Fab@Home:Embedded Circuits
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Embedded Circuits
This page presents a summary of some of the work I (Dan) did with conductive silicone embedded inside structures, generally made of ordinary window sealing silicone. This work was performed somewhat concurrently with Evan's awesome flashlight, but sought to explore slightly more complex circuit designs. I suppose this page could be looked at as a more media-oriented version of the paper that Evan and I submitted to the Solid Freeform Fabrication (SFF) symposium this year.
This is an initial revision of this page, and as such, is rough around the edges. I will refine it as I have more time.
Conductive Material Performance
Initially, I started to work with Caig Laboratories' CircuitWriter conductive ink; it was readily available at Radio Shack, and seemed to have the electrical properties we were looking for. However, its viscosity proved to be too low for easy use with Fab@Home, requiring me to essentially use the syringe tip like a ballpoint pen: only by dragging the tip across the substrate could I get nice, clean lines. Thus, it'd work alright for printing directly on the table, but not so well for printing on objects. It also was difficult to ensure contact between the ink and whatever components were inserted.
Next, I tried some SS-26F conductive silicone from Silicone Solutions. This material holds its shape very well, having a viscosity very similar to the normal silicone that we use. I printed out some traces of the SS-26F, shown below, and tested the cured product on a four-point probe. More data to follow on this test.
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Planar Circuit
I used the conductive silicone to produce a circuit that would allow us to hook up a TS555 Timer chip simply and effectively. The clear silicone was used to create trenches to hold the traces. Once the entire object had cured, the components (resistors, capacitors, etc) could simply be pushed through the traces. Applying power to the circuit caused the LED to flash with a period of roughly 1/2 second, as chosen by the values of the resistors and capacitors. The link below the third picture shows the circuit in action.
 CAD model of silicone structure and traces. |
 Diagram of the timer circuit |
Gumby
Hod has always had this great vision of people being able to build their own action figures. In the spirit of that idea, and given the mechanical properties that silicone exhibits, I thought Gumby would be an appropriate starting point. However, to put a twist on the new Gumby, I decided to create him with embedded conductive traces, allowing him to have LED eyes that lit up when power was applied.
Revision 1
The first Gumby has traces that run from head to foot, meaning that when one inserts wires into his feet, which would cause his eyes to light. The conductive silicone proved to be quite flexible, making it an excellent material for this application. In this version of Gumby, the LEDs were wired in series, and run off of two external AAA batteries.
 A breakout of the Gumby 1 model, showing his layers. |
 Gumby 1 in mid-build. |
 The test setup for Gumby 1 |
Revision 2
After the success with Gumby 1, Evan and I decided that there was no reason not to make Gumby a self-sufficient object, basically inserting the power source inside of him. For this application, a 2032 watch battery fit the bill, being thin enough to fit while still providing the necessary voltage. However, in this case, the LEDs were in parallel.
The primary challenge in this build was making the contact with the upper surface of the battery. The lower surface was simply set upon one conductive silicone trace. Once the battery was inserted, the second trace was completed by building a hollow tube alongside the battery, and the conductive silicone was printed up the tube and on top of the battery.
The video, linked under the fourth picture, shows Gumby's eyes flickering; the conductive silicone does not make good contact when uncured silicone is printed on cured silicone. The two seem not to adhere to each other. In addition, the silicone tends to shrink as it cures, so this can play a big role in how the ultimate product turns out.
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 A look at the lower level of Gumby 2's traces. |
 With the battery & LEDs dropped in, Gumby 2 is roughly half done. |
