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Designing Your Circuit

Blog entry posted in 'Designing Your Circuit', Oct 28, 2015.

5.00/5, 1 vote
  • Determining Your Goals
    The first step in designing the perfect PCB is to have a well-defined set of goals that you would like your design to accomplish. To steal a little bit from the business world, you should always set SMART goals for your project, this means:
    • Specific
    • Measurable
    • Attainable
    • Realistic
    • Time Bound
    As a personal example, I have started working on another side project for my own use. The bathroom in my apartment is too dark in the evening for me to get around in, but when I turn the light on it is way too bright and wakes me up.

    To fix this little issue I thought I would just go buy a small lamp. Unfortunately, I am a little bit too picky and couldn’t find a lamp that I liked. That is when I had the idea to design one of my own. Of course, I’m not talking about just any lamp. I wanted a multi-color, adjustable brightness, wirelessly controlled lamp.

    Sounds cool right? Of course it does! So before the idea was able to leave my head I jotted it down in my notebook and began planning.

    At this point, my goals were pretty broad, let’s take a look at what I had:
    • Multi-color lamp
    • Adjustable brightness
    • Wireless control
    Unfortunately none of these goals are very specific at all. What do I mean by multi-color? Is that two colors, three, or any variable color? What is adjustable brightness? I mean technically on and off would be two different brightness settings right? Wireless control? What, do I want to use Wi-Fi, Bluetooth, infrared, RF, Zigbee, sound? Any of these options would be possible.

    Revising the project goals to be SMART led me to the following list of goals:
    • A continuously adjustable high-brightness RGB LED filtered through a fogged acrylic cover for even light dispersal.
    • Continuously variable brightness control that will allow me to choose any brightness setting between completely off and fully on.
    • Bluetooth low energy 4.0 wireless specification interface, controllable from an iOS or Android devices as well as an optional dedicated controller.
    With the exception of “time bound” these goals meet all of the criteria of a SMART design and allow me to proceed forward with a clear vision of what I want to accomplish.

    By doing your research first and setting SMART goals for your project you place yourself on the right track to create that perfect design.

    Visualizing Your Design
    Now that you have a clear idea of what you want, it is time to start designing it. Before you start scouring the internet for parts or drawing crazy schematics in your notebook, I would advise you to take some time to develop a clear picture of how you want your final design to function.

    Try to determine how your parts will work together to achieve the goals you set. This is a good time to be thinking about your design from a system level.

    [​IMG]

    System Level Design for My Bluetooth LED Lamp, yes I make Visio drawings for fun.

    You may not know specifics like what supply voltages you will need or what connections need to be made, but you will be able to consider how each component will rely on the others and what additional components they will add to your design.

    This is also a good time to consider the aesthetic aspect of your design. Are you trying to fit a certain form factor? Do you need to consider ergonomics (for example if you are designing a game controller)? Will you be able to pick up your design a year from now and understand exactly how it works? These are the types of details that, while seemingly insignificant, can be the difference between a good design and a great design.

    Choosing Parts
    This is perhaps the most tedious step in the design process, but is crucial to a successful design. Choosing the right part for your design could be the difference between finishing your project and giving up in frustration.

    All integrated circuit manufacturers work hard to make their designs robust and perform their function at the lowest price they can, but not all companies are equal. This is especially true when it comes to making their parts easy to use.

    Since there are hundreds of thousands (millions?) of different components on the market, it isn’t possible for me to give a complete rundown, but what I can do is provide some general guidance on how to select the best component for your purpose.
    1. Check availability. The last thing you want to do is put weeks or months into a design only to find out when you go to buy your parts that a crucial component is out of stock and will not be available for a few more months. Choose a part that shows a large inventory and optimally is available from multiple distributors.
    2. Consider where the component is in the product life-cycle. You generally don’t want to get a component that is no longer in active production, but if your project is just a one-off build then this may not be a big deal.
    3. Make good use of the parts filters. Most distributors or part finding tools offer some way to narrow your search criteria. Make use of this not only to reduce the amount of parts you have to look at but also as suggestions for alternative components. As a simple example let’s assume you have decided you want an LED with a millicandela rating of at least 80 mcd. Instead of filtering for components that have a rating of exactly 80 mcd, filter for any component with at least 80 mcd then sort by price, forward voltage drop, or current draw. This method may save you money while also getting you a better performing component.
    4. Be aware of minimum quantities. Some components are only sold in large lots, be aware of this when choosing your components so that you aren’t forced to do a redesign.
    5. Know what package you are getting. All components are delivered in some type of package that allows you to attach them to your board. Some components are offered in multiple package styles that are usually incompatible. If you are planning on making your PCB at home you should try to avoid the very small packages such as no-lead packages or chip-scale packages. These can be difficult to solder without proper equipment.
    6. Understand the part! This is the final and most important guideline that I have. You should always fully understand the part before deciding to use it in your project. Some components can require a microcontroller or microprocessor, an external clock, or a special PCB design. Being aware of these requirements beforehand will help you avoid headaches in the future.
    When it comes to actually finding parts, I like to use the search capabilities provided by my favorite vendors, this way I know the product will be available and I can choose components based on the actual cost to me and available inventory.

    Another way to search for parts would be to go to a particular company’s website and browse their parts catalog for a solution. For example, if I know I need an ADC for a project, I may start with a company that is well-known for their ADC products such as TI. This has the advantage of often leading to a highly usable solution.

    My four personal favorite avenues for finding parts and pcbs are:
    1. 3pcb.com – Mouser is a popular worldwide component distributor that has a wide range of pcbs. Their selection is (in my experience) not quite as large as Digi-Key’s, but I prefer their website design, the better filtering system, and the more logical pcb organization.
    2. DigiKey.com – Digi-Key is another popular worldwide component distributor. They probably have the largest component selection of any distributor, have great customer service, and are fast to ship. Overall I would put Mouser and Digi-Key about even, and certainly at the top of the list.
    3. datasheets360.com – Datasheets360 has a database of over 70 million electronic components, each with a PDF datasheet and is working on scaling the site up to 350 million parts (up to about 180 million in November 2013). In addition to datasheets they also provide inventory and pricing information from some of the major suppliers. This is a great resource.
Winy likes this.
  1. Willen
    Write another article about how to design (where to start and where to end) with a single transistor based amplifier as an example, please!
    Willen, Oct 30, 2015