Portable electronics prototyping

ㅤBackground

ㅤSo, about ten years ago I worked in a company producing industrial equipment. I always tried to place purchased products in control cabinets. But the moment came when I couldn’t find a device on the automation market that solved the problem at an adequate price. The closest solution that could solve the problem looked like a microscope as a nail driving tool. This situation forced me to do some homemade work.

ㅤTo debug the device, a prototype was assembled from several module boards mounted on plexiglass with TFF-M3x12 racks. The obstacle to assembling the prototype on the table was the fact that it had to be mobile to be carried to the workshop for debugging along with the equipment. In the center there was a module with stm32 with minimal wiring, an rs485 module and some other specific ones for controlling industrial equipment.
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Division into modules simplified debugging of the circuitry; in case of errors, not the entire device was corrected. In addition, if during the process there was a need to connect the module to other pins of the microcontroller, this was done instantly, since the modules were connected to each other by dupont jumpers on PLS pins. Further development of circuitry and software was carried out.
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At the moment when there were no questions about the circuit solutions, the final device was developed based on the developments, the PCB was traced and the housing was selected for a DIN rail. Parts and printed circuit boards have been ordered. And while the heavy machine of the supply department was cranking, software debugging continued on the prototype. And by the time the components and printed circuit boards were delivered to the final version, software debugging was completed. The device was assembled, installed on the equipment, the prototype was disassembled and put into a box. Until the moment when the need for homemade crafts arose again.
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Taking into account the previous project, when the prototype withstood frequent transfers from the office to the workshop and back, development proceeded with the same approach. Some modules from the previous project were reused, the missing ones were manufactured. During the assembly process, it turned out that holes were made in certain places, and to change the relative position of the modules, new holes need to be drilled. Therefore, an array of holes was made in a new sheet of plexiglass with a pitch of 5 mm and a diameter of 3 mm. This made it possible to place the modules in any order, since the mounting holes on the modules also had a pitch of 5mm.
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Since then, several more projects have been developed with this approach over a fairly large time interval. Nothing new was introduced in the prototyping approach, due to the fact that the main activity was not electronics development.

ㅤPortable prototyping

ㅤFrom the above story, we can conclude that it is enough to prepare a part with an array of holes, a supply of racks, screws, dupont jumpers and insert a 5mm pitch into the mounting holes of new modules – over time, you will get a kind of set with which you can assemble a mechanically integral prototype.

ㅤBut in the hike described above, I saw some incompleteness. All this time there was an obsession to somehow develop this direction and share our work, since I am sure that there are those who have encountered a similar situation. Not the need for DIY, but the assembly of a portable prototype. Recently I had time to devote some time to the topic of portable prototyping.

ㅤBase

ㅤBase I call a flat part with a grid of holes on which modules are attached. The dimension of the base is indicated by the number of holes on the sides and the distance between the holes. The dimension is indicated by a pair of two-digit numbers with leading zeros.

ㅤIn the first version it was plexiglass with hand-made holes. In a base measuring 2040 (10×20 cm), 800 holes need to be made. By the end of this lesson I was a little nervous, I don’t recommend repeating this. Accuracy is acceptable, but no, not worth it. After some time, the opportunity arose to do laser cutting. Due to the density of the cut, the base suffered from heat, so I had to heat it with a hairdryer and level it. The result is much better than manually, but requires refinement and the availability of laser cutting and material.

ㅤThen recently I finally decided to try printing on a 3D printer. The result is acceptable, one could recommend it, but again availability and cost.

ㅤ PCB base

ㅤRecently, while putting things in order, I was moving printed circuit boards from the table and back, and I realized that this was the solution on the surface that had not been noticed for so long. After all, those who are involved in the development of electronics have experience ordering the production of printed circuit boards. That is, making a base from PCB will probably be easier than looking for cutting or 3D printing. This gives another advantage – a reduction in the number of screws for fastening, since the stand is fastened on both sides with screws of the same length, both with PCB.

ㅤReducing mounting holes

ㅤI recently made a DPT servo controller module, 32×42 mm in size with compatible mounting holes, attached it to the base and saw how much space the screw heads take up.

ㅤAnd also the diameter of the pad on the board is 6.5mm, which is not at all conducive to placing two holes in adjacent cells. I decided that M3 racks, although more common, needed to switch to M2. The low distribution is compensated by the fact that it is enough to purchase 200-500 pieces in bulk once.

ㅤButt connections

ㅤReducing the hole size saves space, which is critical for small modules. It also becomes possible to fasten modules end-to-end, even if the fastener is located at the very edge of the module. Which brings with it yet another opportunity to connect modules to each other not with dupont jumpers, but with jumpers.

ㅤThis option is specific and smacks of proprietaryism, but it is backward compatible, since it allows you to connect the same dupont jumpers to the same theme. Illustrations of possible applications for clarity.

ㅤRevision of base mesh size

ㅤThe idea of ​​a butt connection introduced complexity associated with the mismatch between the 5mm mesh ratio of the base and the 2.54mm perforation of the breadboard.

ㅤIt lies in the fact that in order to match pins, you need to focus on some starting point, which promises difficulties and limitations. The warp pitch of 5mm was chosen without significant justification as a suitable round number in the metric system. There is no reason to hold on to this particular meaning. And I decided that the base mesh should be made with a pitch of 5.08 mm, that is, 100mil. No matter how much I would like to rely on the imperial measurement system, as for me, such a solution has more advantages than disadvantages.

ㅤAdvantages of the new base mesh size

ㅤIn addition to simplifying the development of end-to-end modules, we are able to quickly make a compatible module from a regular breadboard. We drill out the holes to 2mm and get compatible mounting holes. We assemble the module on the breadboard itself.

ㅤFrom the same breadboards, you can make adapters for modules with pins soldered down, when it was originally intended to be used with a solderless breadboard.

ㅤOn the issue of budgeting

ㅤThere are PCB manufacturers who make 100×100 boards as prototypes. When I ordered the first versions of the base in the form of boards, they were exactly 100×100. With a 5.08 mesh, the size is already 101.6×101.6. Which doesn’t fit into a more budget-friendly way to get the base, but where I ordered, this size was accepted as a sample.

ㅤIf the size 101.6×101.6 is not enough, then you can fasten two bases together. I think a separate connecting fastener is unnecessary in terms of budget, it’s enough to connect two small ones by soldering. And those who are not looking for budget can order a larger base, maybe not even the standard thickness of 1.6 but 2 mm to increase rigidity.

ㅤMounting screws

ㅤThere are many screws with different heads. For myself, I chose hex head screws. I find them to be the most convenient to assemble. Such a screw is easier to control and does not get lost, since it can be placed on a hex key without any magnets, as with screwdrivers. Yes, that's the key. Again, in my opinion, it is better to keep a separate tool specifically for assembling prototypes. And a 1.5mm hex wrench takes up virtually no space compared to a screwdriver.

ㅤInter-board racks

ㅤThere is not much to say about the inter-board racks, except that their length is determined by the distance between the printed circuit boards when connecting them with a PLS comb and a mating connector. Which increases the scope of their application, and not just for prototyping kits. Theoretically, it is possible to use other elements, maybe even soldered onto the module board, as a counter fastener for the modules. But the use of solutions that increase the range of the starting set is not included in the concept.

ㅤTool

ㅤI find it useful to keep a separate tool specifically for this task. Use a screwdriver or hex key to assemble only the modules on the base, and store them with the rest of the set. A drill clamped into a collet chuck for drilling mounting holes in breadboards, or another tool that can be used to perform this operation. Also other small tools that you consider necessary and can afford to duplicate.

Minimum starting set

• base – printed circuit board with an array of 2mm holes with a pitch of 5.08mm

• inter-board racks TFF-M2x11 female-female

• double number of screws m2x6

• hex wrench or screwdriver

• dupont mother-to-mother jumpers

• prototype boards of suitable sizes and parts for the manufacture of modules and adapters

• PLS and PBS 2.54 steps

• jumpers on PLS if you plan to use end-to-end modules

• a tool for increasing the size of holes up to 2mm in breadboards (a 2mm center drill will work well, a chamfer will be formed immediately, plus a mini hand drill with a collet chuck)

ㅤBasic principles for further development of the prototyping approach

• maximum budget

• reducing the range of components required to work with a portable prototype designer

• maximum DIY compatibility (if possible, rejection of solutions unsuitable for self-production)

ㅤFurther plans

• study the terminology in more detail.

• create a document with rules and terminology – so to speak, instructions or instructions.

• develop modules with subsequent publication of developments.

• development of illustrative examples of real devices to demonstrate the positive properties of the approach.

ㅤ#pp2508

ㅤHashtag to indicate belonging to the topic described above.

ㅤPurpose of publication

ㅤShare the idea and receive feedback from a wide audience as material for further development.