Bio-medical Pneumatics

The Schematic Medical PneumaticS

Before you even begin to lay out your PCB, you MUST have a complete and accurate schematic diagram. Many
people jump straight into the PCB design with nothing more than the circuit in their head, or the schematic
drawn on loose post-it notes with no pin numbers and no order. This just isn’t good enough, if you don’t have an
accurate schematic then your PCB will most likely end up a mess, and take you twice as long as it should.
“Garbage-in, garbage-out” is an often used quote, and it can apply equally well to PCB design. A PCB design is
a manufactured version of your schematic, so it is natural for the PCB design to be influenced by the original
schematic. If your schematic is neat, logical and clearly laid out, then it really does make your PCB design job a
lot easier. Good practice will have signals flowing from inputs at the left to outputs on the right. With electrically
important sections drawn correctly, the way the designer would like them to be laid out on the PCB. Like putting
bypass capacitors next to the component they are meant for. Little notes on the schematic that aid in the layout
are very useful. For instance, “this pin requires a guard track to signal ground”, makes it clear to the person
laying out the board what precautions must be taken. Even if it is you who designed the circuit and drew the
schematic, notes not only remind yourself when it comes to laying out the board, but they are useful for people
reviewing the design.
Your schematic really should be drawn with the PCB design in mind.
It is outside the scope of this article to go into details on good schematic design, as it would require a complete
article in its own right.

Imperial and Metric Designs :

The first thing to know about PCB design is what measurement units are used and their common terminologies,
as they can be awfully confusing!
As any long time PCB designer will tell you, you should always use imperial units (i.e. inches) when designing
PCBs. This isn’t just for the sake of nostalgia, although that is a major reason! The majority of electronic
components were (and still are) manufactured with imperial pin spacing. So this is no time to get stubborn and
refuse to use anything but metric units, metric will make laying out of your board a lot harder and a lot messier. If
you are young enough to have been raised in the metric age then you had better start learning what inches are
all about and how to convert them.
An old saying for PCB design is “thou shall use thous”. A tad confusing until you know what a “thou” is.
A “thou” is 1/1000th of an inch, and is universally used and recognised by PCB designers and manufacturers
everywhere. So start practicing speaking in terms of “10 thou spacing” and “25 thou grid”, you’ll sound like a
professional in no time!
Now that you understand what a thou is, we’ll throw another spanner in the works with the term “mil” (or “mils”). 1
“mil” is the same as 1 thou, and is NOT to be confused with the millimeter (mm), which is often spoken the
same as “mil”. The term “mil” comes from 1 thou being equal to 1 mili inch. As a general rule avoid the use of
“mil” and stick to “thou”, it’s less confusing when trying to explain PCB dimensions to those metricated non-PCB
people.
Some PCB designers will tell you not to use metric millimeters for ANYTHING to do with a PCB design. In the
practical world though, you’ll have to use both imperial inches (thous) and the metric millimeter (mm). So which
units do you use for what? As a general rule, use thous for tracks, pads, spacings and grids, which are most of
your basic “design and layout” requirements. Only use mm for “mechanical and manufacturing” type
requirements like hole sizes and board dimensions.