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Practical PCB design
15 June 2015
RS Components' Mike Hibbett looks at at using Design Spark PCB to create a simple USB interface for a foot operated switch, enabling a tap of the foot to trigger a series of keyboard commands to a PC.
DesignSpark PCB is RS Components' PCB design tool, part of a suite of applications intended to support rapid prototyping across both mechanical and electronics design. It was released in 2010 following a partnership with Number One Systems, the creators of the Easy-PC CAD program (which unsurprisingly shares a number of similarities with DesignSpark PCB.)
RS Components has built up a thriving on-line community, with great tutorials and a lot of activity - not surprising, as RS is a well-known and respected worldwide organisation, and has put some effort into publicising the package.
The tool is provided free of charge, even for commercial use.
Being free, the engineer is not hampered by tightly controlled licence conditions, limiting the number of PCs to which the program can be installed. It does, however, require a connection to the Internet, which can be annoying, and caught the author out once while writing this review.
Despite being free RS claims it to be a full, professional program enabling the creation of multiple schematic pages, an unlimited PCB area and manufacturing data generation. What's going to be interesting, however, is how easy it is to learn. The author has used several CAD packages over the years, is not a professional PCB designer, and has not used this application before.
In this case we will ignore the software design, although consideration is given for ease of development when choosing the microcontroller. Let's roll a few requirements, and make some design decisions to simplify the design.
We want two inputs that will connect to momentary push to make switches. At the other end, we provide a type B USB socket for connection to a PC. We will select a microcontroller that is available in a DIL package, has a good free USB stack, and a free software development tool chain. We will use USB Full Speed mode since this has a very low data rate (12Mb/s), which will not require any special high frequency PCB tracking
Our favourite microcontroller for this kind of work is Microchip's PIC18F2550-I/SP. It is supported by a great development IDE, free USB stack, and compiler.
The whole design will utilise through-hole components. We only need to manufacture a few boards and we don't anticipate a complex design, so soldering the components by hand will not be an issue. We save on the cost of a solder stencil and should be able to get the boards quicker (or even etch them in-house.)
The foot switch - not really relevant to the design of the PCB - was quickly located on the RS website through a search for "foot switch". We are going to provision the board with two inputs, one for each switch.
The installation application can be downloaded from the RS website at
Only modern Windows OSs are supported; if you use Linux or MAC, you will need to run it with Wine or in a Windows virtual machine. Installation was straightforward (once we discovered you have to run the installer in Administrator mode,) but to complete the process 'activation' was required. This involved registering an account on the DesignSpark forum, and supplying a valid email address. RS is transparent about sending "appropriate marketing material" to your email address. It's very low volume, however.
On running DesignSpark for the first time we are presented with a Start page showing a few adverts. There is no design canvas visible, but this is reasonable, as we should really be setting up a project at this stage. Sure enough, selecting File->New brings up a dialog and we can enter the name for the project, and then create a schematic. At this point we have a dialog as shown in Figure 1. Specialised components can be discovered through ModelSource, an on-line database of selected RS components, or picked from a library of generic devices.
Figure 1: Starting schematic capture, loading a microcontroller from ModelSource. Inset: All manner of files can be included in the project; here, we have a photograph of the original notebook design attached as a jpeg file
Our initial schematic design was sketched out in a logbook and then photographed, downloaded to the PC and imported as a support design file. In this manner all of your notes can be kept with your CAD design in a single project. Creating the schematic - remember, this was our first experience with DesignSpark - was straightforward and took 30 minutes. We even dropped in a few extra I/O pins on headers for future proofing. You can see the results in Figure 2.
Figure 2: Completed schematic. Notice we couldn't help adding some additional inputs, for future proofing
It's great that ModelSource provides access to online datasheet material from a single click of the mouse, accelerating the selection of appropriate specialised parts from the ones available in the program.
Converting the schematic to a board
Satisfied with the schematic, we select Tools->Translate To PCB... to create the board design. At first we are presented with a PCB Wizard dialog that appears to suggest we will be hand held though the process.
First, we select the type of board we are targeting - single sided, the default double sided (our choice,) and so on. This is more intuitive than the common technique of letting us manage dozens of layers ourselves. Next we have the option to refine the list of layers - we accept the defaults. Then we define the board dimensions. We leave this at default, as the board outline can be adjusted as the components are arranged and signals routed.
Finally we have the option to have the components placed automatically, and the signals routed. Automated placement is rarely successful and today was no exception, so we backed the changed out and opted for component placement around the board periphery, for manual placement.
Moving components was just as intuitive as creating the schematic. A particularly welcome feature was that design rule checks were performed dynamically as you moved parts around, and the airwires - lines that indicated connectively between parts - are also recalculated. This stops you making layout mistakes that would otherwise be picked up later on.
Thirty minutes later and we are happy with the component placement, so it's time to route the signals. The auto-router's first attempt was poor, so we manually routed the critical signals - USB and power - and then let the auto router finish the rest. It did a fair job.
Creating the copper flood fill and then generating the Gerber format files was again intuitive, and quick. Going from Figure 3 to Figure 4 took just over an hour.
Figure 3: Initial board layout
Figure 4: Completed board layout, in 3D view
The learning curve for someone with a little previous CAD experience was negligible. Its capabilities would suit many SMEs (it's the primary tool for the author's company) and engineers will be productive with the tool after just a few hours.
Its limitation is the size of the library although, to be fair, you will always be spending time validating and tweaking your libraries.
With its free licence and unlimited capabilities it's a difficult tool to beat.
About the author
Mike has spent the last thirty years designing embedded systems ranging from Chinese character set pagers to electricity meters, data encryptors through to tiny computing platforms for IoT. His current focus is on designing monitoring systems for high performance computing platforms, and teaching embedded software to the next generation of innovators.