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Making our Electronic Masterpiece

The Final Thermistor Circuit.

The Final Thermistor Circuit.

The hard part of our electronic design is over… Well sort of. We are just moving into a new phase of it, but at least we will not be talking theory.  Finally we are actually going to mount parts to something and start thinking about how we will get it all wired up.  This is probably no less complicated, but it is a lot more obvious process.   And no more theory! Well, at least no more theory for awhile.

The first thing we need to do is arrange the schematic diagram so no wires are crossing.  That was already completed when I designed the circuit.   It the “old days” that was a big step forward.  However, since the two amplifiers are in the same IC package, things are no where complete at this stage.  The best thing we can do is write the pin numbers for the amplifiers on the drawing.   We will also have to consider the +Vcc and -Vcc power connections as well as the common, (gnd) connection.

Now the next step is to think how we will use this circuit.   The use of the circuit may determine how we will build the circuit.   For example I have seen circuits build by simply gluing components on the I.C. and soldering jumper wires between the pins.  That can work for a very simple circuit, but this circuit is too complicated to consider that method as an option.   Often circuits are mounted inside an enclosure and some of the components are actually mounted on the enclosure and wired to the rest of the circuit.  An example is volume controls on many audio circuits.   Again that is not the situation for this circuit.   All of the adjustable resistors are just for a one-time calibration and then hopefully never touched again.

The reason for the previous paragraph is to get out of the mindset of automatically creating a printed circuit board.  However, for this circuit something similar to a printed circuit board is how I am going to fabricate the circuit.

BOM The next step is to make a list of the components based on the schematic and any calculations done to this point on the power dissipation rating of the components.   This is the point where it may be necessary to determine where the components will be purchased.   This list is called the Bill of Materials, BOM.   A spread sheet is the ideal method to do this both because it makes a neat list, and more importantly, it is convenient for copying and adding additional information such as supplier, price, minimum quantities, etc.   I often use a form like this to write additional information such as the datasheet pdf file names,  catalog page numbers and so forth.  You will be having to look up the information several times as you continue the process of laying out the circuit, so use the sheet as a summary and index to make it easy to find the same information again.   You can copy the same information to several tabs on the spreadsheet to compare different suppliers as you start looking for a place to purchase the parts.

Some of the items often forgotten at this point are any terminal strips you will be using as well as the circuit board and wiring.   Postage and minimum orders are always a hassle if you are ordering through the mail. Even if you have a local electronics supplier you will want to make as few trips as possible.

When determining the size of the mounting board, again it is necessary to think about what you will be doing with the circuit.  If the board is too small it may be impossible to  install all of the components in the available space.   If the board is excessively large and must be mounted in an enclosure, the enclosure will be more expensive.  Throughout this whole process it will be necessary to cut and try various options and sometimes the available space may require you to go back and change components.   For example, the choice between a capacitor with axial leads or a radial radial leads will often be determined by the space available to mount the capacitor.

Real Estate

Available “Real Estate” for each of my circuits.

I want to build 6 of the thermistor circuits all on one board, so I wanted a fairly large 6.3″ X 4.5″ board.    Each  circuit will have a space on the board of 2.1″ X 2.25″

Once you have the parts lined up it is time to move on to trying to figure out how to place them on the board.  In most cases you will want to design everything based on a grid with 0.1″ spacing.    The pins of a DIP IC circuit is based on the pins spaced at  0.1″ centers.   Small resistors, capacitors, and other axial lead components can have the wires bent to 0.3″ centers.

Laying out the circuit.

Laying out the circuit.

Now it is time to actually place the components on the board. I chose to use CAD to do this and I used LibreCAD.   There are special CAD programs designed just for electronics.  However, I do such a little amount of this work, it was easier for me to do it with standard CAD.   This is one time where I actually turn on the grid and often “snap to grid”

Layers I used while creating this drawing.

Layers I used while creating this drawing.

This is also one time where it very helpful to use layers.  You will need at least two layers, one for the component side of the board, and one for the trace side of the board.  I chose to use several layers for the trace side of the board, the non-component side.  I chose to use different colors for the Vcc+, Vcc- and ground traces. It was helpful to turn off layers while I was working with the drawing.   I also created a layer for labels and even one for holes.

I think this is enough for one post.  In the next post we will talk about how printed circuits are created and the options to think about while determining the trace locations.

As I stated at the beginning of this post, the process is not easy, but it is straight-forward.   It is time-consuming.  It really is not anything that can be taught other than general directions and just trying it and learning from experience.

I hope I have made this worth your time.    I am sure there are things I glossed over so much I have left some confusion.  If so, please e-mail me for clarification.  I will answer back in an e-mail individually, and I will also post the question and answer so everyone can learn.

Gary.


 

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