TROOPERS conference badge

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Electronic badges are all the rage at IT security conferences these days. I think DEFCON started it all with badges made by Joe Grand, but there have been many others, like the Sputnik RFID tags at 25C3, the expandable bunny badges at Easter Hegg, Munich, by lilafisch and friends, and Travis Goodspeed's badge for The Next Hope.

I created 210 interactive badges for the TROOPERS11 IT-sec conference in Heidelberg last month. They feature some ethereal vintage vacuum glassware - IN-16 nixie tubes made at the Reflector factory in Saratov in the former USSR in 1989. I purchased them new-old-stock from some handy guys in Lithuania.

A video of the badge in action:


 

Each conference attendee starts on zero. As they unlock achievements at the con - like sending postcards to their families, meeting the speakers, unlocking the secret in the badge, attending my SMT soldering workshop, etc, attendees level-up. Those who reached the highest levels were entered into a prize draw to win hacker goodies. I designed this levelling-up concept to connect with the central themes of the TROOPERS conference - personal progression, education, and becoming better IT security professionals. Florian did an amazing job of weaving the badge and the game concept into the very fabric of the conference. 

The badge hangs around the neck from a CAT5 cable, rocking the network security image. Red for the speakers, green for everyone else. Here I am wearing the bare, prototype PCB. Have you any idea how difficult it is to find 750mm CAT5 cables? It's quite difficult.

As well as being the LANyard (sorry) the cable functions as the power switch, levelling-up mechanism, programming interface and debug output (and attack vector for intrepid hardware hackers!). When an attendee unlocked an achievement, he took his badge to the information desk where one of the organising staff would plug in a special dongle to update his score. The dongle is simply a TI Launchpad (MSP430G2231) with a CAT5 cable soldered on, running some custom firmware to transmit a magic byte over SPI to the badge.

The batteries and electronics are on the back of the badge. 

Design materials

You can download the design materials here . The hardware is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. The firmware is licensed under the GPLv3 . If you just want a quick look at the schematic, it's available separately here .

Circuit design

The badge is powered from two AA batteries. The nixie tube uses most of the power, so I could basically trade nixie shininess for battery life. I chose a gentle fade-in-fade-out animation, with the duty cycle set for around 40 hours of use. The output voltage of an alkaline cell decreases nearly linearly as it is discharged. Even when it reaches 0.9V or less it still contains a decent amount of energy. To make full use of the energy in the battery the badge must work from a supply voltage of 1.8V or less. The nixie tube needs around 150-170V at 1.8mA. I use an MCP1640 DCDC to boost the battery voltage up to 5V to power the low voltage electronics, microcontroller, etc. Then I use a microcontroller-based DCDC to get from 5V up to an adjustable voltage between 0 and 200V. It's quite difficult to get from 5V to 150V efficiently and cheaply with a standard boost topology, so I use a flyback topology with a small transformer.

Whilst small, high performance, SMT inductors are completely ubiquitous, similar transformers are not. This is due to low industrial demand. Camera flash circuits (which, just like my nixie supply, must step from battery voltage to 150-300V) are about the only application for small transfomers. They are only made by a few large companies, and only leave the East inside cameras. I got mine from Tokyo Coil Engineering, and I really owe them a favour! They usually sell in enormous volume to people like Sony and Casio. 

I used an ATmega48 because it was the cheapest microcontroller around with enough timers, IO and ADC channels. I don't really like AVR's. Their fuse system is a real pain in the arse (fuses in general are a pain in the arse) and they have about 10 different proprietary programming and debugging interfaces. But hey, people seem to like them, and the price was right! The AVR produces a PWM signal that switches the FET on the primary side of the transformer, and the voltage is boosted on the secondary side both by the turns ratio and inductance of the transformer. The voltage is attenuated and buffered, and fed back to the AVR's ADC. The firmware measures the high voltage supply via this attenuation, and adjusts the PWM setting using a PID control scheme. The PWM output is capacitively coupled into the FET gate so that the FET doesn't blow up if the AVR crashes. The efficiency is pretty good; 75-80% depending on voltage setting, etc. 

Instead of using a resistor to limit the current through the nixie tube, I used a 1.8mA active current source. I designed it this way with the intention of using a high voltage to strike the neon then reducing the voltage to minimise power dissipated in the current source. Quite an elegant idea, I thought, but I ended up manipulating the DCDC voltage to fade the nixie in and out, rather than dimming it with another PWM channel. So if anything, the current source was probably a slight hindrance because it made the nixie brightness much less sensitive to supply voltage. Never mind, it all turned out nicely in the end!

The 12 individual nixie cathodes (0-9, left and right decimal points) are switched by high voltage NPN transistors..

The speakers' badges have sound-activated LED's. An electret microphone captures the sound, and it's amplified and fed to the AVR's ADC. Foolishly I biased the mic directly from the 5V rail, which turns out to be pretty noisy when the nixie is at full brightness. I ended up writing a horriffic firmware workaround to dynamically adjust the microphone gain based on the nixie voltage. It worked fairly well, but the sensitivity is still low when the nixie is bright. 

The CAT5 connectors expose the ISP programming interface for flashing the AVR, the SPI port, which is used to increment the score, and a 5V UART which can be used for live debug, etc.There's also a capacitive touch button hidden in the ERNW logo. 

Some lovely graphs

I was, as usual, working right up to the deadline for this project, and ended up writing a lot of the firmware in Heidelberg. I wanted to work around the microphone noise problem, and suspected that I could reduce the noise by improving the high voltage control scheme. Initially I was using a very rough control scheme that either incremented or decremented the DCDC PWM value depending on whether the voltage was below or above the desired setpoint). Since one PWM step makes a significant impact on the output voltage, the whole thing was pretty nasty - too much oscillation when the setpoint was low, and failure to actually reach the setpoint when it was high for only a short period. I decided to hunker down in my hotel room, get the badge to stream live data out over the UART, and plot some graphs in python to see what was really going on.


My original, crappy, control scheme. The green trace is the DCDC output voltage. The red trace is a moving average of this voltage to take out a bit of noise. The setpoint (desired voltage) is in blue, and the value of the PWM signal delivered to the FET is shown in turquoise. The output voltage roughly follows the setpoint, but it is very wobbly when the setpoint is low, and can't quite keep up when it is high.

Note I am ramping the setpoint up and down to get the nixie to fade in and out.The change in gradient near the top of the peak is deliberate - it makes the fading look more consistent to the human eye.

Please ignore the x axis label. It isn't actually graduated in seconds. This whole graph probably shows about 5 seconds of data, not several hours!

  The PID control scheme is a dramatic improvement. The output voltage (green) tracks the setpoint (blue) perfectly. To achieve this, the PWM value (turquoise) is being adjusted in a much more subtle way than before. The red trace shows the error (difference between the true output voltage and the setpoint).
  Next I worked on the microphone noise cancellation. The blue trace is the microphone voltage (recorded in silence). Note the "blob" of noise when the nixie voltage (red) is high. The green trace is a very long moving average used to establish the DC level of the microphone reading. The turquoise trace shows the PWM values being delivered to the FET. I used this signal as the basis of the cancellation function because its shape is very similar to the noise "blob" in the microphone data..
  Here are the results. The blue trace is the input, as before.The yellow trace is the processed signal. On the left I was silent. The DCDC-induced noise "blob" is successfully filtered out, but speech (right half) is largely passed through. Sensitivity is pretty crap when the DCDC voltage is high, but it's the best I can do without a hardware fix!
   

Firmware design

The firmware is written in C, compiled under linux with avr-gcc, and flashed to the badge with avr-dude. I use the AVR Dragon programmer, but there are other options available. At the con, Kevin Redon dumped the firmware binary out using a Bus Pirate, made some modifications, and pumped it back in. He also scored himself an instant 9 by editing the EEPROM image. Respect. Several other guys performed some simple hardware hacks too, shorting segments on, etc., but Kevin's work was the most impressive I saw.

If you're a masochist, you can also  debug the AVR over Debugwire using avarice and gdb. It's unbelievably flaky, though.

The firmware is highly modular and fairly self-explanatory. It is largely interrupt based, making extensive use of the timer peripherals.

Hacking area

Lots of people asked me about this. It's just an empty bit of board with some 0.1" spaced through hole pads and 0.05" spaced SMT pads where you can solder on your own components. The conference attendees are mostly software guys. I wanted to give them a platform that is really easy to hack so that those with budding hardware hacking interests have a low barrier to entry. For maximum flexibility, no electrical connectivity is provided. You can just use wire. I also broke out every pin of the AVR to a pair of through-hole solder points for maximum hackability. So say you want to make your badge into a clock, you could solder a crystal onto the AVR's clock pins. Or if you want to add some radio functionality you could solder your radio chip onto the hacking area, and wire it up to the AVR. Go nuts - totally freeform hacking space.

Shock hazard

The high voltage section is insulated with heatshrink sleeving and self-levelling silicone encapsulant (great stuff, by the way). If you dig your way through it is possible to get a mildly painful shock, but there isn't enough energy stored in the DCDC to do you any harm. Don't take my word for it, though. If you're ancient / part cyborg / whatever, I wouldn't push your luck!

Manufacturing the badge

That was quite some undertaking! I outsourced the manufacture and assembly of the PCB's, of course, but I placed the nixies myself. All 210. This entailed trimming the nixie legs to the right length, soldering all 13 of them to the PCB (6 on the front, 7 on the back), adding heatshrink sleeving and insulating the finished item with silicone. I made some lovely jigs to process the nixie tubes, cut precise lengths of heatshrink and hold everything in place during soldering. Check out these videos:

 


 


 


 

 A closeup of the assembly jig. The perspex assembly (rear) holds the nixie perfectly centred within the cutout in the PCB. The spring steel, aluminium and FR4 assembly (front) clamps the legs down onto the PCB, leaving both of my hands free to solder.

It turns out that 210 is quite a lot of things.

So if you're the lucky owner of a TROOPERS badge, I hope this information inspires you to hack it to do something new! At the very least, please take care of your nixie tube! They are a scarce resource. They haven't been manufactured since the early nineties.

I held a little SMT soldering workshop at the con, where attendees could upgrade their badge to speaker status by soldering on the extra LED's, resistors and microphone. Someone remarked that it was probably the most romantic soldering workshop ever! (Thanks to Insinuator for the photo).

Once again, many thanks to Florian, Enno, Daniel and all at ERNW - thoroughly enjoyed it!

Please ask questions in the comments / by email / twitter, and I'll help you out. If you're running your own con, and would like some similarly insane badges, give me a shout.

Have a look at my flickr account / youtube channel for more media.

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As always a perfect amount of detail.

Pretty amazed you decided to add sound-sensitive LEDs along with everything else!

me too

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Power question

Awesome project! Do you recall what kind of power savings you get with the transformer as opposed to a typical inductor boost circuit?

I don't exactly, no. Due to

I don't exactly, no. Due to budget constraints there was no way I could have used a standard boost topology so I didn't try, and don't have power consumption figures to compare.

One of the main problems is the switching FET. In a standard boost topology, the drain of the FET reaches the high voltage Vout when it's turned off. This demands a FET which can withstand a Vds of, in my case, 170V. Such FET's do exist, but because of limitations in physics and market demand, they typically don't operate with a logic-compatible Vgs of 3-5V. The cheaper ones certainly don't. Instead you need a gate voltage around 8-12V to switch them efficiently. This, in turn, demands an 8-12V supply somewhere in the system, as well as level translation to drive the FET from the microcontroller. Furthermore, these high voltage FET's are typically designed for switching large currents, which makes them physically large - lots of silicon, and packaging, and hence lots of cost. You have to spend even more money to get low Rds(on).

Lots of silicon means lots of capacitance - both Cgs and Cds. High Cgs demands a gate driver circuit which can deliver high current spikes if the FET is to be switched quickly (and therefore efficiently, since the FET dissipates the most power when it is not-quite-on). This means you may need to introduce a buffer between the microcontroller and the FET in order to deliver enough current. High Cds is particularly bad in high voltage situations because, on each switching cycle, Cds must be charged to Vout (170V) and discharged to 0V. A lot of energy is wasted in doing so - 1/2 Cds(Vout^2) on each cycle. Finally, in a boost converter, Vin/Vout is simply equal to D, the duty cycle of the PWM signal switching the FET. This means that in order to boost from, say, 5V to 170V you need a duty cycle of 97%. This means that switching losses are severe, because the FET's switching time is a significant fraction of the off-time. You can reduce the PWM frequency to reduce switching losses, but this demands larger inductors and capacitors to achieve an acceptable output voltage ripple.

If you're just making one or two circuits where cost isn't really a problem, then you can avoid exotic transformers by using expensive (~£1-3) high voltage FET's, big capacitors and inductors, and drive the system at a low frequency. If power consumption isn't a big deal you can get away with murder. If you have a reasonably large input voltage to play with then you can use a BJT instead of a FET as the switching element. Low Vce(sat) BJT's are available with saturation voltages of 0.4V or less. They are much easier to drive because they don't have the Vgs and Cgs problems. But they do consume a reasonable amount of base current. Particularly since high voltage, low Vce(sat) devices tend to have poor current gain.

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