Fighting with computers

Computers are not always friendly.

Wednesday, January 30, 2008

Develop with Arduino



I'm visiting a school in northern Finland next week. This is the presentation I'm giving. It's intended for first year students so you'll find it very basic, and you'll miss the fun of some demos. By putting it here I have a backup plan if I lose my pendrive :-)

Sunday, January 27, 2008

Using eBay's Best Offer feature

I'm still dealing with a non-responsive eBay seller after I've got burned by a non-working 8GB compact flash. I've seen that many people do not like to talk about this type of failures that happen on eBay from time to time. I'm still trying to get my money back from PayPal so maybe not everything is lost.

But now I wanted to buy some cheap control chips for bipolar stepper motors. Nothing fancy with micro stepping or digital current control, just a simple chip that can also be used for other purposes too. I saw this video and I learned about the L293. It is an old chip that several manufacturers mark as deprecated.

Instead of going to a local shop I went online to eBay and I saw several sellers have it available. I tend to favor European sellers if the price is right as I do not particularly enjoy periods longer than two months for delivery (something I've experienced first hand from Hong Kong lately). One of the sellers had a price that seemed ok, but they also did have the Best Offer button. As I was not in a hurry nor in serious need of that part I decided it was good moment to test that feature. Although I'm using eBay since 1999, I have never used the Best Offer feature before.

I've offered something around 60% of the asked price (or a 40% discount if you prefer) for four chips to see what happened. I thought that if the seller was not happy I might get closer to the asking price or I would just use the Pay Now. A few hours later I've got a reply telling me the seller had accepted my offer and the deal was done. I guess I will be paying more attention to this button in the future.

Tuesday, January 22, 2008

On-line math calculation


I know several nice Math software packages, mostly for Windows (Derive, Mathematica, MathCAD, Matlab ...). My university has licenses for some of them but I am not always on campus, using Windows, when I need some help. There are some free math programs too (Scilab, Octave, Maxima, Axiom ...).

What it is not always known is that there is an interesting set of on-line calculators and solvers that sometimes can help you get the job done without installing anything. Quite interesting when you are not even using your own computer.

For easy calculations and units conversion Google calculator is a neat tool. How do you use it? Just type your calculation on Google search box (i.e. "5/sin(32)" or "25 celsius in farenheit" ). Really worthwhile to convert between American and European fuel economy units!

But maybe you already have a calculator so this Google feature while nice does more or less the same. You may need to do something more advanced like online matrix calculation. I have found the linked page very useful for my son to check his homework.

Today I needed to solve some transcendental equations and I have found the online version of Yacas quite useful as one of the samples is a Newton iterative solver. You need Java plug-in though.

There are plenty of other sites I've used in the past but today these are the ones I've used recently.

Oops, I've almost forgotten the cool integrator by Wolfram Research. And also an on-line derivative calculator.

Thursday, January 17, 2008

Troubling iMac



A few months ago I bought a shiny 20" iMac. While the initial impression was good, I was not happy after the trouble I had installing Leopard and certain incompatibilities it brought.

Just when it seemed things were settling down I've got the system just dying on me after opening a large (120MB) PDF file. The screen turned black and then the system rebooted spontaneously. Another computer was on at the same time and it did not experience any problem, so I do not think that was because of a power spike.

Today, when I started the system there was a multilingual message in a centered dark square telling me to press the power button for several seconds to shutdown the system (neither the keyboard or the mouse worked). I have never seen that image before (but I failed to take a photo), it contained the message in Japanse too. It happens to be an OS X kernel panic.

When I powered on the system again I heard the welcome sound and next two beeps and then three beeps that repeated over and over again. It did not sound good. A quick check on the net (using other computer, of course) showed that 2 or 3 beeps signal RAM memory trouble (mine is the original RAM that came with the system and because of the cool and compact format of iMac is not user serviceable).

I removed the computer power cord for some seconds and then I connected it back to try again: No changes, same sequence of sounds; no joy.

Having nothing to lose I removed the power cord again and I applied some of my frustration through my hands (i.e. more than gentle taps to the system). After reconnecting the power the system booted up as normal. My guess is that the RAM is not properly seated on the socket. I'll be paying attention to see if something weird happens again.

Now I'm going to hunt for some memory test software.

Update: Thanks to Jesus coments (and this page) I realized I was wrong: My iMac does have two memory slots that are easily accessible by the user by removing two screws on the bottom part of the screen. Not sure the trouble was caused by memory though, as several diagnostic attempts gave no indication of memory faults. Anyway, if the problem happens again now I know I can access the system's RAM memory.

Monday, January 14, 2008

Other Hacks

While searching the net for possible uses of old hard drive voice coil actuators I've came across this video. A video is worth even more than a thousand words so I'll let it talk.