Fritzing: Custom Components

So I've been playing around with a few more electronic circuits over the last week or so and found myself wanting to move some of the simple logic processing off the Arduino. As an example, I wanted to implement the following logic table

Inputs		Outputs
A	B	Y	Z
0	0	0	0
0	1	0	1
1	1	1	1
1	0	1	1

Now if you look closely at this table then you will see that Y is the same as A, while Z is simply A or B. Now this would be simple to implement using an Arduino, in fact you would just need the following sketch.

void setup() {                
  pinMode(11, OUTPUT); // output Y
  pinMode(12, OUTPUT); // output Z
  
  pinMode(8, INPUT); // input A
  pinMode(9, INPUT); // input B  
}

void loop() {
  digitalWrite(11, digitalRead(8));
  digitalWrite(12, digitalRead(8) | digitalread(9));
}

This could be simplified slightly by not using the Arduino to set Y and just using A directly, but it would still require three pins to read A and B and generate Z. Given the limited number of pins on an Arduino this seems wasteful. Now most logic gates can be built from a combination of transistors, but again they can quickly get complicated, especially if you need more than one logical operation. Fortunately there are a number of cheap integrated circuits that you can buy that implement the standard logical operations (and some not so common). After a quick trip to Maplin on Thursday, 99 pence got me a single chip with four 2-input OR gates: the 74HCT32N from NXP. It was simple to add it to the breadboard and wire it into my circuit instead of using the Arduino to compute the OR.

The only problem arose when I went to record my circuit using Fritzing. Clearly Fritzing can't support every single component in the world, and it didn't have a specific component to represent the 74HCT32N. It does, however, have a number of generic parts which are easy to customize. One of these allows you to represent any integrated circuit by specifying the number of pins etc. The problem is that this generates a breadboard view where the chip is simply labelled IC and a schematic view that just labels the pins on the chip and tells you nothing about the internal workings. Now for a complex chip this makes sense but for something as simple as OR gates it would be helpful if the schematic view included them.

The left hand chip is how Fritzing represented the 74HCT32N when I used the generic IC part, the chip on the right is how I wanted it to look. Now given this is a screenshot I obviously succeeded, and the rest of this post will explain how.

Now you can create parts from scratch, but after an hour or so of trying and getting nowhere, I hit upon a simpler solution.

The first step is to use the parts editor (ignoring the warning about it being bug ridden) to label the pins and properly name the component etc. Once you are happy with those values click to save it as a new part.

The new part will be listed in the "MINE" parts bin. Simply right-click on it and choose "Export Part". This will give you a file with a fzpz extension. This is just a zip file which you need to unpack. You should find that you now have five files: a fzp file which describes the part and four SVG image files for the icon, breadboard, schematic and PCB views.

You can now go ahead and edit the SVG files. Be careful not to edit or alter any of the connectors in the images but otherwise it's safe to make changes. If you are wanting to draw logic gates inside a chip then you can simply grab the appropriate images from Wikipedia which makes life really easy as all you need to add are the connections between the gates and the pins. As well as improving the schematic view I also edited the icon and breadboard views: the breadboard view now gives the full part number and the icon shows OR instead of IC. Note that to avoid any problems try saving to plain SVG rather than any application specific extension.

Once you are happy with the images go back to the part editor and replace the images with the new versions (even though I didn't edit the PCB image I replaced it anyway as a bug in the part editor means it seems to get lost if you don't). Once you are happy you can export the part again if you want to keep a backup copy or share it with other Fritzing users.

I'm going to make any custom components I produce available to anyone who wants them. For now there is just this single chip but they will all appear here.