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Creative Mechanical Ideas for Embedded Systems

Written by Wolfgang Matthes

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If anyone doubts that electronics tinkering as a hobby is on the decline, Wolfgang reminds us that there’s never been more rich resources available for such pursuits. With that in mind, in this article Wolfgang presents several mechanical ideas for embedded systems using small- to medium-sized PCBs and common 19″ hardware such as 3U front panels and subracks.

  • How integrate a system with diverse modules and PCBs
  • How to use an MCU module or PC as a system master or hub.
  • How to use standard 19″ hardware components in your project
  • How to best use a 3U subrack including card guides and ejector handles
  • How to use PCBs as front panels
  • 3u front panels
  • 3U subracks
  • MCU modules
  • HMI modules

Every now and then you hear someone decry the decay of electronics as a hobby ([1] to [5]). But the reality is that there is a plethora of microcontroller (MCU) platforms, shields, HATs and the like available these days. Everywhere you look, there are Internet forums and vendors serving the community of the so-called makers. There’s also a multitude of fairs catering to makers—most postponed now during the COVID-19 pandemic of course, but sure to return to full strength in the future.

When you compare the today’s projects shown at fairs or on the Internet to what ambitious hobbyists of the past have been accomplished, you see a conspicuous difference. More often than not, the maker`s work manifests only in a contraption of modules, breadboards and jumper wires, but not in a piece of impressive equipment like in Figure 1c, for example.

FIGURE 1 – A display module is to be attached to an MCU module. (a) shows a hi-tech solution that the vendor recommends. Now, try to imagine implementing a dual-screen display. (b) is an improvised wire-wrapped module, quick and dirty, but rock-solid. A purpose-designed PCB (c) is more presentable. Free design software and affordable manufacturing services allow even the hobbyist to do it this way.

In some of my previous Circuit Cellar articles ([12] to [15]), I’ve already dealt with projects aimed at sturdy and perhaps even presentable devices. Here, we will concentrate on mechanical design, depicting some ideas that are not that common in today’s realm of tinkering, experimenting and prototyping. References [6] to [11] are a small selection of relevant articles and web content.

We want to stay within affordable limits. With that in mind, the proposals here are centered around small- to medium-sized PCBs and ubiquitous 19″ hardware—especially 3U front panels and subracks. These PCBs and front panels are not overly complicated or large. They could be developed using free design automation software and manufactured by any appropriate service provider. With modest equipment and effort, they could be turned into presentable showpieces—even at the proverbial kitchen table.

The basic ideas in this article occurred over my years of project work. Instead of relying on the ubiquitous small modules, I preferred medium-sized PCBs with interface connectors located somewhat thoughtfully, so as to ease assembling more complex devices. To build them sturdy and presentable, I make good use of 19″ hardware, above all of the subracks, chassis and front panels.

Becoming familiar with the 19″ system, I revived the venerable technology of small PCBs to be plugged into a backplane. Occasionally, it turned out that PCBs make sufficiently rigid 3U front panels, at least for educational modules with small connectors. The concluding idea to be discussed here is to employ ZIF sockets as general-purpose component adapters, being a somewhat expensive, but much more reliable alternative to the white breadboards.

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MORE THAN ONE PCB
A PCB typically carries one module—a functional unit. Typical hobbyist or educational modules should be neither too expensive nor too complicated. But most projects won’t get by with only one module. As a result, problems arise on how to arrange and fasten the modules and how to interconnect them.

It is important to select the principal mechanical design early because each one has its particular requirements concerning the selection, placement and pinout of the connectors. Figure 2 introduces some of the well-proven approaches to how machines could be built from more than one module or PCB. Our preferred solutions will be illustrated in some photos. Additional photos can be found on Circuit Cellar’s article materials web page. Still more photos and drawings are available on my own webpages (see “About the Author” box for links). On those webpages, the different approaches shown in Figure 2 will be illustrated and discussed more closely.

FIGURE 2 – Shown here are some well-proven mechanical design principles to build machines from more than one module or PCB. (a) stack one upon another, (b) plug them into a baseboard, (c) plug them into a backplane, (d) plug them against each other, (e) set them up individually and connect them by cables. 1 – upstream interface; 2 – module-to-module connections; 3 – I/O connectors.

Figure 3 shows a generic block diagram of a typical project. Imagine, for example, a programmable logic controller (PLC) built for educational purposes and experimenting. In this respect, it makes sense not to strive for compactness and miniaturization, but to make all functional units easily accessible. The machine is centered around an MCU module as its central processing unit, connected to human-machine interface (HMI) modules, input/output modules and upstream and downstream interfaces. Furthermore, Figure 3 hints to additional modules to emulate or substitute the real-world process environment, here dubbed the diagnostic front-end.

FIGURE 3 – A typical project depicted as a generic block diagram.

Trying to implement such a project by selecting suitable modules in an ecosystem like Arduino or Raspberry Pi is an instructive exercise. Appropriate shields, HATs and the like are easy to find. But what’s the best way to arrange, fasten and interconnect them to implement the complete machine? Usually, only one peripheral module can be stacked on top of the MCU module—remember, it is not an industrial-grade form factor, like PC/104. Three, four or more modules could be connected as shown in Figure 1a. That approach, however, is not very inviting.

An obvious solution is to arrange all the modules or PCBs next to each other, for example, by plugging them onto a baseboard. The modules, shields, HATs and the like must be equipped with appropriate pin headers, as shown in Figure 4. A quick-and-dirty approach could be wire-wrapping with a sufficiently large prototyping board or perfboard as the baseboard [14]. The ultimate solution would be, of course, the purpose-designed PCB. By the way, a baseboard with thoughtfully placed wire-wrapping sockets can be altered and even re-used multiple times, provided you employ an appropriate unwrapping tool and work somewhat cautiously. In principle, however, the shields, HATs and so on are not marketed to be used in projects outside their particular ecosystem. They are deliberately not meant as OEM components.

FIGURE 4 – The header (a) is the type of connector to be soldered into the modules. The baseboard is to be populated with strip-line sockets (b). Modules with headers (c) are inserted into the sockets (b).

MORE COMPLEXITY
Alternatively, you can base your projects on modules of medium size and complexity [12], or even on industrial-grade hardware. Figure 5 shows how the block diagram of Figure 3 can be implemented with such modules.

FIGURE 5 – Here, the project of the Figure 3 block diagram is implemented with medium-sized educational modules.

The modules are expressly designed for building systems of two or more modules. That intention drove the selection of the form factor as well as the type and arrangement of the I/O connectors. Connections should be inexpensive and reliable. Besides, they should allow for maximum flexibility. Therefore, we prefer shrouded pin headers, ribbon cables and terminal strips. The modules can be mounted on top of separate enclosures (Figure 6), inserted into housings to be clipped onto DIN rails (Figure 7) or fastened on a chassis (Figure 8).

FIGURE 6 – Here, two of our modules are shown, as described in [12].
FIGURE 7 – Modules clipped on a DIN rail. (a) illustrates the mounting principle, (b) shows an educational PLC consisting of three modules. The human interface module 1 is stacked atop the MCU module 2. All modules are connected by ribbon cables.
FIGURE 8 – Four modules mounted on a chassis

In most of the applications, the modules will be placed next to each other in order to make all components and interfaces easily accessible. Figure 9 shows a preferred arrangement. It serves as a good template to place the components on the boards, especially the connectors.

FIGURE 9 – An MCU module in a programming and application environment

If the MCU board is in front of the user, the upstream devices are on the left and the downstream devices are on the right. An upstream device can be, for example, a personal computer (PC) or an MCU module acting as a master or hub. Downstream devices are MCU modules acting as I/O or slave processors, display modules, modules with power stages and so on.

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The MCUs aboard the modules are connected via serial interfaces. The upstream device is the master. A downstream device is a slave. This results in a star topology with a hub in the center. It’s the same principle as USB, just a lot more straightforward.

Figure 10 depicts how the connectors are to be placed. The connectors to the left and to the right are preferably shrouded pin headers. Serial and programmer interfaces have 6 pins, I/O ports 10 pins. Some modules support serial communication via an RS-232 interface or the USB. Accordingly, they carry D-sub or USB connectors. The connectors at the rear edge are terminal blocks or shrouded pin headers.

FIGURE 10 – Connectors are placed according to the principle depicted here.

At the front edge, some modules have a so-called multi-purpose connector to attach additional PCBs. Figure 11 shows an example. The ribbon-cable connection can be opened like a book, thus allowing access to all components on both boards. The PCB layout of the multi-purpose connector can be thought of as some kind of standard, providing a maximum of 64 holes (in two rows of 32) to be populated with different pin headers.

FIGURE 11 – (a) One of the more advanced modules connected to a historical joystick. The operating and display panel on top of the module is attached via the multi-purpose connector. (b) shows the 40-pole pin header.

SOME MORE HARDWARE OPTIONS
Using 19″ hardware: 19″ hardware has its origins in the realms of telecom and measuring equipment. Figure 12 shows a few examples of ubiquitous 19″ hardware components. Chiefly, we will rely on 3U subracks (Figure 12a) and the chassis (Figure 12b). Humble educational and hobbyist projects allow for a simplified mechanical design. Frame-type plug-in units (Figure 12c) will not be required. The front panel alone yields a sufficient mechanical platform.

FIGURE 12 – Some examples of ubiquitous 19″ hardware components

Small pluggable modules: Decades ago, electronic devices were built from small modules plugged into a backplane (Figure 13). Why not revive this well-proven principle? Inspired by the small boards of IBM’s SMS and SLT technologies, of DEC’s modules, of the Control Data (CDC) computers and the like, we have chosen half a Euro-board (100mm × 80mm) with a DIN 41612 connector as the basic form factor. The boards shown in Figure 14 carry CPLDs, MCUs (AVR, Arm and 8051), SRAMs and dual-port RAMs.

FIGURE 13 – Two historical examples of how devices are built from modules plugged into a backplane (Digital Equipment Corp.).
FIGURE 14 – Examples of today’s modules thought of for experimental, educational and ambitious hobbyist projects. (a) CPLD Xilinx 95108, (b) two ATmegas 1284, (c) three SRAMs 128kx8, (d) two dual-port RAMs, (e) MCU 80C51RD2, (f) CPLD Xilinx CoolRunner XC2C384, (g) MCU ARM NXP LPC2220.

The modules are plugged into a backplane that is to be wire-wrapped. The mechanical platform could be provided by a 3U subrack including card guides and ejector handles (Figure 15a). A straightforward solution could be based on a perfboard with soldered-in connectors (Figure 15b). If the device is not mechanically stressed, card guides and handles are not necessary, because the cards will be held firmly in place by the connector’s friction alone.

FIGURE 15 – Modules in a 3U subrack and on a jerry-built experimental platform

Front panels as chassis: Industrial-grade 19″ modules are typically designed as a front panel with an attached PCB or as a frame-type plug-in unit. The mechanical design of small, humble modules, however, may be centered around the front panel alone. All components are attached to the front panel. Figure 16 shows a 3U rack with modules built this way. The modules presented in Figure 17 are examples of mechanical designs that are somewhat more demanding.

FIGURE 16 – The modules in this rack are built this way. The front panels have been manufactured by a service provider, the mechanical design and wiring are homemade.
FIGURE 17 – Two examples of modules. (a) shows a temperature trainer containing different temperature sensors that can be heated up or cooled down. (b) is a switchable load resistor, providing four different resistance values.

PCBs as front panels: Epoxy PBCs (FR4, 1.6mm thick) make sufficiently rigid 3U front panels—at least for educational modules with small connectors. The modules shown in Figure 18 and Figure 19 have 2mm jacks, so insertion and withdrawal forces are not that high. All modules carry an AVR MCU. They are intended to be programmed as digital or analog simulators. In this respect, the modules are quite similar to the modules shown in the left picture of Figure 13. In the example of Figure 18, the modules are programmed to simulate an up/down counter. The jacks may be labeled by a strip of paper or by an LCD display (the luxury variant).

FIGURE 18 – These MCU modules can be mounted in 3U subracks like front panels. The modules (a) and (b) have an AVR Xmega MCU. They are just different enough in their width, that one is 14 HP and one is 7 HP. Module (a) is that wide to allow for mounting a frame to insert a strip of paper. All the jacks are connected to freely programmable I/Os. Module (c) is a graphic LCD display of 32 x 180 pixels, controlled by an ATmega MCU.
FIGURE 19 – Some of our modules, accompanied by a 7″ Windows tablet

ZIF sockets as component adapters: ZIF (zero insertion force) sockets can accommodate arbitrary components, provided they are to be attached via pins or wire. This fact has led to devices intended as an alternative to the ubiquitous white breadboards. The devices are essentially PCBs with ZIF sockets and jacks.

The IC trainer shown in Figure 20a provides five ZIF sockets with 16 pins each. The DIL-40 baseboard (Figure 20b) has a ZIF dual-in-line (DIL) socket with 40 pins, chiefly to accommodate an MCU or a CPLD. It provides somewhat like an infrastructure, comprising a crystal oscillator, RS-232 and USB attachments, and pin headers to connect it to other modules. SMD components are inserted via interposer boards. The general-purpose adapter shown in Figure 20c carries a 16-pin ZIF socket connected to different jacks and terminals. Up to two 8-bit ports from other modules or starter kits can be attached and forwarded to 2mm or 4mm banana plugs, stripped wire and arbitrary components fitting into the ZIF socket. Figure 21 and Figure 22 illustrate how these devices are used in the lab.

FIGURE 20 – ZIF sockets as component adapters. (a) IC trainer, (b) DIL-40 baseboard and (c) general-purpose adapter
FIGURE 21 – A somewhat more sophisticated analog circuitry on three IC trainers
FIGURE 22 – Two general-purpose adapters and a DIL-40 baseboard used intensively

SUMMARY AND SUGGESTIONS
Even today it is still possible to pursue projects that are sturdy, somewhat more ambitious and sometimes even presentable. The preeminent approach is to seek components showing an appropriate level of handiness. They should not cost too much. You should be able to handle them in your workshop or even at your kitchen table. It all depends on your inventiveness. Some technological challenges can be met by taking advantage of the vast range of accessories the market offers. Typical examples are interposer boards to accommodate SMT components.

Furthermore, you should not hesitate to make good use of components or modules outside their native ecosystem. In that respect, even complete tablet PCs could be employed as components, for example, to serve as HMI devices, thus substituting homemade front panels carrying LEDs, keys and switches [15]. A basic tenet is to reduce complexity by building somewhat larger.

For example, it may be tempting to build your own computer from scratch. It goes without saying that FPGAs are around that could easily accommodate the complete project. But by pursuing such an endeavor, you will depend on a development environment that will cost months alone to become familiar with. Therefore, maybe a more adequate hobbyist or educational computer should be built from small modules, each containing a functional unit implemented in a tiny FPGA or even CPLD. This way, you will obtain not just a virtual, but rather a real testbed and playground—a machine you can bring up and troubleshoot hands-on with the oscilloscope or the logic analyzer. 

RESOURCES

References:
Some musings concerning electronics as a hobby:

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[1]   Frenzel, Lou:  Whatever Happened To The Electronics Hobbyist? Electronic Design, March 04, 2007.
https://www.electronicdesign.com/archive/article/21752153/whatever-happened-to-the-electronics-hobbyist
[2]   Frenzel, Lou: Electronics Still Thrives as a Hobby. Electronic Design, May 17, 2018.
https://www.electronicdesign.com/technologies/analog/article/21806520/electronics-still-thrives-as-a-hobby
[3]   Electronics Research Study Hobbyists State Electronics Skills Are Critical to Fueling The American Economy. The Great American Electronics Hobbyist Study.
https://www.jameco.com/Jameco/workshop/inthenews/American-Electronics-Hobbyists.html
[4]   The Electronic Hobbyists of America. The Great American Electronics Hobbyist Census.
https://www.jameco.com/Jameco/workshop/rollcall/Who-Are-American-Electronics-Hobbyits.html
[5]   Reader’s Comments: Why I Love Electronics. What We Love About Electronics.
https://www.jameco.com/Jameco/workshop/rollcall/readers-love-electronics.html

Some reflections about and examples of tinkering and prototyping:

[6]   Best Electronic Components – Worst Electronic Components. What are the Most and Least Favorite Components?
https://www.jameco.com/Jameco/workshop/RollCall/best-electronic-components-worst-electronic-components.html
[7]   Cong, Robert: How to Design an Electronics Project. Electronic Projects for Electronic Designers.
https://www.jameco.com/Jameco/workshop/TechTip/how-to-design-an-electronics-project.html
[8]   Dung Dang  (Interview:) TI’s Dung Dang Talks About Modular Hardware For Rapid Prototyping.

Electronic Design, January 31, 2014.
https://www.electronicdesign.com/technologies/microcontrollers/article/21799064/interview-tis-dung-dang-talks-about-modular-hardware-for-rapid-prototyping
[9]   Rako, Paul: What’s All This Pease Prototype Stuff, Anyhow? Electronic Design, August 16, 2019.
https://www.electronicdesign.com/technologies/analog/article/21808434/whats-all-this-pease-prototype-stuff-anyhow
[10]   Rako, Paul: What’s All This Stuff Stuff, Anyhow? Electronic Design, July 12, 2019.
https://www.electronicdesign.com/technologies/analog/article/21808270/whats-all-this-stuff-stuff-anyhow
[11]   Protoptyping with purpose. EiT eBook Volume 4, Issue 2.
https://www.mouser.com/news/ebi-ebook2/mobile/index.html

The author’s previous articles related (more or less) to mechanical design:

[12]   Matthes, Wolfgang: Microcontroller Modules for the Ambitious. Circuit Cellar, Issue 312, July 2016, p. 24-33.h
[13]   Matthes, Wolfgang: Emulating Legacy Interfaces. Circuit Cellar, Issue 327, October 2017, p. 14-23.
[14]   Matthes, Wolfgang: Wire Wrapping Revisited. Circuit Cellar, Issue 336, July 2108, p. 14-19.
[15]   Matthes, Wolfgang: Using Small PCs in New Ways. Circuit Cellar, Issue 350, September 2019, p. 12-21.

The author’s project homepages:

https://www.realcomputerprojects.dev

https://www.controllersandpcs.de/projects.htm

19″ products and documentation:

NVent/Schroff | www.nvent.com
Fischer Elektronik | www.fischerelektronik.de
Hammond Manufacturing | www.hammondmfg.com
Rittal | www.rittal.us

Mechanical components, prototyping boards, etc.:

BusBoard Prototype Systems | www.busboard.com
SchmartBoard | www.schmartboard.com
Vector Electronics & Technology | www.vectorelect.com
Vero Technologies | www.verotl.com

AN APPENDIX TO THIS ARTICLE WITH ADDITIONAL PHOTOS IS PROVIDED ON
CIRCUIT CELLAR’S ARTICLE MATERIALS WEBPAGE FOR MAY 358:
https://circuitcellar.com/article-materials-and-resources/may-issue-358-circuit-cellar/

PUBLISHED IN CIRCUIT CELLAR MAGAZINE • MAY 2020 #358 – Get a PDF of the issue


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Wolfgang Matthes has developed peripheral subsystems for mainframe computers and conducted research related to special-purpose and universal computer architectures for more than 20 years. He has also taught Microcontroller Design, Computer Architecture and Electronics (both digital and analog) at the University of Applied Sciences in Dortmund, Germany, since 1992. Wolfgang’s research interests include advanced computer architecture and embedded systems design. He has filed over 50 patent applications and written seven books. (www.realcomputerprojects.dev and
www.controllersandpcs.de/projects).