceswith a robust construction technique and interactive trace lay-out interface. Circuit Design Tools
There are many circuit layout design tools for planar circuitboards. For example, commercial packages such as Eagle TM ,AutoTRAX TM and DipTrace TM provide comprehensive envi-ronments for design and simulation for PCB. Autodesk 123DCircuits [1] provides a layout design system for breadboards.Autodesk Project Wire provides 3D circuit layout for theVoxel8 printer [3]. Our SurfCuit 3D circuit design tool en-ables novices to quickly design 3D traces constrained to thesurface of an arbitrary input mesh. SURFCUIT CIRCUIT FABRICATIONConstruction Procedure D print soldering a b c d e
Figure 3. Workflow of SurfCuit. The user first draws a 2D schematic diagram of a circuit (a), then positions the electrical components on a 3D shapeand connects them with curved traces (b). SurfCuit automatically generates channels and holes on the surface (c) to guide the user in placement ofcopper tapes and tubes. Finally, the user solders the copper pieces together to achieve a robust circuit on the 3D print.
The robustness of electrical connections is very important forpermanent circuit construction — disconnection of a singletrace can disable an entire circuit. However, constructing ro-bust, highly conductive traces on a curved 3D-printed sur-faces has been difficult. In this paper, we take advantage ofthe fact that PLA and ABS plastics that they melt into sticky,glue-like viscous fluids at around 200 ◦ C. Since the meltingpoint of solder is also around that temperature, soldering ontraces and pins melts the surrounding 3D plastic and strength-ens the mechanical bonds between them. The result is thatafter cooling, the highly conductive copper traces are firmlyaffixed to the surface of the 3D print. Note that our approachis inspired by recent works that exploit melting behaviors forfabrication [14, 18]. However here we use melting for bond-ing, not for forming.This melting technique creates strong connections, but ac-tually using it on 3D surfaces requires some pre-planning.Computational design tools are necessary to plan the spatiallayout for complex circuits. To guide the user in fabricatingtheir design, we computationally generate channels and holesin the 3D surface before printing, which also helps to increasecircuit robustness. Hence, the workflow of SurfCuit fabrica-tion is as follows (see Fig. 3):1. In the SurfCuit design tool, the user first draws a 2D cir-cuit schematic diagram, then lays out the electric parts andtraces on a 3D surface.2. SurfCuit generates a 3D mesh with channels and holes thatguide the user to install copper tape and tubes on a 3D print.3. Then, the user covers these copper traces with solder, andsolders the traces, pins, and electric parts together.4. Finally, a thin layer of clear lacquer spray electrically pro-tects the traces.With SurfCuit, a novice maker can easily create complexfunctional 3D objects using widely-available single-materialFDM printers. The soldering process requires exactly thesame skills as fabricating a 2D circuit, which we alreadyknow that nearly anyone can learn to do. Applying solderto the copper tape is not difficult, as the solder naturally flowsinto the trace channels due to surface tension (see accompa-nying video). This soldering step also thickens the traces,further increasing mechanical robustness and electrical con-ductivity.
Construction Detail
Our circuit fabrication process is intended to be used withthough-hole parts. Though-hole parts are desirable becausethey are widely available and are easy to manually solder.More importantly, through-hole parts achieve mechanicallystronger bonds compared to surface mounted parts. Thus,they can be left exposed on the surface. The interval betweenpins is typically 1/10 inch = 2.54 mm for though-hole parts.Our technique maintains sufficient isolation between neigh-boring traces by keeping the trace width and pin diametersmaller than this interval (see Fig. 4).
Figure 4. (left) Dimensions of holes and channels generated by our toolto guide copper tape and tube placement. We used 1.5 mm-wide coppertape and copper tubes with 1/16 inch (1.6 mm) diameter. The imageson the right show the channels and holes in a 3D print (top), and thepost-soldering result (bottom).
The channels and holes generated by our design tool are es-sential for the fabrication process. We arrived at this processafter multiple iterations with simpler techniques. The benefitsof this method include: • The channels and holes help the user to accurately re-produce their complex virtual designs • The channels and holes provide a large contact area be-tween the copper traces and the 3D print. • The holes help to temporarily hold components in placewhile they are soldered • The channels partially enclose the traces and prevent themfrom peeling off. • Since copper tape is placed in a V-shaped channel, it is easyto cover the channel with solder – surface tension makesthe solder naturally flow into the channel.The use of solid copper, rather than conductive inks or similaralternatives, is desirable for three reasons. First, copper has3omparatively high conductivity / low resistance (less than . / m in our traces). Thus, we are not limited by parasiticresistance when using the large electric currents typically nec-essary for driving small motors or incandescent lights. Sec-ondly, its high heat conductivity makes it possible to bondcopper to 3D-printed plastics via the application of heat. Fi-nally, the copper is solderable. Solder quickly spreads on thesurface of copper and creates mechanically and electricallyrobust bonding.Compared to silver-based conductive inks (the most conduc-tive), copper is also inexpensive and widely available. Tomake 1/16 inch traces, we split widely available 1/8 inch cop-per tape (about 10$ for 50 m) in half using a rotary cutter. Forcopper pins, we used 1/16 inch copper tubes manufacturedvia K&S Engineering .Inc (three dollars for 1 m). Resistance Comparison
During the development of our fabrication process, we ex-perimented with various conductive inks and copper paints.However we found that most are difficult to apply to plas-tic surfaces. Liquid-based materials were also unreliable asa small crack results in electrical disconnection. This isespecially problematic on the rough surfaces of 3D FDMprints, where the ink will tend to pool into the small cavi-ties and channels produced by the printing process, creatinghighly variable thickness in the conductive layer. Further-more, as reported in the previous studies, the electric resis-tance in the conductive liquids are very high (e.g, 11.48 Ω for a 28cm x 0.5cm trace using silver conductive ink [13]and 2 Ω /inch for a 3mm diameter tunnel filled with copperpaint [21]).We also tried drawing traces with conductive PLA filamentusing a hand-held plastic extruder, however again the resis-tance is significantly higher than typical PCB traces, and thuscannot support many common circuits. For example, BlackMagic 3D’s Conductive Graphene Filament, which has oneof the highest conductivity among the filaments on the mar-ket, still has 0.6 Ω / cm volume resistivity. To achieve sim-ilar resistance as our traces (0.5 Ω / m ), the cross-sectionalarea should be at least 120 cm (=11cm x 11cm), vastly largerthan the through-hole component pitch (2.54 mm). In otherwords, if we use conductive filament for traces, and the traceshave a 1.5mm x 1.5mm cross-section (small enough to con-nect to through-hole components), then a 20 cm long tracehas more than 0.5 k Ω resistance. Such resistance causes overa 1V voltage drop with only 2mA current, which is barelyenough to light an LED. Generally, conductive filaments aresufficient for capacitive touch sensors or blinking LEDs, how-ever they fall short when attempting to drive common micro-controllers, actuators, and transducers. Robustness Comparison
The user of copper tape in circuits-on-surfaces is not entirelynovel, in particular copper tape is widely used in wearablesand fashion tech. However these circuits are generally veryfragile. To demonstrate the mechanical robustness of ourapproach, we made a qualitative comparison with a na¨ıvemethod using destructive testing (see Figure 5). In the na¨ıve construction method, traces are just copper tapesplaced on the 3D prints without the channels and solder. Thetape generally has a sticky backing which is sufficient to holdit in place. The test circuits light a LED using a trace pat-tern that consists of more than twenty copper tapes (see Fig-ure). We then brushed the circuits for one minute with ny-lon brushes to observe the robustness of the circuit. We firstused a relatively soft nylon kitchen brush designed for wash-ing dishes, and then switched to very hard nylon brush meantfor scraping off rust.The circuit with na¨ıve construction immediately stoppedfunctioning when the soft nylon brush was applied. This isbecause the connection between overlapping copper tape seg-ments is particularly weak and breaks under small mechanicalforces. After few additional seconds of brushing, the cop-per tape segments in the na¨ıve construction start to peel offthe plastic. After one minute, the copper traces in the na¨ıveconstruction were severely damaged, to the point where thecircuit would need to be entirely rebuilt. On the other hand,there was no visual damage to the SurfCuit traces even afterthe brushing with the hard brush. The circuit with SurfCuitconstruction stayed functional during the entire experiment.Please refer to the accompanying video for the detail of thecomparison. before brushing after brushingtypical construction our construction
Figure 5. (left) LED lighting circuits with na¨ıve construction and Sur-fCuit construction. (right) After brushing for one minute, the traces inthe na¨ıve construction were severely damaged while the SurfCuit traceswere intact.
SURFCUIT DESIGN TOOL
Designing the trace layout for a complex circuit (e.g., 10+connection points) typically requires planning before startingconstruction. Each trace that is added constrains the designspace of future traces, because no traces can intersect. For the2D flat circuits, we can plan on a paper or in vector-graphicsCAD tools. However, since our circuits are on 3D free-formsurfaces, we cannot lay out traces on a 2D flat geometry. Itis also not practical to lay out traces as 3D space curves, askeeping them ”on the surface” is very cumbersome. Hence,we developed a domain-specific interactive CAD tool whichallows users to arrange parts and traces on arbitrary 3D sur-faces.
User interface of Surfcuit design tool
Our SurfCuit design tool has two modes: 2D schematic de-sign mode and 3D part and trace layout mode (see Fig. 6).The schematic mode lets the user specify electronic parts andtheir connections in the form of a 2D diagram, while the 3Dlayout mode lets the user arrange the parts and traces on the4D model’s surface. The user can switch back and forth be-tween these modes during circuit editing. While the user editsthe circuit in one mode, a small window highlights the elec-tric parts or traces currently being edited in the other mode,making 2D/3D correspondence easy to understand. Note thatwe are inspired by existing works showing highly abstractedschematic diagram while editing complex models [26, 12].
3D layout windowchanging connection2D schematic windowprint button clickrelease
Figure 6. (left) SurfCuit 3D trace design interface. The user designstraces on a 3D object by dragging and rotating parts. The 2D schematicwindow is shown at the same time to facilitate understanding of the cir-cuit structure. (right bottom) The user can change connectivity of tracesby a simple gesture.
The schematic diagram is desirable as a circuit input ratherthan drawing traces directly on a 3D surface becauseschematics are symbolized and thus easily comprehensible.In our schematic editor, the user places symbols of electricparts and specifies these connections. To make the schematictidy, the user also can add or delete a point on a trace andswitch connectivity of points inside connected traces (seeFig. 6-right bottom). Note that thousands of schematic dia-grams are widely available on the internet, for virtually anykind of circuit. Thus, inputting such diagrams does not re-quire sophisticated electronics knowledge, a novice designercan simply copy an existing schematic.The electric parts and their connections, specified in the 2Dschematic window, are imported to the 3D layout window.The user lays out the parts by dragging and rotating them onthe 3D surface. The traces are automatically generated on thesurface in real-time during editing, making it easy for the userto lay out parts while avoiding intersecting traces. Similar tothe schematic editor, the user can also add/delete points onthe trace and change connectivity of points inside connectedtraces. Note that the 3D operations maintain the topologi-cal connection between pins of parts. These connections arespecified in the 2D schematic window and are automaticallyreflected in the 3D layout window.Finally, when the user presses the “print” button, the sys-tem creates the necessary channels and holes on the meshthat correspond to the design, and then exports the geome-try for use in 3D printing software. Channels on the surfaceare generated using the stroke parametrization technique [22],which generates texture coordinates with minimum distortionaround a stroke. We simply displace points of the mesh in thenormal direction with respect to the distance computed fromthe parametrization.
Algorithmic detail of SurfCuit trace computation user adding a new point trace generation stuckannotation of failure
Figure 7. Surface trace generation algorithm. (left) Starting from twoendpoints, the line segment between the two points is projected on thesurface to find tangential directions for each point. We then incremen-tally slide each point along the surface in that projected direction, untilthey meet each other. (right) If the algorithm fails we annotate the fail-ure to prompt the user to add an additional point.
SurfCuit updates the routing of traces interactively during theuser’s editing. A trace is computed as a curve on the surfaceconnecting two end points, each of which is either a pin of auser-specified trace midpoint. To make manual constructioneasy, traces should be as short and straight as possible, asthis will introduce the smallest amount of twisting (torsion)in the copper tape. The shortest (and thus straightest) path ona surface connecting two points is called a geodesic .There are various existing methods to computegeodesics (e.g., [24]), however true geodesics are ex-pensive to compute. Thus, we use a heuristic method toestimate geodesics in real time (see Fig. 7-left). The basicidea behind our method is to start with the two endpoints p and q , and move them towards eachother until they meet.The direction of movement is defined by a vector in thetangent plane at each point. We compute these directinosby first finding a 3D direction, and then projecting into thetangent plane and normalizing. For a pair of points p i and q i ,the tangent-plane directions are: (cid:126)t pq i = ( I − (cid:126)n p i ⊗ (cid:126)n p i )( (cid:126)q i − (cid:126)p i ) | ( I − (cid:126)n p i ⊗ (cid:126)n p i )( (cid:126)q i − (cid:126)p i ) | (1) (cid:126)t qp i = ( I − (cid:126)n q i ⊗ (cid:126)n q i )( (cid:126)p i − (cid:126)q i ) | ( I − (cid:126)n q i ⊗ (cid:126)n q i )( (cid:126)p i − (cid:126)q i ) | (2)where I is an identity matrix, and (cid:126)n p and (cid:126)n q are unit normalvectors at (cid:126)p and (cid:126)q . We then update the positions of points( p i → p i +1 , q i → q i +1 ) by taking small steps in the direc-tions of (cid:126)t pq i and (cid:126)t qp i , respectively. To keep the resulting pointon the triangle mesh, we use discrete parallel transport [6].First, a point on a triangle is updated in the direction of thetriangle’s tangent vector (cid:126)t until it hits the boundary (edge) ofthe triangle. Then, at the boundary we transform (cid:126)t into theplane of the next triangle using a minimal rotation around theedge of the triangle, i.e. the rotation that takes the currenttriangle normal to the neighboring triangle normal. We stopthis update when the p i and q i move 1 mm along the surface,and call them p i +1 and q i +1 .This simple algorithm can fail when the normal of the sur-face (i.e., (cid:126)n p or (cid:126)n q ) becomes parallel to (cid:126)p i − (cid:126)q i (see Fig.7-5 M386 9V NE555NE5556V6V LDRNE555 chirping bird smartphone stand police car
Figure 8. Fabricated SurfCuit examples and their schematic diagrams. right). In such cases, we annotate failure as a line connecting p and q in order to prompt the user to add another pointalong the trace. Clearly this algorithm does not produce abounded approximation to a geodesic, however it does pro-duce exact geodesics for simple surface geometries such asplanes or spheres. And in practice, we have found that it ishighly effective at producing low-torsion 3D curves which areideal for trace fabrication. SURFCUIT EXAMPLES
To demonstrate the effectiveness of our approach, we presentseven different examples created using our SurfCuit designtool and fabrication method. Each example is chosen to show-case various properties of SurfCuit. Specifically, we demon-strate integration of many different sensors (for light, touch,and sound), controller ICs, and transducers (for light, electro-magnetic actuators, sound, and radio wave) into small spaces.Note that all the examples are fully self-contained and workwithout external controllers or power sources. Please refer tothe accompanying video for more detail.We can fabricate SurfCuits on a wide variety of in-put meshes. In these examples, the input mesheswere taken from the Thingiverse TM Enriching 3D Prints with Sound and Lights
Christmas Tree
Figure 1 shows a Christmas tree (thing:608606) that blinks13 LEDs in an asynchronized timing using a 16 pin timer IC,CD4060. This example has many components on a relativelysmall volume. 21 electric parts, 20 traces, and one 9-volt bat-tery are integrated into the volume (12cm x 6cm x 6cm). Thisexample also demonstrates the inclusion of an IC with manypins using SurfCuits. Such complex circuits typically needcables over the traces when constructed on a single-sided 2Dboard. However, in the Surfcuit, we can take advantage of thethree dimensional structure of the object to avoid such cables.For example, if we cannot connect two parts on the front sidewithout intersection, the trace can go around the back side.Because the circuit construction is three-dimensional, thereare more degrees of freedom in the trace layout.
Chirping Birds
SurfCuit enables the fabrication of complex circuits in a verysmall volume. The chirping bird (Fig. 8) integrates a lighttheremin circuit, which uses a 555 timer IC and photoresis-tor, into a 3D bird shape (thing:359531). The light theremincircuit modulates the pitch of the sound according to the in-tensity of light received by an LDR sensor. Thus, a usercan create chirping sounds by waving a hand on the top ofthe bird. This behavior significantly enhances the static birdgeometry– not only does it generates sounds, it makes thebird a playable instrument. This example also demonstratescircuit integration into a small space. The part volume is verysmall (2.5cm x 3cm x 6cm) but because we use the full 3Dspace we can fit both the thermemin circuit and 2cm-diameterbatteries. Our interactive layout tool allowed us to avoid ob-6tructing semantically-important features such as the bird’sface, and place the batteries and switches in the occluded areabehind the tail.
Smartphone Stand
The speaker-embedded iPhone stand (Fig. 8) augments asmartphone stand shape (thing:642881) by integrating a cir-cuit using an LM386 timer IC to amplify the sound signal.This smartphone stand exemplifies the integration of geomet-ric and electrical functionality. While the original geometryprovides the function to hold a smartphone, the circuit ampli-fies the audio signal. 3D printing makes it easy to fabricatethe precise shape needed. Achieving the same geometricalfunctionality is very difficult with flat circuits.
Police Car
The police car example (Fig. 8) integrates a circuit that blinksa LED beacon while a magnetic speaker generates sirensounds, which is modulated by two 555 timer ICs. Aside fromthe functionality of making sounds and lights of a police car,the surface mounted circuit also gives mechanical appearanceto the 3D printed shape. The input mesh (thing:806770) isclearly a car but it lacks any detailed texture, in large part be-cause the printer is limited to a single material. The circuitryon the car body gives the shape some definition, creating amore interesting machine-like appearance that would not bepossible with 3D printing alone.
Dynamic 3D Prints with High-Current Circuits
As previously discussed, our traces have only small amountsof parasitic electric resistance, and thus can handle largeamount of electric current (up to 1 Ampere, possibly more).This is enough current drive electro-magnetic actuators, al-lowing us to create mechanized objects.
Octopus Fan
Our Fan example (Fig. 9-left) demonstrates a touch-sensitiveUSB fan, where the user can toggle a DC motor fan on andoff by touching specific locations of the print. The shape ofthe fan is designed to be clipped to the top of a computermonitor. Upon making physical contact, a 555 timer IC de-tects a small current transmitted through body, and togglesthe motor control. We use a MOSFET to amplify the out-put and drive the 5V DC motor with about 1.0 A current.SurfCuit is convenient for fabricating touch-sensitive objectssince the traces are naturally exposed to the surface. We suc-cessfully mounted long curved traces on the octopus’s tenta-cle (thing:158069).
Cat Robot
In the waving cat example (Fig. 9-right), an ATtiny85, whichis an Arduino-compatible programmable micro controller,drives two servo motors which wave the arm and shake thehead of a cat statue (thing:163032). This robot also drawseveral hundreds milliamperes of current at peak load. Theuse of a programmable micro-controller makes the inter-action/behavior design of this object much more flexible.Again, SurfCuits highly conductive traces are critical to al-lowing the micro-controller to drive the various outputs.
Concealed Circuit waving cat
ATTiny859V servo 1 servo 2LM78055V NE555 octpus USB fan
Figure 9. Examples of high-current electric circuits. (Left) Octopus USBfan with a touch switch. (Right) Cat robot waving its hand and its head.
So far, our examples have placed the circuit components andtrances on the exterior surface of objects. These circuitshave increased the functionality and interactivity of the 3Dprints via sensor-controlled transducers. However, adding on-surface circuits does involve modifying the original surface,which may be undesirable if the surface has specific func-tional or aesthetic purposes. An example of such a require-ment arises in cases where we may wish to obscure the func-tionality of the circuit. Fig. 10 shows a FM transmitter thatis concealed inside a shape of a squirrel (thing:11705). Thissquirrel spy could be used in covert recording or nanny-mictype applications. To create this object, we split the squirrelgeometry along a curved partition surface, and then imple-
Figure 10. A covert Squirrel Spy which contains a concealed FM trans-mitter.
Circuit as a Design Element
While in some cases we might wish to hide the circuit, inothers we can actively use the circuit as part of the designaesthetic. Many people find beauty in circuits, as seen inthe circuit jewelry (e.g., Circuit Breaker Labs [2]) and wear-able fashion shows. In fact, many ground-breaking worksof industrial design have integrated the internal engineeringmechanisms into aesthetics of the design. Notable examplesinclude Swiss watches showing the gear work or movement,the iMac G3’s translucent body, and the intentionally-exposedfunctional and structural elements of the Pompidou museumin Paris. Although we cannot claim our own results as worksof art, SurfCuit enables this aesthetic by making it easy forcreative users to integrate circuitry elements into the externaldesign of complex 3D shapes. For example, our featurelesscar above was turned into a police car with more interestingsteampunk styling.To further illustrate this concept, we created a circuit that il-luminates EL (electro-luminescent) wires whose placementis designed based on an existing circuit-like facial tattoo (seeFig. 11). The core of the circuit is an inverter that converts 9VDC current to 120V AC current using 555 timer IC and a mi-cro transformer. Although most of the traces do not contributethe function of the circuit, the texture of the metal traces com-pletely changes the aesthetic of the otherwise smooth andmonochrome 3D-printed head (thing:33503). This examplealso demonstrates the use of quite high-voltage circuits withSurfCuit, where insulation between traces is critical.
Figure 11. Facial tattoo model uses circuit’s trace patterns as a designelement (Left). This model is inspired by an artistic circuit tattoo byFaeriegem (Right).
FUTURE WORK
We are interested in making the circuit design system moreintelligent by incorporating a circuit simulator (e.g., SPICE),a physics engine (e.g., Open Dynamics Engine) to simulateprinted characters’ dynamics, a schematic image recognitionsystem [4], or an interactive sketch beautification system [10]to facilitate the user’s creative circuit integrated 3D object de-sign.
CONCLUSION
We presented SurfCuit: a system that integrates circuits into3D prints by mounting them on the printed surface. Ourconstruction method enables building rather complex, highly-conductive circuit patterns robustly on FDM-based 3D prints.Our interactive design system enables intuitive input and 3Dlayout of electric circuits on 3D geometry.
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