Avian’s Blog

These days I usually buy my groceries at a Slovenian supermarket chain Mercator - they opened a new store recently just across Zemanta's offices. Since it's a new building it boasts a row of automatic check-out machines.

It must have been gut feeling, but I've never went near those things until a couple of weeks ago, when store employees started insisting that you try to use them.

Well, I'm not an expert in the field of usability, but I did my share of interface designs and I think that system can go down as a wonderful example of how not to make a human-machine interface. Especially not one that is intended for such a broad use.

It's downright confusing: you have no less than 6 ways of interaction, each one on a visually very distinct piece of the machine:

• The bar code scanner,
• touch-sensitive LCD,
• coin and banknote slot,
• returned change slot,
• card reader with its own keypad and
• scale for verifying the weight of the merchandise

You have to use all of these in correct order - even if you pay with cash, you have to use the smart card reader to insert their customer loyalty card.

I don't know about the average shopper, but my brain is only able to process one input channel at a time. When a machine is trying to tell me something in voice, plus displaying something different on the screen (plus there's an employee near-by, helpfully telling you his own instructions), I get annoyed in the best case.

Then there's paperwork. You get 6 different pieces of paper back (printed by two distinct printers). Some of which you are supposed to throw back into the machine! Don't ask me which ones - I just stuffed in pieces that didn't look like the master receipt. Or do they expect me to completely read those two meters of paper on the spot while a queue is building behind me?

And finally, the straw that broke the camel's back and the reason I'll never use the machine again: they force you to get a new plastic bag for each visit. No, you can't bring your own. I tried, successfully triggered an alarm and got told that that is a feature, not a bug (why, everyone likes a free plastic bag!).

Great. At the time where everyone is talking about sustainable economy and how to change the throw-away society, they introduce a service that gives out plastic bags in the thousands which will be thrown away the moment the happy shoppers get home.

I can understand a plan to take over the world by replacing people with cold, heartless machines, but this is ridiculous!

Here's another piece of electronics I saved from being dumped in a landfill: it's a wireless temperature and humidity sensor from one of those home weather stations. My sister gave it to me after I expressed my interest in it, but unfortunately I was too late to save the broken base station with the receiver. Now I'm playing with the idea of building a new receiver from scratch.

On the outside the sensor has a couple of buttons for setting up its connection with the receiver, self-test and choosing temperature display in Celsius or Fahrenheit (Kelvin fans are left out in the cold). There's a small LCD display on the unit that shows the current sensor readings.

Top side

Bottom side

There are no markings of any kind that would identify the manufacturer. That could be intentional, guessing from the sloppy way it's soldered together. Looks like the cost of assembling one of these things together was the primary concern of the designers: there's a single IC in the middle that uses direct chip attachment. Everything is SMD except for a couple of through-hole components that appear to be soldered by hand - in fact, whoever did it forgot to solder one of the pins of the trimmer capacitor (so I'm wondering if the base station was actually working just fine). Battery, buttons and LCD display all connect to the PCB by just being pressed against it - connectors really must cost a fortune these days.

Here's a schematic of the RF part. It appears to be a classical case of a cheap transmitter in the 433 MHz band. Basically it's a tuned oscillator: you have an amplifier and two tuned resonant circuits on both sides. One is a R433A SAW resonator and the other is a LC circuit that can be tuned with the trimmer. Because of the resonance the feedback can be minimal. In this case just the parasitic capacitance in the transistor's base-collector junction (C_bc) appears to do the trick. The antenna is just a fancy trace on the circuit board. I'm guessing there's only a simple amplitude modulation going on: either the transmitter is on, or it's off. But I have yet to hook up an oscilloscope to the circuit the verify that.

I'm planning to build a simple super-regenerative receiver, reverse engineer the data protocol and hook it up to an Arduino or some similar microcontroller. I know I could probably get a receiver module already built, but I want to get my hands dirty and finally try working with some RF circuit designs.

What I found so far on the internet seems encouraging. There's a really nice document about super-regenerative receivers by Eddie Insam and the protocol for weather stations has been broken before.

Yesterday I attended the opening of the astrophotography exhibition in Tivoli, Ljubljana (part of the events celebrating the international year of astronomy).

I don't know about you, but I still get goosebumps whenever I see a picture like this.

Image by NASA

Having grown up watching Star Trek and building LEGO moon base sets, these photographs remind me that at least some small part of the promises we were given in elementary school did come true. There's a permanent international outpost in the Earth's orbit!

Now imagine seeing something like this with your own eyes. And if our unfashionable corner of the galaxy has wonders like that to offer, what kind of unimaginable sights are there to see around other stars?

And then comes the realization that all progress in space flight has been achieved with what, one tenth of a percent of resources put world-wide into senseless wars and destruction?

There are a number of technologies that could bring at least the rest of the solar system within our grasp, if only some more effort could be put into it.

Here's a nice video shot from a camera aboard my father's model airplane:

(can't see embedded video?)

The airplane is EProjekt4, a fully aerobatic radio-controlled model airplane. It was designed and built from scratch by dad two years ago. The plane is propeller-driven and uses an 1.2 kW electronically commutated brushless DC electric motor. Power is provided by a 5-cell 4000 mAh lithium-ion polymer battery pack.

Video was recorded on a SD card using the tiny ACME FlyCamOne2 which was mounted under the airplane's fuselage (you can just see the propeller at the top of the frame).

The recording shows a panoramic view of a few villages around the model airfield on Planinsko polje, Slovenia, and a first-person view of a couple of aerobatic maneuvers.

In my last post I was trying to figure out an optimal strategy for guessing a combination for a lock that only checks the last m symbols entered.

It turned out my venture into the graph theory was a bit of a dead end. As Luka Rahne pointed out to me (on Facebook, no less), there's a different branch of mathematics that offers a much better approach to this problem and even has a connection to my home field of electronics.

Linear feedback shift registers are actually direct solutions to such a problem, albeit only if you limit yourself to two characters (n = 2). These are digital circuits that will generate all 2^m-1 different sequences of length m in a shift register (i.e. for each step they insert a single digit at the beginning of the register and drop one at the end). Exception is the state that contains all zeroes, but that is easy to correct with a special case.

It turns out that there exists a generalization of the LFSR for n states. Basically you replace the XOR operations in the feedback loops with arithmetic multiplication and addition modulo n. There is quite a bit of theory behind, of course, but I had trouble finding a higher-level description of it on the internet. A great deal also appears to be patented judging by the ratio of patents that came up in the search.

Just like with ordinary LFSRs, where you need to find the correct locations of the feedback taps, you need to find the correct set of m+1 multiplicative constants in order to get the maximum length sequence (or M-sequence) out of it. This is not really straightforward, as it involves finding a primitive polynomial for the finite (Galois) field GF(n^m). Luka posted his Mathematica code to calculate the polynomial's coefficients, but I found it easier to use COS.

Here S_i are memory cells holding register states and a_j are coefficients of the polynomial:

My knowledge of this field is a bit superficial and I don't completely understand the connection between this polynomial and the fact that its coefficients will produce the M-sequence. I admit I only skimmed the most interesting parts of an introductory book yesterday. Even my curiosity has limits.

There was only one obstacle left: a finite field only exists for prime characterics n, which means that you can't get a M-sequence for, say, 10 symbols directly. The answer is to factor n and produce a M-sequence for each prime factor (2 and 5 for 10 symbols). You then run two sequences in parallel and assign a symbol to each unique ordered pair you get in this way. So for ordered pairs where the first element can have one of 2 different values and the second element can have one of 5, you get 10 different pairs.

It's easy to show that two sequences ran in parallel with periods

have a composite period of

if n₁ and n₂ are prime.

So, the final result, a sequence of 10003 keypresses that checks all 4-digit combinations, is below. C code that calculated it and checked for its correctness is here if anyone wants to play with it.

00007559254372858084599981873643682873396507388560205737258390890406577109057085
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08627537838732874062995703057263482819914165760180461959450774474688955345095201
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28883759030110654824537385819449888097374369386328641739856130472624939189051647
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42049159870332490022537741095023604299936581300106449355272958094488991963653683
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48227519074958638202317147451021640273770547746522267977036910878080937125035481
66625117636992014206791365457825060019334723786524043355450596836846597749219645
28061397872574054020939901413805242219932929544326221537016370616420591589613225
80023537650194032604399927491201006459354363948184489991872752692862397507299461
20463735838198050646711905619485406655756989542540843113430312850860977305459601
44843391078154218466753961411029864895356961788766607353852756254628135769215083
42487571670912832006193169699023882837778949906940049515056916216224199963019869
44601371894178232846319101477287684837312763322124847953834992470482375181659487
28203317056550030640373761457647422277976127900968081937034134490666351167279821
04207791274556834060119325633687424053354541586926847597658318654280713969635641
44021939810512814242319923839445226231536107360706421591498712234800335367411841
22605399836590210006559345273849084499990963742782863397416398470204737349291881
40647711814718494406755747899443440853112521302940861977214558610448533901691443
08467753870510038864995347871689666617352943746344629135678314092424975707619029
22007193078798032882937769859807840059514147906306225199872118878446113709851683
22847319010576296684937303673223024857952925982560483375090758496282133161475401
20641373670556656422377967037801868091936125124580667351076378830042177903655469
24061119234732696424153345451487826857596749308744281713878734650440319389015029
04243319832938454226331527017261606431590589702324801335276510850226153989275803
000

In conclusion, it turned out that there is quite a lot more than one way optimal way to guess the combination (as each finite field has multiple primitive polynomials and each polynomial yields an unique M-sequence). Still, you won't hack into a beer store anytime soon with this, as any practical implementation surely locks you out after some number of tries or adds an delay.

A brand of electronic locks marketed by a certain security company in Slovenia has a peculiar way of recognizing the correct code.

To unlock it, you need to enter a four digit PIN. However, instead of the usual way where you have to press a confirm key after entering four digits this one unlocks as soon as you enter the correct sequence.

So basically, if the correct code is 1-2-3-4, it will unlock after pressing 9-8-1-2-3-4, or whatever sequence of digits comes before the 1-2-3-4.

It's certain that such a design reduces the time needed to guess the right combination. In theory you can check a new combination for every keypress where a normal lock would require four plus one for confirmation. For example, entering 9-8-1-2-3-4 checks 9-8-1-2, 8-1-2-3 and 1-2-3-4 with 6 presses while a normal lock would require 15 presses).

I'm wondering. What is the optimum strategy for guessing the combination? Is the theoretical one combination per key possible? Or is there necessarily some overlap between sequences entered in this way?

In the general case of n digits and m places I'm picturing this as a directed graph with n^m vertices, one for each possible combination, where each vertex has n outgoing edges (corresponding to n possible keys you can press next). So the problem translates to finding a Hamiltonian path in such a graph (which is a NP-complete problem in the general case).

Path for n=5 m=2

Path for n=2 m=3

I have been able to find nicely symmetrical paths for simple case of m = 2. At least from those examples it seems that you can construct the path incrementally, by first exhausting all combinations with n digits before moving to n + 1

It appears possible that an analogue way exists for more places, but I'm having problems drawing three and four-dimensional grids on paper.

It seems even hardware is not immune to heisenbugs.

A week ago my wireless access point in Logatec (an old version of SMC Barricade G) started behaving weirdly. A network scan showed the SSID broadcast, but association was not possible. I tried several different devices (among those my Eee 901 with Atheros card and the work laptop with an Intel IWL), but none was able to connect. iwconfig just showed not associated and an ever increasing Rx invalid nwid counter.

Interestingly, kismet found the network and showed constant traffic on it. It also warned of broadcast disassociate requests which it seemed to indicate a denial of service attack on the network. But I just couldn't believe that someone would keep such an attack going for that long in that part of the country.

The problem fixed itself after a couple of days. But since it started working just after I moved the access point around, I thought that maybe there's a bad connection somewhere. So I took it apart.

The only two things that caught my attention were a loosened nut on the left antenna connector and plenty of dust on the RF part of the circuit.

I tightened the connector and vacuumed the dust and the access point has been working flawlessly since then.

I'm not sure I actually did anything. It's conceivable that the loosened nut did something to the grounding of the circuit or that there was something nasty in that dust that shorted a trace or changed its impedance enough, but I find that hard to believe.

Here's a close-up of the RF front-end of the transceiver (minus the dust). The chip in the middle is the receive/transmit and diversity switch.

The microstrip line carries microwave frequencies, so the correct characteristic impedance is very important. There's no solder resist over he line to prevent it from affecting its characteristics and the upper and lower ground planes are well connected with a lot of vias.

Proper use of units is another one of my pet peeves. I usually can't keep my mouth shut when I see someone throwing around numbers without clearly defining what unit of which quantity they represent.

A while ago I stumbled upon the Electronics Engineering videocast, and unfortunately the very first episode I watched left a very bad impression in this regard. While the show was informative, I just couldn't forgive the presenter the way he did his calculations live on a piece of paper.

It was a bit of a shock to see an engineer have amperes squared on one side of the equals sign and watts on the other, or the result presented in watts per second. Of course I sent a comment and the answer I got back was that we're not in school and this is how real-life engineering looks like.

It's engineering, Jim. But not as I know it.

I find that being strict with notation is most helpful especially in quick back-of-the-envelope calculations. When you're skipping two steps at a time in solving some equation it's easy to make a mistake. And if you're only working with dimensionless numbers, it's nearly impossible to spot the error. Well, if you're lucky and you've made a big one you have a chance that your gut feeling will tell you your result is off by a couple of orders of magnitude.

However, if you keep units attached to all the values you write down all the way through to the final solution, you have a number of ways to easily spot mishaps early on. A sum of values of a different quantity (e.g. you've come to a point where you would have to add 3 volts to 1 ampere) is a reliable indicator that you should recheck your calculation. At least in my experience when I'm expanding polynomials in my mind I often miss a factor somewhere and conflicting units in a sum make the error obvious.

Another useful rule is that the argument of exponent or trigonometric functions may only be a pure number. That's of great help when you can't exactly recall an equation, but know the general form.

Once you get the result you also have an additional means of boosting your confidence in it. If you want to calculate the amount of work required and you arrive at watts per second that should start ringing some pretty big bells.

I agree that there are situations where a system of measurement is agreed-upon and well defined and the field of electric engineering is full of them. Just think how many circuit diagrams say 10 k instead of 10 kΩ. But there such notation is unambiguous because it's accompanied with a symbol for a resistor and giving resistance in ohms is a de-facto standard.

On the other hand if you're writing such a document for long-term use, you're (hopefully) doing that with much more care than some napkin calculation at the last minute.

Anyway, being strict with your calculations is certainly most useful outside of the classroom. And in fact, my advice would be not to believe anyone that says that a particular skill is only useful in school.

Back when I was a student I designed and built an UTP cable tester. It's a battery operated hand-held device that uses finite state machine (more exactly a Moore automaton implemented with ROM and some logic - no microcontrollers with unreliable software here) to control some simple analogue circuits that check for proper connections and polarity of pairs in a standard Ethernet cable.

It was designed from the start to have two separate parts that need to be connected on the opposite ends of the cable. One part plays an active role and has all the logic and the diagnostic lights, while the other is a simple passive terminator with a diode circuit.

At the start I implemented both in a single case for simplicity, but that meant that you had to be able to bring both ends of the cable into one spot. Since I'm planning to lay some cables in my new apartment in Ljubljana I went on and made a second, separate terminator so I will be able to test cables once they are already fixed on the walls.

I didn't bother to make a circuit board. It's going to be sealed shut and filled with hot resin, so nobody will be able to see the rat's nest anyway.

On the second though it would probably be better if I made a metal box that would provide some shielding. But I had this piece of plywood at hand and no conductive paint. It works this way just as well and it's not something that will be in continuous operation anyway.

Finally I gave it a coat of black paint to match the metal box of the tester.

I guess Power cylinder with a spherical cap just doesn't have the same ring to it.

There's an article on Wired about SpaceX. The text is a bit light on details, but there are plenty of photos that are worth a look.

SpaceX is a startup company that developed the first privately-funded launcher capable of reaching orbit (they had their first successful launch last September)

While the Falcon production line looks like something I would expect in a shop like that, there's one photo on the first page that really surprised me. They seem to use an open-plan type of an office. That's just incredible. These are not telemarketers or some people typing in PHP from 9 to 5, but engineers doing (I hope) some serious thinking.

I would love to see their company policy on Blackberry pings and beeps and general distraction-making in the big room (you can see several people chatting on the photo).