Shallow Thoughts : : Jun

Beginning Python, Lesson 2 posted

I've just posted Lesson 2 in my online Python course, covering loops, if statements, and beer! You can read it in the list archives: Lesson 2: Loops, if, and beer, or, better, subscribe to the list so you can join the discussion.

I hope everybody has fun writing loops!

Beginning Programming in Python course starting

I'm about to start a new LinuxChix course: Beginning Programming in Python.

It will be held on the Linuxchix Courses mailing list: to follow the course, subscribe to the list. Lessons will be posted weekly, on Fridays, with the first lesson starting tomorrow, Friday, June 17.

This is intended a short course, probably only 4-5 weeks to start with, aimed mostly at people who are new to programming. Though of course anyone is welcome, even if you've programmed before. And experienced programmers are welcome to hang out, lurk and help answer questions. I might extended the course if people are still interested and having fun.

The course is free (just subscribe to the mailing list) and open to both women and men. Standard LinuxChix rules apply: Be polite, be helpful. And do the homework. :-)

Wiring up a surplus-store LCD display to an Arduino

Every now and then I think it might be handy to have some sort of display on the Arduino ... something a little more detailed than an LED that either blinks or doesn't.

Adafruit's 2-line LCD text display comes with a great Character LCD tutorial, but it's quite heavy, and includes a backlight I don't need. I wanted something more minimal.

The local surplus store always has lots of cheap LCDs, but they unfortunately tend to be unlabeled, so you can't tell which pin is which. But the other day I spied a very lightweight little display for $2.95 that actually had a label on it, so I grabbed it, figuring I'd be able to get the pinout from google. It said:

DENSITRON 2
617ASNG0441
0201 TAIWAN

Alas, googling produced no useful information for any of those numbers. Foiled again! It might as well have been unlabeled!

Wait -- let's not give up quite so quickly.

Adafruit's LCD Shield tutorial says most parallel displays have either 14 or 16 pins, while this one has 15. That's close, at least ... but comparing the two Ada tutorials, I could see that the pin assignments for the two displays were completely different even though both were 16-pin. I wasn't going to get pin assignments there.

Searching for just densitron 15-pin lcd found lots of displays that clearly weren't this one. But apparently a lot of them were similar to a display called an LM50. Perhaps mine used that pinout too.

So I tried it, and it worked with only a little experimentation. Here's the pinout:

LCD pin	Function	Arduino pin
1	Gnd	Gnd
2	+5 V	+5 V
3	Contrast	pot
4	RS	7
5	RW	Gnd
6	EN	8
7	D0
8	D1
9	D2
10	D3
11	D4	9
12	D5	10
13	D6	11
14	D7	12
15	(nonexistent backlight)

Or I can use the nice cable with the 8x2 connector that came with the display, which maps to these functions:

1 = Gnd	Contrast	RW	D0	D2	D4	D6
+5V	RS	EN	D1	D3	D5	D7

The Arduino LiquidCrystal library works just fine with it, using this initialization:

LiquidCrystal lcd(7, 8, 9, 10, 11, 12);

Works great! I went back and grabbed another $3 display. So the moral is, even a complete hardware klutz shouldn't give up too easily: with the right web search terms and a little fiddling, you might just get it to work after all.

Update: apparently something has changed in the LiquidCrystal library, and you now need a trick to get this to work. Specifically, to see the rightmost 8 characters, you need to call lcd.setCursor(40, 0). See this discussion: 16x1 LCD showing only first 8 characters (lcd.setCursor(0,1) not working).

Make your own Saturn sketching template with GIMP

My SJAA Ephemeris planetary astronomy column for next month will discuss Saturn, among other topics, since Saturn is the main planet visible in the evening sky right now.

Saturn has some storms visible right now in the north polar and equatorial bands, and a great way to focus your attention to see more detail through a telescope, especially on subtle details like Saturnian storms, is to take pencil and paper and sketch what you see. I've recommended sketching in my column many times before, but don't talk about it on the blog very often.

When sketching Saturn, it helps to start with a template, so you can concentrate on the interesting details of the rings and bands rather than fussing over trying to get the exact width of the rings right. Saturn's tilt changes with time -- right now it's tilted at 8° to observers here on Earth -- so sometimes the rings are open wide, sometimes they're narrow, and sometimes (as last year) they're edge-on and invisible to us. That's a hassle to try to get right in a sketch, when you'd rather be focusing on the gaps in the rings and the pastel colors of the cloud bands on the planet.

ALPO, the Association of Lunar and Planetary Observers, makes templates for sketching Saturn; but I had trouble finding any online that showed a tilt appropriate for this month's Saturn. You can get observing materials by joining ALPO, but sheesh! you shouldn't have to join an organization just to get a simple sketching template. And I wanted one for my column. Besides, the ALPO templates fill in too much detail -- they don't really give you a chance to do your own ring sketch.

So here's an easy way to make a Saturn sketching template with GIMP.

Start with an image

You can calculate the aspect ratio you need for the planet from the ring tilt, but why go to all that trouble? I started with an image of Saturn I got by running XEphem. Call up View->Saturn, then make the window as big as you can. Of course, you may substitute any planetarium program of your choosing, as long as it shows Saturn with the right ring tilt.

I used GIMP's screenshot facility to open this as an image: File->Create->Screenshot..., then Select a region to grab.

You can also use a recent photo of Saturn. The point here is to get something that's the right shape: it doesn't matter if it's beautiful or large.

Fix the rotation and size

You want the rings horizontal, if they're not already. Use GIMP's Free Rotate tool to do that. You can eyeball it to make it approximately right, or if you want to be more accurate, use the Measure tool (the icon looks like a drawing compass) to measure from one edge of the rings to the other and note the angle in the status bar at the bottom of the window. Then when you use Free Rotate, type in the number you measured.

You'll be printing this out on sketching paper, so if the original image is small, use Image->Scale to expand it. Remember, you won't be looking at this original image -- it's just for tracing -- so don't worry if the image comes out fuzzy after you scale it up. I made mine about 1000 pixels wide.

Make a white background layer

Layer->New Layer... to make a new layer; check "white" in the dialog. Then click the eyeball icon next to it in the Layers dialog to make it invisible. You'll want it later.

Outline the planet on its own layer

Layer->New Layer... to make a new layer; this time make it transparent, not white. I named mine "planet", since this is where I'll draw the ellipse for the planet. (Yes, Saturn is an ellipse, not a sphere. So is the Earth, for that matter, but Saturn is a lot less spherical than Earth is.)

Choose the ellipse selection tool and drag out a selection that matches the outer edges of the planet. Use the resize handles to adjust the selection until it fits as closely as you can manage.

In the Toolbox or the Brushes dialog, choose the smallest hard brush, "Circle (01)".

Then Edit->Stroke Selection.... Click "Stroke with a paint tool", and click Stroke.

Tip: You may notice my template ended up with very jaggy lines. That's a common artifact of GIMP's Stroke Selection. I'm not worried about it for a sketching template, but if the jaggies bother you, you can get a much smoother line by converting the selection to a path and stroking the path instead of the selection.

Preview your work so far

Go back to the Layers dialog and make that white layer visible again, so you can see the outline you just made. You may want to do Select->None and click on some tool other than ellipse select, so the selection outline disappears and you can see the line better.

If you're not happy with your planet outline, Edit->Undo and repeat with a different selection, a thicker line or whatever.

Outline the rings on their own layer

Repeat what you just did for the planet, this time for the rings. I recommend using a new layer for just the rings (you'll see why in the next step).

I outlined just the outside of the rings, so the sketch can show the ring thickness. ALPO's templates don't do this, but how much ring you can see can vary based on seeing conditions. If you want the inner edge of the ring on your template, add it now.

Erase the hidden parts of the ring and planet outlines

You can't see the rings where they go behind the planet, or the part of the planet hidden by the rings. And you don't want your template lines spoiling your sketch in those regions. So use GIMP's eraser tool and a large brush to erase the appropriate parts.

This is a little easier if you used separate layers for the rings and planet: you won't have to be as careful with the eraser. But it's not a big deal: this is a template, not a finished artwork, and you're going to be drawing over it anyway. So don't sweat it too much.

Optionally, make the lines fainter

I made the template lines fainter using the Opacity slider in the Layers dialog on the planet and ring layers. Of course, you can just draw in grey in the first place, but I like being able to decide afterward what color I want, or change it later.

Label the template

Trust me, you'll be really annoyed if you decide in 2026 that you want to make another Saturn sketch, find your old template but can't remember what ring tilt it's for. So use the Text tool to label either the current date or the approximate ring tilt. Or put that information in an image comment under Image->Image Properties..., or in the filename.

Save your template as XCF.gz, save a copy in some other format like jpg, png or gif, and you're ready to print templates on paper. Then go out and sketch Saturn!