Programming TrueSTUDIO .hex files over DFU on linux

So you have an Intel HEX (.hex) file from your microcontroller firmware (from Atollic TrueSTUDIO for a STM32 micro, for example), but you need a .dfu file in order to program your microcontroller using dfu-util? And can’t (or don’t want to) use the Windows-only DfuSe software? I hope this guide helps you out!
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Do beeswax candles produce negative ionisation? Nope.

This short note is heavily based on Are Ions Good for You, a beautiful exposition of the actual science of ionisation and pollution of the air.

There are lots of claims around the internet, mostly by beeswax candle makers, that beeswax candles are “natural ionizers”. The theory goes that beeswax candles emit “negative ionisation” which spreads through the room and causes pollutants and allergens to bind together because, apparently, those things are positively charged. No source or explanation is ever provided, and I very much believe that’s because the claim is bogus.
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Using MOSFETs as blocking diodes

Connecting a battery backwards to an electronic circuit can rapidly do a lot of damage — current will flood through (and destroy) many integrated circuits when powered up the wrong way, and electrolytic capacitors have a famous tendency to explode. For this reason, it’s common to use a blocking diode in a circuit to provide reverse polarity protection:


If the battery is connected correctly, as shown, current flows through the diode to the circuit, and the circuit operates normally. If the battery is reversed, the battery tries to pull current through the diode the wrong way, and the diode refuses to conduct — protecting the load from damage.

Find out the disadvantages of this circuit, and how to do better…


XKCD’s ‘Click and Drag’ comic in Google Maps form

I just threw together a zoomable view of xkcd’s latest comic using the Google Maps API and US$0.27 of Google App Engine quota: Click and Drag. It helps to find some of the more off-the-beaten-track easter eggs, view it here!

If I get enough (i.e., any) interest, I will clean up the source code and make it available.


Deriving magnetism from electrostatics and relativity

Imagine a world where there was no magnetism at all — just electrostatic forces. It turns out that by adding Einstein’s laws of special relativity to that world, and looking at the effects that special relativity has on moving charges, the laws of magnetism appear — just by mashing electrostatics and relativity together.

In other words, the moving electrons in an electric motor’s coils see the world from a different frame of reference to the copper atoms making up the windings of the motor — and this imbalance is what makes the motor turn!

There’s a bunch of stuff around on the internet on this, but I find it hard to process because it deals with general relationships between magnetic and electric fields, not specific examples. I’m sure this is actually very general and useful, but it’s also beyond my immediate understanding as well. So, I tried approach the problem in my own way using just special relativity and high-school level physics, and it works!

See how it’s all connected…


Automatic vs Manual transmission cars

Don’t worry, this post is not advice on how to live your life. Actually, on second thought, it is. Well, no, not really, it’s mostly about cars. And life.

Maybe I should start off with my main point:

Things which are “easy-to-use” often come with hidden drawbacks.

Some examples:

  • Point-and-shoot digital cameras (vs Digital SLR cameras) – Due to their larger sensors, SLR cameras can give much sharper images and more beautiful out-of-focus backgrounds. As well as much better performance in low-light situations, and more versatility with a variety of lenses. However, the compact size and low cost of point-and-shoot cameras make them a perfectly valid choice for casual photographers.
  • Automatics cars (vs manual cars) – Unlike point-and-shoot cameras, there is no valid reason (special circumstances aside) I can think of or find that justifies wanting a car with an automatic transmission – if you’re blessed with two legs.

At this point, I’m going to drop the illusion of making a general argument about “easy things” and just rant about automatic cars instead.

What’s the actual difference between auto and manual?

Find out…


Where’s the North Pole on Google Maps?

I’ve seen several posts on the internet asking about the North Pole on Google Maps: where is it? Why isn’t there any snow there? Where’s Santa’s house?

There’s a couple of reasons why the ice around the North Pole is not shown on Google Maps.

Find out why…


The Galaxy Nexus PenTile display: a reasoned take on the debate

There has been a lot of discussion on the ‘net about the infamous PenTile display. Lots of sites are throwing numbers around, comparing the ‘subpixel resolution’ of competing phones such as the Galaxy Nexus and iPhone 4/4S. Some commentary has been insane, comparing apples with oranges and declaring PenTile an abomination regardless of resolution. Others, like this one, are a lot more reasoned, but still seem to ignore the well-proven fact that the eye’s visual acuity depends on the colour in question. Let’s rattle off a couple of undeniably true facts:

  1. Hypothetical 1000ppi RGB and PenTile displays can both look absolutely sharp and perfect.
  2. The acuity of the human eye varies significantly between colours. Acuity is particularly poor with blue light.

Take the following example: these four squares consist of fine lines. From left-to-right, they are: #1: white/black, #2: red/cyan, #3: green/magenta, and #4: blue/yellow.

Stand well back from your monitor, until you can’t tell the lines apart. Now slowly move back in. Notice how the blue/yellow (#4) lines look almost the same as white/black (#1), and red/cyan (#2) look very similar to green/magenta (#3)? Very roughly speaking, this is because our eyes smear out blue light slightly, so we can’t tell the difference between white lines (yellow+blue), or alternating yellow and blue lines. Similarly, the only difference between red/cyan and green/magenta is where the blue lies.

I started this not-terribly-scientific experiment with the “hypothesis” that the eye had a higher acuity to green than red and blue. This is apparently the reason that is used to justify PenTile’s use of twice as many green subpixels as red or blue. However, although my pictures above suggest that green is more visible than blue, my eyes are more-or-less equally acute with respect to red and green light. This is backed up by at least one actual proper study, as well. [Interestingly, the original PenTile technology was RRGGB, which makes much more sense!]

So, we can conclude that the resolution of red subpixels might be the limiting factor for an RGGB “Retina” PenTile display, whereas the resolution of green subpixels might be the limiting factor for an RGB Retina iPhone 4 display. Comparing the two of these numerically is very difficult and highly subjective. The sharpness with which the eye can see these colours is different, and even this difference varies according to the brightness of the image, and the individual. To see numbers calculated with no apparent regard to these factors and presented to five significant figures is just laughable.

The closest you can come to a meaningful comparison is stating what sort of displays would be equivalent if green sub-pixel resolution was all that mattered, and what would be equivalent if red/blue sub-pixel resolution was all that mattered. The linear green sub-pixel resolution is the same for both RGB and PenTile displays, because both feature full green sub-pixels. The linear red/blue sub-pixel resolution of PenTile is compromised by a factor of sqrt(2), because red pixels are diagonally separated. So, here we go:

The effective equivalent RGB resolution of a 1280×720 PenTile display probably lies somewhere between 905×509 (if red/blue is the limiting factor by far) and 1280×720 (if green is limiting factor by far.) That is, somewhere between the-highest-resolution-ever-seen-on-a-smartphone (except the iPhone 4S by a hair) and the-highest-resolution-ever-seen-on-a-smartphone (by a mile, although some [dirty LCD] RGB 720p devices have been launched since.)

And we’ve come all this way without mentioning display size. The bigger the display, the bigger the font can be and the further you can comfortably hold the device from your eyes. A bigger display should therefore mean a lower linear resolution limit before the display be reasonably considered a ‘Retina’ display.

The proof of the pudding is in the eating

My conclusion? It is virtually impossible to pre-judge a display that we haven’t even seen before. I would love to hear subjective comments from people who have seen the device in person in Hong Kong; otherwise, I’m not interested.

Continue reading, including simulated images from a 720p PenTile display…


Rattling Nikon MH-18a charger: how to fix/prevent

During my holiday in Thailand, I was silly enough to drop my Nikon MH-18a battery charger on the floor from a height of a metre or so. As soon as I picked it up, I heard a rattle. When I looked inside, this is what I found:

Nikon MH-18a shattered inductor

The core of the inductor had broken into several pieces. The pieces are shown here, below the copper coils of the inductor. Even though inductor cores are made from ferrite, a notoriously brittle ceramic, I was pretty disappointed to see that the charger was so fragile. After failing to find any suitable replacement parts, I decided to figure out what it was for.

What is the inductor’s job? (Don’t care? Skip to ‘How to fix to fix the problem.’)

A reverse-engineered schematic of the mains electricity conditioning part of the circuit is shown below, with the broken inductor core depicted in red:

The flyback converter draws pulses of current at a frequency much higher than the 50 or 60 Hz of mains electricity. The two capacitors and inductor combine to form an LC filter, which prevents this high-frequency energy from leaking back into the electricity supply and interfering with other devices.

What effect does the broken core have?

If you plug in the charger while ferrite fragments are still sitting in the box, all sorts of nasty short-circuits could happen.

Even with the inductor effectively absent, the voltage in the two capacitors is still topped up 100 times per second by the mains electricity supply. The flyback converter should therefore have plenty of energy to run the rest of the circuit with, and the battery charger should still function normally. As mentioned in the previous section, the only issue I can identify is that the current waveform being drawn from the electricity supply will be different and noisier, which may cause interference to other devices nearby, or a slight reduction in efficiency.

How to fix the problem

High Voltage

Do not open the charger while it is plugged in, or if it has been plugged in for the past few hours. Capacitors inside the device are charged to potentially lethal voltages during operation, and may maintain this charge after being unplugged. Do not plug the device in unless it has been completely re-assembled, and test it under close observation at first as overheating/fire are possible with any mains electricity device. Do not open the charger if you don’t completely understand all the points mentioned in this box.

Opening up the charger and removing the shattered pieces should remove loose bits of shrapnel that might have caused short circuits, and leave your charger completely functional — the inductor core is completely unnecessary.

To disassemble the charger, simply remove the sticker on the bottom (which is hiding one of the screws) and undo the two screws. The screws are designed to take a funny 6-point star driver, but fortunately a plain small flathead screwdriver did the trick for me.

How to prevent the core from breaking

If you’re very clumsy, or want to cover all your bases, you may wish to open up your charger and preemptively pot (surround) the inductor with epoxy (e.g. Araldite) or similar. Make sure you pay attention to all the notes in the yellow box above, though.


Orewa college requiring laptops: Good. Recommending the iPad: Bad.

UPDATE (August 2014, 3 years since I published this article): I hate to brag, but I told you so…

There’s been a story going around the media about Orewa college adding laptops to its compulsory stationary list. Furthermore, Orewa college has recommended the iPad as the best ‘laptop’ to use for this purpose, due to its ‘ease of use and long battery life.’ Apart from journalists managing to confuse the two previous statements to invent the headline ‘Orewa college requires iPad on stationary list’, my main gripe is with the specific recommendation of an iPad.

In order to recommend a device, you have to keep in mind what the purpose of the device is. With the points mentioned below in mind, I would love to see the recommendation of an iPad defended in the context of a normal school day.

What’s wrong with the iPad?

For the sake of argument, I’m going to weigh up the Apple iPad ($779) against the Asus Eee PC 1005P ($536).

It’s expensive. With well over $200 separating the two, the iPad has to have some meaningful advantage over the Eee PC to be a sensible recommendation. Those on a tighter budget could go for a Acer Aspire One D255E ($375), which is just as capable as the other two as far as schoolwork is concerned, although it does represent a compromise in terms of battery life.

It’s optimised for anything but schoolwork. Let’s compare the two in the context of some common subjects:

    History — Laptop use would presumably be focussed on accessing web references to research and bolster arguments. No particular advantages either way, just about any device these days can display a webpage.

    English — Research is no different as above, but when it comes to writing essays, the Eee PC has an actual keyboard, whereas the iPad would fall back on a touch-screen keyboard which is much slower and fatigue-inducing to use. Eee PC wins.

    Maths — Pop OpenOffice spreadsheet or GNU Octave (both free software) on the Eee PC and you’re drawing graphs, playing with linear algebra, whatever you like. iPad’s calculator is inferior to a standard calculator that has been in maths class for the past many years. Eee PC wins by a wide margin.

    IT — Software development on an iPad? Apple put up perplexing barriers to software development on their devices, completely barring free experimentation with programming, and knocking out any nascent software engineers before they can even start. Eee PC wins.

    Workshop/electronics/graphic design — Apple iPad? Just ridiculous; unheard of. Windows/Apple PC? Yes; fantastic, free, open source software all over the internet. Eee PC is the only solution worth even considering.

    Wasting time posting facebook updates — Apple iPad is definitely quicker for this one.

In short, the social-network-centric nature of the iPad versus the maturity and popularity of Linux/Windows means that the iPad is just useless for freely exploring the possibilities of computing, programming, mathematics, electronics, graphic design, engineering, etc. Considering that an iPad might replace a desktop PC in the home due to some family’s tight budgets, this seems completely ridiculous.

The iPad is a luxury toy for people who already own a proper computer, and want a fashion item to show off to their friends.

What’s wrong with Orewa college’s purported reasons for recommending the iPad?

According to the story, Orewa college has recommend the iPad due to its “ease of use and long battery life.” Other advantages claimed by Orewa college (at this link) are also covered below.

Ease of use? — The Apple iPad is so easy to use because, as mentioned above, it doesn’t really do anything other than take mediocre photos, display web pages, and a bunch of random games and web services. It’s only hard to get anything wrong on the iPad because you can’t get anything remotely useful done at all.

Battery life? — The Eee PC boasts a battery life of 11 hours, longer than any school day for sure.

No viruses — Anyone who knows anything about computer security knows that this is like saying a ship is unsinkable – extremely ignorant. Also, it’s wrong.

“Abundance of apps!!!!” — There’s a great abundance of “apps” for PCs too…

“Voice recording apps useful!!!” — I can’t believe someone actually wrote this; Windows comes with a sound recorder built in. If you want some fancy program that automatically organises the recordings or whatever, there is going to be some open source software out there to do this for you. The idea that Apple apps are some amazing new idea is astonishing. Yes, it’s new for phones. But PCs have supported software written by anyone since they were introduced in the 1970s.

“Useful for all the family” — This looks like it is just copied-and-pasted marketing gibberish straight from Apple.

What I find so amazing about Orewa college’s list of advantages of the iPad is that it all looks very convincing and exciting, until you analyse the contrapositives – i.e., what they’re implying about PC laptops:

“PC laptops don’t have battery lives of 10 hours.” Wrong, some do.
“PC laptops don’t have cameras built-in.” Wrong, most do.
“PC laptops don’t have auto-update of system software.” Um, yes they do, it’s called Windows Update.
“PC laptops are not useful for all the family.” This one deserves some elaboration to say the least.
“PC laptops don’t have an abundance of freely available apps.” This is just idiotic, as pointed out above.
“PC laptops can’t record voice.” What, is the microphone just for decoration???

Thanks for reading. Do you think I’m being too harsh on the iPad? Let me know in the comments below!