Embedded Systems

May 2013 Bryan Bergeron

In case you haven’t noticed, just about everything can be networked with embedded hardware. Take automobiles. Besides the obvious wireless connectivity for your cell phone and GPS, the brakes, headlights, wipers, radio, and transmission are all monitored, controlled, and connected by microcontrollers. At home, there’s a wireless monitor for humidity in my music room, with an alarm set to sound if humidity drops below 45%. There’s also a wireless network of smoke and CO detectors that sound at the first hint of a fire.

I’ve assumed for years the major cost of ubiquitous embedded system networks is low-level radiation from Wi-Fi hot spots and Bluetooth devices. That’s not the only cost, however. The problem with networked embedded systems — as they grow more powerful and more plentiful — is the potential for harm.

It’s one thing for a government to remotely destroy the equipment purportedly used to make nuclear weapons, and quite another for someone to change the setting on your IV drip while you’re in the hospital. Or, to cause your car’s anti-skid brake system to lock up as you accelerate to pass. Or, by someone who remotely shuts off the oxygen to your aircraft cabin. What if someone parked in a car outside your home or office could shut down your pacemaker?

The problem with malicious embedded system crashes is that they can result in physical crashes, as opposed to the soft crashes on a computer screen. Recognizing this, DARPA and other government agencies are funding research to develop means of automatically detecting and patching vulnerabilities in networked, embedded systems.

This is no small task. Think about the difficulty in handling malware on desktop computers. You have to first identify the malware with a program such as McAfee or Symantec. Then, you have to get rid of the malware and patch the corrupted software.

As you may have experienced first-hand, it’s rarely straightforward. I can recall having to format my hard drive and reinstall software at least once in the past few years because of malware I couldn’t remove by other means.

So, what are the practical implications of this reality? I suggest you consider the worst-case scenario. Let’s say everyone in your family has a tablet computer with GPS and video cameras. What could someone do with the location information and perhaps a few real time snapshots? Certainly, these would be an advantage to a would-be burglar.

What about that quadcopter you’ve been building, complete with waypoint software? What if, on your next flight, someone usurps your uplink, and they fly the quad into a moving car? Or, simply force it to land and take your investment with them?

For now, the operative term is vigilance. To my knowledge, there isn’t a standard ‘security ‘ library for the Arduino, Propeller chip, or other popular microcontroller capable of automatically identifying and eradicating malware. Of course, as with malware for the big iron, as soon as protection becomes standardized, the malware makers will adapt.

Perhaps it’s a good time.

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Kickstart Your Project

April 2013 Bryan Bergeron

Kickstart Your Project Okay. You’ve invented the perfect mouse control device. Let’s say it senses a rodent’s heat signature and sterilizes the critter — otherwise unharmed — with a blast of radiation from a cavity magnetron. Your prototype is based on a repurposed microwave oven, a PIR sensor, and an Arduino microcontroller which seems to do the job. If only you had, say, $50K for parts and printed circuit board (PCB) design and fabrication, you could produce a few hundred units and just maybe change the world.

Problem is, $50K is too little to interest a venture capitalist, too much to put on your credit cards, and too much to borrow from the bank. If you’re motivated, have a clear vision for your product, and have a good handle on production cost, then there’s another, relatively risk-free option: Kickstarter (www.kickstarter.com).

Basically, it’s an eBay for DIY funding in which you pitch your product plan to the world. Anyone or any company can back your project. The cost is 5% of the funding, but only if the project is fully funded. Unlike a venture capitalist or silent partner, you get to keep all of the intellectual property.

I recently backed a Kickstarter project that transforms a smartphone into a realtime IR camera. Sure, I could just buy a more expensive stand-alone unit off the shelf and not worry about warranty, customer support, or wonder if the designer will be around in two years, but that isn’t the point. It’s fun to support someone’s dream and — at least vicariously — be part of the action.

Back to your dream. Let’s say you need that $50,000 to produce 100 mouse control units at $500 each. As on eBay, you set up an account so that you can get paid. You’ve got to describe your project (including a video or at least photos) and set up categories of backing. Support isn’t an all-or-nothing proposition.

Typically, the first level of support is a thank you email for a pledge of $1. At, say, $25, a backer receives an official project t-shirt. At $100, you might offer a bare-bones PCB with full schematic and instructions.

At $250, you might offer an unassembled kit with all the parts. A pledge of $500 gets you the full product (shipping extra). A pledge of  $750 gets one of 10 custom units, in dusty blue, signed by you. 

At $1,200, you offer to hand deliver and install a unit at the customer’s site. 

You can use Kickstart to fund just about any reasonable project. I’ve seen projects ranging from $1,500 to use drones to search for Sasquatch to $120,000 for a speaker system, to $7,000 for a ghost detector that plugs into an iPhone. I’ve seen projects funded at up to 1,500% over target. For their 5% cut, Kickstart hosts your product website and handles the financial transactions.

In addition to raising money to pay for your dreams, Kickstart is a great test marketing tool. Let’s say that after 30 days your rodent control device has only $3 in pledges. That’s great marketing feedback for a relatively small investment in time and effort.

Perhaps you can redesign the unit so that you can offer it at, say, $200. Or, perhaps you need to invest time in making a quality video that will attract more backers.

Even if you opt to bypass the funding and mortgage your home, the Kickstart site is worth visiting and studying. Take a look at what’s selling and what isn’t. 

Most importantly, study the how-to guides on how to put together a killer video, how to put the best marketing spin on your DIY project, and how to plan for the details that tend to fall through the cracks — such as the hidden cost of  bubble mailers.

Finally, be careful for what you wish for. If you find you project funded at 300%, then you’d better have time (and space) to build a few hundred of those mouse control units. NV

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Component Specifications: Trust or Verify?

March 2013 Bryan Bergeron

It's easy to forget that no two components or devices are exactly alike. This is fairly obvious with common leaded resistors, given the tolerance marking is hard to miss — gold 5%, silver 10%, none 20% for four-band resistors. In other cases, it's less obvious.

For example, in working with batches of Arduino microcontrollers, I've noticed small but significant differences in the analog-to-digital conversion accuracy from one microcontroller to another. This may be due to differences in the ATmega chip but, more likely, it's due to variations in the crystals or other external discrete components.

Often the exact or absolute value of a component isn't as important as having two or more components of the same value. For example, I just put together an MP3 player with the SparkFun MP3 player shield and their little TPA2005D1 audio amplifier breakout. The class D amplifier has a fully differential input that interfaces nicely with the output of the MP3 player. It also has the advantage of relative noise immunity.

However, I had to increase the gain of the amplifier for my application, and this meant using a pair of matching resistors to keep the performance optimized. So, I sat down with a few dozen 25K 5% resistors and my lab-quality Fluke DMM and identified a matching set. I only wish it were this easy with other components.

Take vacuum tubes. I own several vacuum tube amps — one DIY and a couple vintage guitar amps. One guitar amp uses a matched pair of 6V6 output tubes, and the other a quad set or 'quartet' of 6L6 tubes.

In each case, the amplifier designs assume that the tubes are matched — in terms of transconductance — which is roughly the amount of amplification provided by a tube. It's actually the change in plate current divided by the corresponding change in grid voltage/plate voltage held constant.

The problem with tubes is that they're expensive, and there doesn't appear to be an industry standard for what "matched set" means. Moreover, matched sets command a 50%-100% premium over single tubes. As a point of reference, a single Groove Tubes 6L6 sells for less than $20, while a quartet sells for $140. So, I naturally look to online sources such as eBay for affordable sets.

Unfortunately, there's no way to know from mere visual inspection whether someone simply put four tubes in a box and called them matched. A few months ago, I snagged a 6L6 quartet on eBay for $60. Quite a deal — until I plugged them in. The amp just didn't sound right.

My next purchase was an old but operable tube tester, shown in the accompanying photo. Given the age of the unit (a Hickok 6000A), absolute calibration probably isn't what it should be. However, because I'm looking for relative differences in tubes, calibration doesn't really matter.

As I suspected, the tube tester showed marked differences in tube transconductance in the quartet. Since the meter purchase, I've amassed a small collection of bargain priced tubes and created my own matched sets. My amps are purring with delight. 

Clearly — at least in the case of vacuum tubes — it pays to assume that "matched" is meaningless. I've encountered a similar 'lack of honesty' when it comes to "matched" speakers sold for DIY amplifier cabinets.

As analog electro-mechanical devices, speakers of the same make and model can vary significantly from one unit to the next. This becomes problematic if you want to combine two or four speakers in a cabinet while maintaining an impedance match for the amplifier output circuit.

The bottom line is that it often pays to verify component specifications, especially when you're paying a premium for supposedly premium components. NV

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The Stuff Dreams Are Made Of

February 2013 Bryan Bergeron

It seems like déjà vu all over again. I endured a noisy, 15 cps dotmatrix printer for years until I could afford a quality laser printer at home. Although Apple sold me a LaserWriter at a discount, I paid well over $1,000 for the new device. No more walking to the corner printer with my disc for sharp printouts. Personal publishing reached the tipping point, and, well, the rest is history.

Recently, I made the same sort of move, for the same relative investment in 3D printing. I was tired of paying the setup fees and waiting days for the commercial printing services. Besides, affordable 3D printers have been around for well over a decade. More importantly, the software tools for creating 3D objects have matured, in part because of competition in the software game design market. Besides, high-res 3D scanners are almost affordable.

So, why now? Why not wait for the next model in six months? Well, part of the motivation to move on the technology was a chance meeting with Ken Gracey, President of Parallax. He's using a home 3D printer to prototype next-generation quadcopters and other drones. Turns out that having immediate feedback on the merits of a design do wonders for turnaround time. It's like walking to the corner store to use a laser printer versus having one on your desk. You can try four or five iterations of a design where you may have had time for only one before.

Making the jump to 3D printing is relatively painless, checkbook permitting. One of the key decisions you'll need to make up front is the working material or filament. The two main types of filament are ABS plastic — used in just about everything — and the theoretically more eco friendly PLA.

One of the many advantages of ABS plastic is that the colors are gorgeous and it's easy to work with the printed objects. PLA, in contrast, has a milky translucent appearance when printed and is more brittle, so therefore can be more difficult to work with.

Both plastics should be handled in a well-ventilated shop, but PLA appears to have fewer toxic fumes. On a positive note, if you're used to working with a computer numerical controlled (CNC) or manual milling machine, you'll appreciate the quiet, dust-free 3D printing process.

From a cost perspective, don't get the idea that you're going to start printing toys and selling them on eBay. At-home machines are great for rapid prototyping, but compared with what's available from the commercial printing houses, you'll produce the equivalent of dot matrix output compared with their laser printer output. Moreover, just as the old dot matrix machines had a super-slow letter-quality mode, you can do the same with at-home 3D printers. Just don't expect to see any output for a while.

The greatest economic challenge at the moment is the cost of source material. I've found both PLA and ABS on eBay for about $20/lb plus shipping, when purchased in 5 lb rolls (such as the two rolls of PLA in Figure 1). It pays to shop around for materials, as I've seen adds on 3D printer sites asking for $35 or more for 1 Kg spools. I suspect that somewhere in China, someone is planning a $300 3D printer and $10/lb plastic filament for the US market.

So, why not wait for the inevitable announcement of a price drop? Why not let the 50-60 3D printer suppliers duke it out until the Apple and Microsoft equivalents appear? For one, you'll miss out on all the fun. If you lived through the days of the Commodore-64, TRS-80, Commodore Amiga, and NEXT, you know what I'm talking about.

Second — and most importantly — it takes time to wrap your head around 3D printing. Think back to all of the worthless flyers and incomprehensible manuscripts that were generated with the help of the first home laser printers. Simply because you own a printer doesn't mean you can write or format a document correctly.

The same holds true for 3D printing. You don’t just 'print' a quadcopter. You need to know, for example, about methods of cutting weight with minimal loss in strength, and how to arrange your objects in 3D space so that printing is quick and waste is minimal. Then, there's finishing the printed object to remove support splines or ripples from the low-resolution printing. It's art and science.

Even if you don't buy into the world of 3D printing today, at least check the status of the industry and then recheck it in six months. Pay attention to the hardware printer proper, but also the software tools and the all-important peripherals, including 3D scanners and associated software.

What's the old saying about the future and plastics? Clearly, it's what dreams are made of. NV

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Cosmic Ray Detector

January 2013 Bryan Bergeron

One of the perks of my position as editor of Nuts & Volts is that I get to work with experimentalists, hobbyists, engineers, and scientists who are eager to share their experiments and projects with other readers. While there is the occasional perpetual motion machine or circuit that seems like something repurposed from a 1970's electronics magazine, most are right on. And sometimes there are contributions that are intriguing but don't have the scientific underpinnings to make it into print — it just wouldn't be fair to readers.

For example, I recently had to turn down an article that described how to build a "cosmic ray detector." The author was an exceptional writer and builder, but he couldn't back up his assertion that the device he described actually detected cosmic rays. In fact, he couldn't specify the type of cosmic rays his device was designed to detect.

It apparently detected something, but it could have been static electricity just as well as gamma or beta radiation from the cosmos or from radon gas. Unfortunately, the author didn't have a standard radiation source or radiation detector at his disposal, and so there was no way to verify what — if anything — was being detected.

Although the article didn't make the pages of Nuts & Volts, I hope that the author continues his investigation. Many notable inventions and discoveries come about without a solid or even incorrect scientific backing, and often with no homage paid to the scientific method. Take the Crookes radiometer as an example.

As you may be able to see in the photo of my radiometer, the device consists of four vanes — each of which has one blackened side and one white side. It is encased in a clear glass bulb that has a near perfect vacuum. Crookes believed — incorrectly — that light falling on the device turned the vanes so that the black surfaces were pushed away by the light.

The problem with Crookes' logic is that light falling on the black side should be absorbed, while light falling on the white side should be reflected. Meaning the white side should be pushed away by the light. It turns out that the radiometer only works in a near vacuum, which supports the current theory that the movement is the result of gasses passing over the edges of the vanes. In a perfect vacuum, the radiometer doesn't operate.

Crookes’ radiometer remains a curiosity that's popular with the science crowd, but as a detector it has been supplanted by quantitative, calibrated instruments.

The point is that it's possible for someone to create a device that's operational and yet operates on principles unknown to the inventor. With this in mind, I had waves of second thoughts about the author's cosmic ray detector. However, lacking any evidence supporting the claim that the signals being detected were — in fact — of cosmic origin, I was forced to place the article in the rejected pile.

By chance, I set the article aside just before the discovery of the long-awaited Higgs Boson. The particle was verified nearly 50 years after its existence was postulated, using multi-million dollar advanced instruments run by teams of scientists.

I suppose I'll never know whether the 'cosmic ray detector' (constructed with perhaps $50 in parts) could have detected the Higgs Boson.

I hope, however, the experimenter who was temporarily turned away continues with his experiments. Sometimes it takes science a while to catch up with what experimentalists develop by intuition, sweat, and most of all, persistence. NV

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The Lost World of Tubes

December 2012 Bryan Bergeron

Despite the popularity of the iPod and other svelte electronic music gadgets, the bulky, hot, temperamental tube amplifier is alive and well in the world of guitarists and bass players. Solid-state amplifiers can be had for about a dollar a watt, but you can pay $800 per watt or more for a quality tube amp. The difference is the 'tone' produced by a tube amp — the unique distortion that's created when a tube is driven near saturation.

The tube sound can be simulated on a chip and, of course, there's an app for that. However, the pros insist that there's still a gap between what a processer and an old fashioned tube can produce. I own both tube and solid-state guitar amps and admit that — while the tubes do sound subjectively better — they're a pain to maintain and use properly.

One of the universal truths of tube amps is that the volume level associated with saturation is more than neighbors or roommates will bear. At low volume settings, tube amps make annoying hisses and pops. Solid-state amps, in contrast, sound about the same at any volume level.

A common work-around is to put a tube amp in a closet, close the door, and run a microphone to the amp. I haven't tried this because of the obvious fire hazard posed by clothes hanging over and on a hot piece of electronics (I don't have a spare closet). The second approach is to purchase an attenuator that's placed between the amp output and the speaker. The idea is to dissipate the unwanted audio power as heat.

The problem with the attenuator approach — it will be obvious to readers who have experience building their own ham radio transmitters — is that placing high powered wire wound resistors, capacitors, and other components in the output circuit of a power amplifier has the potential for encouraging RF oscillations.

That is, you can transform a tube audio amp to an RF transmitter by placing inductance and capacitance in the output circuit. The potential for this problem seems to have been lost to the current generation of attenuator developers.

Another approach is to simply place series resistors in the speaker circuit. For example, in the attenuator shown in the accompanying photo, there's an eight ohm resistor in parallel with the output, and either zero, 100, or 200 ohms in series with the speaker.

This does the job of decreasing the speaker output, while matching the output impedance of the amp. Tube amplifiers don't tolerate significant mismatches in output impedance — tube life is shortened.

Another issue with tube-type components is price. Believe it or not, the attenuator in the photo sells for $200, and that's on the low end of what's commonly charged for a few power resistors in a box.

A better option would have been to purchase a 100W eight ohm 'L Pad' from Parts Express or Amazon for about $14. L pads are designed to vary the power delivered to a speaker while maintaining a constant load on the amplifier — typically four, eight, or 16 ohms.

Because knowledge of working with tubes is so limited, there are amazing products on the market that do very well. A popular attenuator that's based on two power resistors and a wire wound rheostat sells for $380 — quite a profit margin. There are similar markups for other peripherals for tube-type amps.

The takeaway is that there's money to be made if you know yourway around a vacuum tube circuit ortake the time to learn.

A painless way to rediscover the lost world of tube circuits is to pick up an inexpensive guitar amp (meaning cosmetically lacking) or a DIY tube amp kit from any number of suppliers such as diyaudio.com.

Of course, the definitive tube amp and audio reference — the Audio Cyclopedia — is a must-read for the tube audio enthusiast and experimenter. NV

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Hand-Wired Electronics

November 2012 Bryan Bergeron

One reason I follow the music electronics industry is that it's usually on the leading edge. There seems to be a constant stream of embedded systems for adding sound effects to percussion, string, and wind instruments, and the latest generation of keyboard synthesizers is not only technologically advanced, but amazingly affordable.

One reason I follow the music electronics industry is that it's usually on the leading edge. There seems to be a constant stream of embedded systems for adding sound effects to percussion, string, and wind instruments, and the latest generation of keyboard synthesizers is not only technologically advanced, but amazingly affordable.

On the other end of the spectrum is the market for tube-type amplifiers for guitar and bass. Despite the availability of sound modeling software and firmware for solid-state amps, no one has managed to capture the nuances of a tube-type amplifier. If you're into these kickbacks to the ‘60s and ‘70s, you know that the best — or at least most expensive — amps use point-topoint wiring. We're talking terminal strips, solder tabs, and discrete components — no circuit boards or chip carriers.

In the past few months, there's been an odd movement in the industry toward hand-wired and point-to-point circuitry. Everything from effects pedals to amplifiers and headphones are available with the "hand-wired" label. For example, take a look at the insides of my guitar effects pedal in the accompanying photo. It's an odd mix of directly soldered transistors and an op-amp IC with carbon composite resistors and a few caps.

Companies charge a premium for the handwired option. For example, the effects pedal in the photo sells for almost three times the price of the model based on a printed circuit board (PCB). Similarly, if you follow any of the big bands, you've probably noticed massive Marshall tube-type amplifiers on stage. While they use tubes, they also use modern PCB construction; in part to withstand the rigors of road work, and in part to reduce production costs through automated parts placement and soldering.

Marshall — along with Fender and other top-tier amplifier manufacturers — recently released a hand-wired line. You can expect to pay more for a one watt hand-wired tube-type Marshall amplifier than for one of their 100 watt professional amps you'll see on stage.

The marketing departments behind the hand-wired and point-topoint wired electronics emphasize the hand-wiring is performed domestically (in the case of Marshal, that means the UK). Supporting the local work force is certainly a good thing. However, they'd also have you believe that there's an audible difference in hand-wired electronics — something to do with the 3D arrangement of components and the interaction through the coupling this allows.

It's difficult to support or refute such claims because it's difficult to find devices that differ only in how they're wired. However, the effects pedal shown in the photo is available in both standard PCB and point versions. The components and schematics are theoretically identical.

So, I hooked up both pedals to a sound source and a spectrum analyzer. No difference. I tried the A/B switch on some musician friends who have trained ears. No difference. I even switched the "hand selected" op-amp — which was soldered in place — with the socketed op-amp from the standard effects pedal. No difference on the spectrum analyzer or to the ears of my musician friends.

Of course, I'm generalizing from one circuit, but the results make technical sense to me. Although there might be some coupling between components, at 3,000 Hz or so, the capacitive and magnetic coupling would be insignificant. I'm not saying products with hand-wired circuits aren't worth some extra money — they do tend to be higher in quality.

So, what's the relevance to you? Well, if you're looking to make some extra spending money, this might be a great time to offer "hand-wired" circuits to your musician friends. This is probably the only time in recent memory that you can compete with the overseas electronics assembly industry from your basement.

You could offer audio cables with hand-wired connectors, or any number of DIY amplifier and effects circuits. I've even seen hand-wired replacement cables for studio-quality headphones listed for $200+ on eBay.

If you have an entrepreneurial bent, this might be the time to cash in on the fad. NV

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Full Power to the Shields

October 2012 Bryan Bergeron

Daughterboards — as a means of expanding hardware platforms — have been around for decades. They really came into their own in consumer electronics with the advent of the PC. Daughterboards enabled the expansion of the motherboard and video cards with memory and coprocessors. Unfortunately, most of these arrangements have been proprietary and limited to a single model of motherboard.

Today, thanks largely to the microcontroller market, many daughterboards — better known as shields — are plug-and-play devices. They provide an efficient, costeffective means of transforming a generic microcontroller into a servo controller, RF transceiver, sensor array, or what have you.

The Arduino exemplifies what a standard shield can do for experimenters. Not only are there hundreds of shields available for the Arduino, but the pin configuration of the Arduino shields (Uno and Mega) has been duplicated for other microcontrollers.

For example, Parallax offers the StampDuino, a BASIC Stamp development board designed to be compatible with most Arduino shields. Parallax also offers a sort of reverse shield — the Board of Education for the Arduino. Whereas most shields are the same size as the Arduino and sit above the microcontroller, the Board of Education is significantly larger than the Arduino and accepts the Arduino from above. Digilent — my go-to company for high performance Arduino compatible microcontrollers — also has one of the best assortments of shields on the market. I’ve used their Basic I/O shield — a real life saver when you have more important things to do than figure out how to connect motors, servos, and I2C devices to your microcontroller.

Digilent also offers 60+ minishields — Pmods™ — based on sixand 12-pin connectors, supporting everything from Wi-Fi and organic LED displays to a Bluetooth. I’ve used the audio input mini-shield which is a real time saver.

Together with the added processing power of the chipKIT microcontroller, the mini-shield and processor hold up to just about anything my PC with sound card can handle. The Pmods — while not a standard shield configuration — are designed to work with their Arduino compatible shields.

SparkFun — the source of countless red shipping boxes in my workroom — offers some of the coolest shields available, especially if you’re into kits. I’ve built several of their MIDI and joystick shields for various projects. SparkFun also offers the Netduino which — at first glance — could pass for an Arduino Uno but uses the .NET Micro Framework. The hardware is compatible with most Arduino shields.

There are exceptions, of course. Because of size and weight limitations, there are a number of Arduino compatible boards on the market that can’t accept shields. For example, I’m working on an ArduPilot Mega 2.0 from 3DRobotics which is essentially a Mega-compatible shield chocked full of SMT sensors for making flying, diving, and walking robots.

The open source board is expensive ($199), but crams an amazing amount of sensor technology into a very small, lightweight package. A regular sized Mega would be too heavy and too large for a small flying vehicle. I’ll be reviewing this shield with a DIY quadcopter in an upcoming issue of SERVO.

I’m all for shields. Look what the Arduino has done for the add-on industry and for experimenters. There is a downside, of course. There’s a lot of unused space on the latest generation of Arduino compatible shields.

While ASICS and SMT components continually reduce component count and the space requirement, shield size has remained constant. So, even though it’s possible to shrink the form factor with no loss in functionality, the standard size and pinout dictate otherwise.

At some point, the standard Arduino will evolve to a new size, and you’ll have to buy new shields or some sort of adapter — perhaps akin to the Pmod mini-shields. But that’s the price for progress. For now, full power to the shields. NV

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Traveling Light

September 2012 Bryan Bergeron

I’m a fan of tablet computers, the seemingly endless library of apps, and the growing body of hardware add-ons. For example, I’ve worked with the open source Hijack interface and TechBasic to make a heart rate monitor on my iPad. Also in my addon library is the suite of virtual instruments from Oscium — a digital oscilloscope, logic analyzer, and spectrum analyzer. These three instruments — each only a bit larger than the standard iPad connector — have (until now) been collecting dust.

The Oscium instruments are first rate — sturdy miniature hardware and robust easy to use software. However, since I have a fullyequipped lab, there’s no reason for me to snap on a connector and boot up an iPad app when I have full-scale instruments within arm’s reach. I suspect that’s the case with many experimenters who are already stocked with test gear.

My virtual instruments no longer collect dust, however. Last week, I was called to make a day trip across country to examine a series of medical devices for a patent infringement case. I had no idea what to expect in terms of equipment on site, and given the tight airline schedule, this was a carry-on flight only. Forget checking a trunk with test equipment, or of putting a standard oscilloscope or spectrum analyzer through the TSA’s screening process. So, I grabbed the set of Oscium instruments and put them in the small outer pocket of my backpack. Total weight: about 6 oz and less volume than a miniature USB charger for my phone.

When I arrived on site, I was almost happy to find it ill-equipped. I used the WiPri spectrum analyzer (see the figure) to verify Wi-Fi activity and the scope and logic probe, in turn, to look at sensor output and data rates. All in all, lifesavers that paid for themselves in a single trip. So, now I keep the set of instruments in a small camera bag, with the oscilloscope probe and charger in a separate zip-up case — all set for the next flight out.

Although I still use my benchtop gear on a daily basis, I’ve started using the virtual instruments whenever I’m away from my bench. For example, in tuning up my latest quadcopter, I didn’t want anything tall enough to be struck by a spinning blade — that means no workbench. The kitchen table became my test area, and virtual instruments on my thin unobtrusive iPad became my bank of instruments.

I suspect that I’m not alone in my initial excitement over app-based virtual instruments for tablet computers. The question is, when will they make sense for most experimenters? I can see the obvious utility of these hardware and software apps in high school and college physics and electronics courses — forget the antiquated lab gear and bring your own modern digital gear to class. But what of the average experimenter?

I suspect it’s a matter of budget and travel habits. With the TSA and airlines forcing smaller and smaller carry-ons and frowning on big boxes loaded with wires and circuit boards, a tablet with add-ons is the way to go.

Another option is to go with several stand-alone, palm-sized instruments. I have one of those cell phone-sized digital oscilloscopes, for example. Unfortunately, I used the $99 scope only twice, in part because it took too long for me to figure out the convoluted menu system, and in part because the case was so poorly constructed that the battery had a tendency to pop out. An advantage of a standard interface panel (that is, tablet or smartphone) is that, well, it has a standard interface. Learn one instrument and you learn to use them all. It’s the same approach Apple used with the Macintosh which has had a measure of success.

If you have a favorite app and hardware combination for your tablet, please let me and the other readers hear about it. NV

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How Much to Invest?

August 2012 Bryan Bergeron

I have a general rule when it comes to tools, electronics, and accessories: I buy the best that I can afford at the time. There’s nothing more frustrating than having to buy another widget because I didn’t spend the extra dollar to get a feature that I didn’t know I needed. This defensive purchasing sounds simple, but in practice, it’s a difficult rule to follow.

I have a general rule when it comes to tools, electronics, and accessories: I buy the best that I can afford at the time. There’s nothing more frustrating than having to buy another widget because I didn’t spend the extra dollar to get a feature that I didn’t know I needed. This defensive purchasing sounds simple, but in practice, it’s a difficult rule to follow.

Let me give an example. I’m preparing a series of articles for our sister publication, SERVO, on autonomous aerial vehicles. Thanks to open source hardware autopilots and ground control software, it’s possible to build a fully autonomous quadcopter (a helicopter with four motors) capable of carrying a 2 lb payload for about $1,000.

The ground control software — often referred to as waypoint software — is linked to GoogleEarth, enabling just about anyone to plan a graphical route for the UAV and have the payload delivered there autonomously. That is, you literally direct a drone to travel two blocks down Main Street, take a left at the corner, and then land at the doorstep of the fifth house on the right. Some think this is a great scheme for pizza delivery. Others think cruise missile.

Regardless of what the technology and capabilities bring to mind, it all depends on reliable battery power. So, as part of building and testing various quadcopter systems, I had to identify and purchase the ‘best’ batteries and charger that I could find and stay within my budget.

Simply going for the most expensive chargers and batteries and ignoring my budget wasn’t an option — even if I had unlimited funds — because a huge, high voltage, high capacity lithium battery wasn’t compatible with the airframes I was testing. No, I had to do my homework.

It turns out that lithium batteries generally offer the best energy density, weight, and cost combination, compared with the other common battery chemistries on the market. However, when you get into it, you’ll discover that there are various formulations of lithium batteries — including LiPo, LiLo, and LiFe. I won’t go into details here (as you can tell, I’m trying to entice you to take a look at SERVO), but each formulation has different costs and capabilities.

Then, there’s the physical packaging. Some lithium batteries are — because of safety regulations — encased in hard, heavy plastic. I discovered this type of battery on the Parrot AR Drone 2.0 (the ‘low end’ vehicle in my study) and immediately ordered a replacement battery with the same weight and dimensions, but that provided double the capacity. The new battery is encased in shrinkwrap.

The tradeoff is that lithium cells are known to catch fire and explode when damaged in a crash — a tradeoff that I’m willing to make. There are lightweight, fireproof battery wraps available, if you want to lower the risk of fire somewhat.

The problem with the battery upgrade for the Parrot AR Drone was that the original charger couldn’t handle the load, so I was forced to buy a new charger. The ‘best’ charger (in this case) had to handle not only the relatively minor demands of the AR Drone battery, but those of the batteries capable of powering the meatier motors and electronics of drones from DIYDrones (www.diydrones.com) and Parallax (www.parallax.com).

What’s more, because it’s illegal to fly a potentially lethal drone over a crowded city like Boston, I have to travel an hour outside of the city to test the drones. That means a supply of batteries. It also means that the ‘best’ charger for me had to include the ability to charge/discharge several battery packs simultaneously. So, I was forced to consider $100+, 1,000W battery chargers and multiple, $40+ LiPo batteries.

Then, there’s the ‘best’ power supply to power the charger — which has to be powerful enough to meet the demands of charging a half dozen batteries simultaneously.

Repeat the above exercise for the R/C transmitter and receiver (check out the Spektrum DX7) and onboard camera system (see the GoPro HD Hero2), and we’re talking a lot of research and a significant investment. Not the most expensive or capable components I could find, but I expect the investment in batteries, charger, camera, transmitter, and receiver will outlive the review.

For example, had I gone with a simple upgrade charger and battery pack for the Parrot drone, I would have had to buy yet another charger for the higher capacity batteries on the other drones.

I hope this helps you with your purchase planning. Do your best to look ahead to your next few system upgrades — whether it’s test equipment or a microcontroller development kit — to determine the best investment for you. NV

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