Aws's Tumblr-ings

• 2012-02-01

  A Swarm of Nano Quadrotors (by TheDmel)

  Source: youtube.com

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  Clear: A To-Do List App With A UI From The Future
  By John Pavlus, fastcodesign.com

  This simple gestural interface breaks the usability rules of today, but may help set the ones for tomorrow.

  To-do list apps are lame. Why? Because man­ag­ing the system—learning how to input items, strike them out, sync them, tag them—is often…

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  More Systemd Fun: The Blame Game And Stopping Services With Prejudice
  Carla Schroder, linux.com

  Sys­temd, Lennart Poet­ter­ing’s new init sys­tem that is tak­ing the Linux world by storm, is all full of lit­tle tricks and treats. Today we will play the slow-boot blame game, and learn to how stop ser­vices so com­plete­ly the poor things will…

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  The Ever-Changing Linux Filesystems: Merging Directoris into /usr

  Joe ‘Zonker’ Brockmeier, linux.com

  If you don’t like change, work­ing in IT has to be a har­row­ing expe­ri­ence. That’s par­tic­u­lar­ly true in open source, where few stand on tra­di­tion and things move at break­neck pace. The lat­est change that has a few folks excit­ed? Fedo­r…

• 2012-01-30

  Time to move from SVN and learn Git

  Tech Talk: Linus Torvalds on git (by Google)

  Source: youtube.com

• 2012-01-29

  » Capacitive power supply – part 2 JeeLabs

  http://jeelabs.org/2012/01/30/capacitive-power-supply-part-2/

  Capacitive power supply – part 2

  Yesterday, I tried to get to grips with how a capacitive power supply works. Real samples are a bit messy, though.

  So let me try something else this time, and simplify a bit:

  

  The setup is very similar, but I’m leaving out the zener diode and the rest, and more importantly, I’m going to feed a real 50 Hz sine wave signal into this circuit, using my little sine wave generator.

  Here again, because scopes need to measure with a common ground, I’m placing that common ground in between the resistor and the cap, and I’m using the scope’s internal “invert” feature to treat this as if the channel 1 probe were connected the other way around. Here’s what we get with this new setup:

  

  The vertical scale of the resistor was adjusted to display the same amplitude for the resistor as for the capacitor.

  • the yellow line is the voltage over the capacitor
  • the blue line is the voltage over the resistor
  • the red line is the sum of the yellow and blue lines

  The scale of the red line is not quite accurate, but its shape is. So the red line is in essence the input signal.

  So what’s going on here?

  As you can see, all these signals are 50 Hz sine waves. That’s quite remarkable already. Obviously the red line is a 50 Hz sine wave, since that’s what we’ve been feeding in. But so is the voltage over the capacitor, the voltage over the resistor, and hence also the current through this circuit!

  What you see, is a set of sine waves which differ only in phase and in amplitude:

  • the voltage over the capacitor (yellow) lags the input signal (red): it’s forever trying to catch up
  • the current through the capacitor, i.e. the voltage over the resistor (blue), is leading in phase

  And something else, as we saw yesterday: the current through the capacitor is related directly to the slope of the capacitor’s voltage change (i.e. its derivative). When the yellow line is steepest, the blue line is at its highest.

  Let’s throw one more calculation into the mix: power. Power is input voltage (red) times input current (blue):

  

  Looks very sine wave’ish again! There is some amplitude variation, which can probably be attributed to signal asymmetry or amplitude differences between the different sine waves. The phase of this wave is different from the ones already shown, but note that it’s also twice the frequency, i.e. 100 Hz.

  Let’s step back for a moment. With a purely resistive circuit (as used in a resistive transformer-less supply), the current and voltage would be in lock step, i.e. sine waves with exactly the same phase, and the power consumption would be maximal (high current at times of high voltage).

  With this capacitive setup, currents get “moved around”. That means power consumption will be less than when voltage and current match up (since this gives the largest possible results).

  I’ll include one more screenshot, this time using the same vertical scale for all signals (except power):

  

  This puts things more in perspective: the voltage over the capacitor (yellow) is slightly lagging the input signal (red) now. And the input current (blue) is out of phase w.r.t. the input signal (red). So the power consumption (red x blue) is substantially lower than with a resistive circuit: when the current is maximal, the input voltage is only a fraction of its maximum range. IOW, we’re drawing current when it “costs” little.

  This is why a capacitively coupled supply is cheaper: the electricity company charges us for real power (i.e. V x I). We’re being charged for what happens inside a pure resistor.

  But we’re doing a lot more: we’re taking charge out of the AC mains line on one half of the cycle and pushing it back on the other half. It might seem as if that doesn’t use up energy, but it does: current through a wire causes resistive losses (in the form of heat), and “returning” that energy one half cycle later causes those losses again! So the electricity company sees its electricity turn into waste heat, and they can’t charge us for it.

  Here’s a thought experiment: suppose you had a huge capacitor, and hooked it up directly to AC mains, next to the electricity meter. According to what we’ve seen, it’ll track the 230V cycles, with current exactly 90% out of phase with AC mains voltage. Huge currents would flow at the time of zero volts. You’d be charged relatively little for the large out of phase current that flows (not quite zero because a practical capacitor has some internal losses that look like a resistive component from the outside). But your garden would be nicely heated by that current in the resistance of wires between the electricity company and your meter…

  You can read more about “real”, “reactive”, and “apparent” power on Wikipedia.

  (with a tip of the hat to Martyn for helping me understand this stuff a little better)

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  » Capacitive power supply JeeLabs

  http://jeelabs.org/2012/01/29/capacitive-power-supply/

  Capacitive power supply

  (Thanks for your overwhelming understanding w.r.t. not making the Low-power supply available as kit!)

  The concept of the transformer-less capacitive power supply still puzzles me – intuitively I still don’t get it:

  

  (the above image was copied from this excellent site with a web-calculcator for it all)

  So what exactly is going on here? Tomorrow, I’ll simplify that circuit in an attempt to really get it, but for now let me show you what I see on the oscilloscope. What I did was feed a ≈ 100 Vpp signal into the above circuit, which is essentially the same as in the Low-power Supply.

  To interpret the graph, you need some info:

  • the scope ground was connected between the capacitor and the resistor
  • the yellow trace is the voltage over the resistor
  • the blue trace is the voltage over the capacitor
  • the yellow trace is inverted, i.e. negative voltages at the top, positive at the bottom
  • zero is the middle of the screen for both signals

  Here’s the scope capture:

  

  Ignore the fact that these waves are hideously complex, ignore the red lines for now, and also note that watching voltage over a resistor is the same as watching current through that resistor. Voltage and current are always proportional in a resistor, that’s what defines a resistor (Ohm’s law: voltage = current x resistance).

  So what are we seeing here?

  Well… when the yellow line is high, the blue line rises sharply. When the yellow line is zero, the blue line is flat.

  That makes a lot of sense: the resistor is charging the capacitor. And similarly for negative values, it’s discharging the cap (and then charging it negatively).

  So ignoring the zener and the rest of the righthand side of the circuit, this is really all that’s going on: when the input voltage is highly positive, current flows in one direction, charging the cap and dropping to zero, and when the input voltage is highly negative, the whole process unfolds in the opposite direction.

  One more piece of the puzzle: current is “charge per time unit” (in units: Amperes is Coulombs per second).

  In other words, the capacitor accumulates the charge pushed into it, in either polarity. And while it does so, the resistor “takes the heat”, so to speak: it limits the current by creating a voltage drop over itself.

  Please let this sink in, dear reader. It’s essential to get a solid intuitive grasp on what’s going on.

  Note also that it really makes no difference at all how complex the input signal is. The resistor and capacitor work in tandem, sharing the task of dealing with that signal. In other words: the capacitor is always trying to catch up!

  This is where it gets interesting.

  You may or may not have given up in high school when it came to advanced maths / calculus – derivatives and integrals, in particular. If so, then get ready to finally get to grips with these incredible concepts.

  Let me explain the red lines in the above image – they are generated by the built-in math functions of this scope. One red line is the integral of the value measured across the resistor, and the other red line is the derivative of the voltage across the capacitor. But here’s the big surprise:

  • the integral of the voltage over the resistor is the same as the voltage over the capacitor!
  • the derivative of the voltage over the capacitor is the same as the voltage over the resistor!

  What’s the point? Well, this means that I didn’t have to measure both signals to see what’s going on. I could have omitted the blue trace, because it can be calculated from the yellow trace (and vice versa). Even though these traces have completely different shapes, they are in fact totally inter-locked and inter-related.

  As I said before, the voltage over a resistor is proportional to the current through it. So the derivative of the voltage over the cap is the same as the current through it (the current flowing through the resistor and the capacitor is always the same, since they are connected in series).

  The derivative is the rate of change, i.e. the slope of the graph. Integrating the current (i.e. the derivative) is like adding “all the little currents together over time. A capacitor is no more and no less than a “current integrator”.

  And that’s exactly the same as saying that a capacitor accumulates charge. It’s like a tiny rechargeable battery, it takes the current pushed through it and it stores that current (as charge). As the charge accumulates, the voltage rises. Loosely speaking, this is the same as saying that it pushes back harder and harder against the incoming current. At some point it pushes back so hard, that no more current comes in. At that same point, there will be zero volts over the resistor, and the voltage over the cap stays constant. Check the graph to see where that happens.

  One last observation is that the blue line is a lot smoother than the yellow line. That’s not surprising: when you integrate (accumulate) a jittery signal, things tend to smooth out. That’s why capacitors are also a fundamental component in filters, i.e. circuits which let some frequencies through more and others less. That schematic we’ve been looking at here is also known as an RC circuit – if you ignore the zener and the rest. One way to look at an RC filter is to see the capacitor as the sluggish part, and the resistor as taking up the slack. So with any input signal, the voltage over the cap is related to the low frequencies, while the resistor follows more the high frequencies.

  Did this explain how a capacitive power supply works? Probably not. But first we need to get to grips with what a capacitor does, and hopefully this little experiment helped you get some intuition for what’s going on.

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  7 Awesome Apps For Discovering Great Music
  appchronicles.com

  Sat­ur­day, Jan­u­ary 28, 2012

  With­out music, life can get pret­ty bor­ing. But even with music, things can get repet­i­tive if you’re lis­ten­ing to the same old thing, over and over again. Let’s be real here, dis­cov­er­ing a new band or soun…

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  Jeff Carlson: One of the coolest things ...

  twitter.com

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  A Veteran of SEAL Team Six Describes His Training
  By Howard E. Wasdin and Stephen Templin, vanityfair.com

  politics | WEB EXCLUSIVE

  The navy SEALs Team Six is so elite and secre­tive that its very exis­tence has never been acknowl­edged by the mil­i­tary—even after its mem­bers led the suc­cess­ful assault on Osama bin Laden’s com­pound in Abbot­taba…

• 2012-01-28

  Hands on with Remarks note-taking and PDF annotating app for iPad

  Rene Ritchie, imore.com

  Remarks is a brand new hand­writ­ing note-taking, and PDF anno­tat­ing app for iPad from Read­dle. I’m con­vinced the team at Read­dle never sleeps because they release new apps, and update their cat­a­log of exist­ing apps, at pret­ty fast pace.…

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  F-BOMB $50 surveillance computer hides in your CO detector, cracks your WiFi
  Myriam Joire, engadget.com

  What hap­pens when you take a Pogo­Plug, add 8GB of flash stor­age, some radios (WiFi, GPS) and per­haps a few sen­sors, then stuff every­thing in a 3D-print­ed box? You get the F-BOMB (Falling or Ballistically-launched Object that Makes Back­door…

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  How to Get a Good Night’s Sleep
  By Kayleen Schaefer, details.com

  It’s time to quit being tired, once and for all. Here’s a step-by-step guide to improving the quality of your rest.

  STEP 1
  Deter­mine Your Level of Tired­ness

  “Peo­ple don’t accept that being tired is not normal,” says Dr. Ba…

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  Hip-Hop’s Future Billionaires
  Scott Goodson, forbes.com

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  MacWorld 2012: The iPro Lens System Turns Your iPhone Into A Pro-Grade Camera
  Lory, appadvice.com

  At this year’s Mac­World event, Schnei­der Optics showed off their cam­era lens pack­age, the iPro Lens Sys­tem. The pack­age includes a clever pro­tec­tive cover, fish­eye and wide-angle lens­es, and a case that dou­bles as a tri­pod mount or a…