Note: the switch design on this page is intended only as a proof of concept (POC) or as an existence proof. Every design element and decision could be improved upon and I hope it will. The purpose of this design is only to get the conversation started.
The POC was modeled using Fusion 360. A real Volksswitch design should be modeled in a more accessible tool like Tinkercad. Fusion 360 and STL files can be obtained from this URL on Thingiverse ().
The Volksswitch has two subsystems: the switch core and the activation surface. The activation surface screws onto the switch core. The image above shows the switch core and four activation surfaces – three of which are square and one of which is round.
The switch core is comprised of a base which encloses two breadboards (one medium size and one small), a printed spring with variable thickness and variable length contact, and components for two circuits (internally powered and externally powered). Each circuit has its own micro-switch and the two micro switches are joined together with a switch gang that is engaged by the spring contact which simultaneously closes both circuits. The switch cover has two posts that constrain the movement of the spring, one post that constrains any movement of the small breadboard and its attached headphone jack, a slot for removal and replacement of the battery driving the internally powered circuit, and a hole for the bolt that screws to the activation surface and translates downward movement of the activation surface into downward movement of the spring.
The switch core looks like this when assembled:
It looks like this in cross-section:
Process of Assembly
1.) Place components on the two breadboards.
2.) Print and install the switch gang over the two micro-switches.
3.) Print the switch base and install threaded inserts on both the top and bottom surfaces.
4.) Use the included double sided tape to attach the medium sized breadboard to the base and insert the two posts of the small breadboard into the provided holes in the switch base.
5.) Print and place the spring across the two pedestals – flat side up.
6.) Print the switch core cover and bolt collar. Gather the bolt and cut a short piece of PTFE tubing.
7.) Insert collar into cover and glue in place.
8.) Place PTFE tube over bolt and glue in place.
9.) Insert bolt through collar from below then place the cover on the base and bolt in place.
10.) If desired, print GoPro mount and bolt to bottom of switch base. (The design for the GoPro mount is based on work by bonapaz.) If placing the switch on a flat surface, I prefer to paint a thin coat of clear Flex Seal on the bottom which creates a good amount of friction between almost any surface and what would otherwise be a very slippery plastic switch bottom.
11,) Print choice of activation surface and install threaded insert. Then screw activation surface to the bolt passing through the switch core cover.
Testing the Device
1.) Plug a male/male stereo headphone cable into the headphone jack on the Volksswitch and then into another jack on a powered circuit (e.g., adapted toy) – in this case a simple LED circuit is used.
2.) Press down on the activation surface until the internal circuits close. The LED inside the switch core and the LED in the external circuit should light at the same instant.
Note that transparent filament (which is what I used above) is, at best, translucent when printed using FDM 3D printing techniques. Because Fusion 360 has a cool rendering engine here’s what the switch base would look like if constructed using a technique that produced an injection molded acrylic kind of result (sweet!):
Volksswitch POC Design Adherence to Requirements
- supports being constructed by individuals closest to the individual with a disability (e.g., therapists, AT professionals, family members).
- can be 3D printed on a consumer accessible printer (i.e., printer cost <= $250)
- All 3D printed parts were printed on a Monoprice Select Mini 3D Printer retailing for $200 – $220
- each instance can be printed and assembled for less than $20
- See parts list below – total cost $xx.xx
- can be assembled and disassembled with no special skills or equipment (e.g., does not require soldering skills and equipment, components bolt/snap together)
- By choosing to use breadboards as the backbone of the electrical circuits, building a circuit is a simple as pushing a component into a couple of holes. Instructions for building the circuits can simply be pictures of which holes to insert which components into. All plastic components are held together with bolts and threaded inserts which won’t strip like a bolt/screw into plastic would. This means that disassembly is also simple and results in no degradation of the parts resulting in
- supports trouble shooting via simple part-replacement techniques
- This is another case where the solderless breadboard facilitates the process of troubleshooting. There’s little cost associated with pulling a part and replacing it with a comparable part. The process is a simple one and if I determine that the part that I’ve replaced wasn’t the culprit I haven’t destroyed it along the way and can reuse it in the next switch.
- can be 3D printed on a consumer accessible printer (i.e., printer cost <= $250)
- supports personalization of aesthetics (e.g., user-specified colors and shapes, activation surface texture, and tactile activation travel characteristics)
- For the most part this requirement is met by keeping the design of the activation surface mostly independent of the design of the switch core. The activation surface has to provide a mounting point that matches the bolt embedded in the core but other than that it just needs to have a center of gravity that doesn’t fall too far from the center of gravity of the core.
- One problem with 3D (FDM) printed is that they don’t have smooth surfaces – particularly those surfaces printed in the Z dimension and sliding two such surfaces against each other produces a grinding sound and feel. You can sand and fill them but the process is labor intensive – which negates one of the major the advantages of 3D printing in the first place. In the POC design, I was concerned about how the bolt would articulate with the collar of the switch core’s cover. By first putting a sleeve of Teflon tubing over the bolt, the bolt moves smoothly up and down adding little friction and no annoying tactile feedback.
- supports simple customization of features and functions:
- activation distance and activation pressure
- I struggled to come up with a design that would provide simple control over both the distance that the activation surface traveled and the amount of pressure required to finally close the circuits. [Note that the micro-switches themselves have a nominal pressure requirement that can exceed the pressure required by the rest of the design. This is unfortunate.] I experimented with coiled spring-based designs and leaf spring (based on thin lengths of spring steel) but the options in both cases are limited and spring steel is difficult to work and prepare. Instead I borrowed aspects of a leaf spring but 3D printed the leaf instead of purchasing it as flat steel. That way I could embed a variable length engagement post directly and independently into the leaf. The thickness of the leaf controls the activation pressure and the length of the post controls the activation distance. Just a few variations provide a significant number of options (see the distance/pressure chart below). There is little cost to printing several copies and keeping them on hand when fitting the switch to the user and swapping them out is as simple as opening the core and dropping in an alternative configuration.
- support for externally powered elements (e.g., adapted toys)
- The POC design incorporates a micro-switch that is wired simply and directly to the headphone jack.
- support for internally powered elements (e.g., visual /auditory / tactile feedback, wireless interfaces)
- The POC design incorporates a micro-switch that is wired in series with an internal battery (in this case a 3 volt coin cell battery) that can power other micro components like an LED, a buzzer, and a vibrator. It will be necessary to determine what the latter circuits would look like and whether they could be implemented simultaneously (LED + buzzer, LED + vibrator, etc.) given a fixed number of holes on the breadboard. Finally, the presence of an internal, powered circuit begs the question of whether wireless connectivity is possible.
- support for common mounting and positioning solutions
- A GoPro compatible mounting face can bolted to the bottom of the switch core and/or a rubberized coating like Flex Seal can be applied to prevent the switch core from sliding when placed on a flat surface.
- supports simple scaling of the components to meet a variety of applications and abilities
- The POC design falls short when it comes to this requirement. The incorporation of the breadboard sets a minimum size for the switch core in the X and Y dimensions. The thickness (Z dimension) of the breadboard and the requirement that components be mounted to its upper surface further limits the degree to which the design could be shrunken in the Z dimension. Expanding the design in any of the dimensions would not create a problem for this design.
- activation distance and activation pressure
- supports simple re-implementation to better fit the needs and desires of the individual with a disability without the destruction of the component that is being replaced
- As stated with respect to the troubleshooting and maintenance section, the solderless nature of the POC design makes reconfiguration of the internal circuitry non-destructive for the electrical components. The fact that major components are assembled using bolts and threaded brass inserts makes their replacement non-destructive as well.
- supports simple, end-user replacement of components with limited life (e.g., batteries)
- It would be a simple enough process to unbolt the switch core cover and access the coin battery that way but but cutting a slot in the cover, it’s possible to remove and replace the battery. Additional slots have been added to support battery removal and replacement using a tool like a pair of needle nosed pliers. [Note that the pliers create a short between the two poles of the battery as soon as they make contact so the period of time in which the pliers and battery are in contact should be brief at most.]
- exhibits high reliability simply by avoiding known, low reliability configurations and components
- The POC attempts to meet this requirement by using “off the shelf” electronic components.
- It also incorporates a headphone jack in the switch core so that standard stereo and monaural headphone cords can be used to connect the core to external, powered devices and the interface from the cord to the switch core can leverage strain relief features of every headphone cord. Many switches fail because the cord was “hardwired” to the switch and the wires in the cord fatigued due to repeatedly being wrapped around the switch body.
- supports simple and low/no-cost design customization if the supplied designs fail to fit the needs and desires of the individual with a disability
- The POC was designed using Fusion 360. Autocad currently offers a free license to Fusion for students and teachers for a three year period. After that a significant price has to be paid. Additionally, Fusion is non trivial to learn to use. I think that this makes Fusion a bad choice for the final Volksswitch design. I think a better choice would be a tool like Tinkercad which is free to use and relatively simple to use (I doubt that 3D modeling will ever be trivial). The POC design could be re-implemented in Tinkercad though some features would be difficult to represent. For example I think that Tinkercad supports champfers but not fillets.
- many aspects of the design should be reusable and extensible across new switch types, new form factors, and alternative technologies
- This requirement is still a bit vague to me – and I wrote it! I’m thinking that an optimal design will have reusable elements that facilitate porting the concepts (or at least don’t hamstring the porting) to other implementation technologies (e.g., injection molding, laser sintering, stereolithography) or other form factors – e.g., how many of the POC design elements could be reused if a switch was created based on this form factor:
POC Component Costs and Overall Cost
Because vanilla WordPress doesn’t provide a table tool, I’ve created the table in Word and provide a link to a PDF version below. The total cost is $xx which meets the Volksswitch requirement that the total cost to produce the switch be less than $20.
Chart of POC Activation Pressure and Activation Distance Options
Eight spring options were printed with different values of spring thickness (1 mm, 1.5 mm, and 2 mm) and post lengths (4 mm, 6 mm, and 8 mm).
Post length is measured from the flat side of the spring so the resulting activation distance is actually independent of activation pressure parameters but it will affect the initial height of the activation surface.
Activation pressures range from 0.65 kg to 2.75 kg while activation distances range from 1 mm to 5 mm. The following links lead to pressure and activation measurements a scatter plot of the two values for several printed spring options.
The data shows that a wide range of options are available by making small changes to the thickness and post length.