Autobahn

Autobahns are controlled-access highways. The photo above reveals some design details that make safe, fast travel possible. Rather than pouring out asphalt and see where it sticks, there's a bridge built across a rather shallow valley, the hill in front has been cut into to lay the road through. Built in seven layers, the road surface is flat, smooth and provides grip, drainage puts away with rain, side rails keep you on track if necessary. Autobahns are built for high throughput and high speed. That makes them expensive and different from roads in much bigger countries that must stretch across large distances at economic expense.

Speed Limits

You'll find speed limits where they make sense:

• around densely populated areas, i.e. in vicinity of cities (sound pollution)
• when approaching interchanges (German: Autobahnkreuz)
• when approaching construction zones
• multiple on-ramps in short distance after each other
• weather conditions inflict a risk of water collection during heavy rain
• the landscape makes tighter curves or steep grades necessary
• a local administration needs funds, thus sets up speed traps

Speed limit signs are round, white background, black font, red outer ring:

Speed limit: maximum 60 kph

Take speed limits extra seriously if you see the same speed limit three times within a very short distance. Expect a speed trap right after the third sign.

Recommended Maximum Speed

German Autobahns have a recommended maximum speed: 130 kph. Sometimes you will see a blue, square sign:

Recommended maximum speed: 80 kph

This sign means nothing else than "recommended maximum speed of 80 kph". Exceeding this is legal.

Minimum Speed

In some places you'll find a blue, round sign like this:

Minimum Speed: 30 kph

This sign means nothing else than "recommended minimum speed of 30 kph". Exceeding this is legal. Going slower is not. Imagine you are traveling on n older three-lane Autobahn approaching a steep hill. You can expect heavy trucks to slow down while trying to get up. You can also expect more powerful trucks going 52 kph trying to overtake less powerful trucks going 49 kph while you zoom in. We call that situation an elephant race (German: Elefantenrennen). Putting slower vehicles in place, the above sign is often combined into one of these:

Mimimum Speed Lanes

Go at least 50 kph when in the center lane, at least 80 kph when in the left lane.

Weather Conditions

In some places you will find a speed limit accompanied with another sign on the same post:

Speed limit only applies during wet road conditions

The sign literally says: "when wet" (German: bei Nässe). The speed limit is binding when the road is wet. Some aged Autobahns may have groves on the right lane (made by heavy trucks) in which water will collect during rainy weather. In some places water can collect on the road surface during heavy rain falls and lead to aquaplaning. Look out here for your own safety.

No Overtaking

This is not a speed limit sign but important to understand another sign:

No overtaking

The above image relates to all vehicles: No overtaking (German: Überholverbot). The following sign applies to you if your vehicle has a permissible maximum weight of over 3.5 metric tons, e.g. trucks:

No overtaking by trucks - passing limit for motor vehicles with permissible maximum weight of over 3.5 metric tons, including trailers, except passenger cars and busses/coaches.

The following two signs end the above passing limits:

End of passing limit

End of passing limit for motor vehicles with permissible maximum weight of over 3.5 metric tons, including trailers, except passenger cars and busses/coaches.

No limit

Oh yes! This sign ends all speed and passing limits. Shift down, floor it and get into the fast lane. The friendly driver in that Audi R8 or Mercedes AMG or BMW M5 behind you will do that. Be fast or please get out of their way.

End of all speed and passing limits

The fast lane

When traveling on the Autobahn, stay in the right lane. That is not the emergency stop lane. Just stay right. The left (and middle) lane are for overtaking.  Usually, trucks will stay on the right lane. They will usually keep a pace that looks close to 100 kph on your speedometer (I'll spare you the details). If you need to go faster than them, overtake. You will see many motorists staying in the middle lane at their chosen cruising speed between 120 kph and 160 kph. Use the left lane if you want to go faster than them. When you're done overtaking, go back to the middle or right lane. Always overtake on the left. Overtaking on the right is illegal and savagely fined, unless you are in a lane to or from a ramp. There are fast cars roaming German Autobahns. You might find yourself going 200 kph and that Porsche zooming past you makes you feel like you're at stand-still. Especially when it has flashing blue lights on top. Please be aware of that before entering the fast lane. Double check your mirrors.

Distance saves lives

You're probably used to keep your distance to the vehicle ahead. Higher speed, higher distance. Here's a rule of thumb for speed beyond 100 kph: chose your distance - in meters - to be half of what your speedometer shows. Example: your speedometer shows 100 kph, then your distance should be at least 50 meters. What distance covers 50 meters? On Autobahns, it's the distance between two of these delineators (German: Leitpfosten):

Delineator

These are usually equidistantly placed on the right side of the pavement every 50 meters. Spot them in the photo above? Neat! The above rule of thumb is proportional. For 150 kph, chose at least 75 meters. For 200kph chose at least 100 meters.

Sundays

On Sundays and public holidays, trucks aren't allowed to use the Autobahn. That is, from Saturday night 10pm until Sunday night 10pm. Which makes an excellent time for joy rides.

Copyright Notice: All road signs were copied from Wikimedia Commons. These images are in the public domain according to German copyright law because they are part of a statute, ordinance, official decree or judgment (official work) issued by a German federal or state authority or court (§ 5 Abs.1 UrhG).

Thermistor temperature sensor

Recently, my friend w0z visited after 28c3 and we ended up checking out Segor, a brick-and-mortar electronics shop in Berlin. It's to me like a candy shop is to kids. Among other things, a thermistor module piqued my interest, mostly because I had never used one before.

The thermistor module I got came as a thermistor and an op-amp on a PCB. I hooked it up to an Arduino's analog input pin and sure enough I could see the input data change when I exposed the sensor to warm air.

Documentation

Of course, there is an article on thermistors on Wikipedia. While reading it, you'll learn:

• the ratio of the thermistor's resistance to its temperature is proportional but non-linear
• for accurate temperature measurements, the resistance/temperature curve of the thermistor must be described
• someone already solved that: Steinhart-Hart equation
• the coefficients necessary for the Steinhart-Hart equation are usually provided in the thermistor's data sheet.

Unfortunately, the module came with no documentation at all. The thermistor itself just said 4k7, which means it has a resistance of 4,700 ohms at 25°C. A bit frustrated, I de-soldered the thermistor and decided to make my own circuit and calibration data.

Half-bridge

I searched the web a bit for thermistor temperature sensor schematics. Then w0z pointed me to the course material of her electrical engineering studies, namely 6.121J at MIT. Not quite able to find it on the website of MIT, I found her course material on a Chinese mirror. After reading it, I decided to use a half-bridge for my schematics. The thermistor would be part of a voltage divider. I would have a micro controller measure the voltage drop across the thermistor and determine the thermistor's current resistance, ultimately computing the current temperature.

Thermistor half-bridge (voltage divider)

Given the above schematic, the thermistor's resistance equals Rntc = { 4k7 ohms / (Vcc - Vntc) } * Vntc. Vcc is known. Vntc is determined by measurement. Thus Rntc can be determined.

Steinhart-Hart

The simplified Steinhart-Hart equation is a model of the resistance of e.g. thermistors at different temperatures. It uses three coefficients that are specific to the thermistor at hand. Knowing the three coefficients, I can use the thermistor's measured resistance to get its current temperature. The three coefficients unknown (no data sheet), I can expose the thermistor to three known temperatures and measure the according resistance. That would leave me with a set of three linear equations with three unknowns, i.e. the coefficients. The thermistor says 4k7 for 25°C, so I already had one equation. I had to take two more measurements at known temperatures. Not having a thermometer at hand, I chose:

• 0°C (water freezing at sea level) and
• at 100°C (water boiling at sea level).

Water-resistant sensor

I intended to submerge the thermistor in water. Tap water is usually not salt-free. In an effort to make my sensor insensitive to water conducting between the two pins of the thermistor while submerged, I encased it in shrinking tube. I used pliers to clamp the open end shut while the shrinking tube was hot. At the other end, I added a three-pin connector. The pull-up resistor of the above circuit is encased in the shrinking tube around the connector.

Calibration Method

I hooked the sensor up to an Arduino for measurement. A simple ADC measurement would suffice. We'd put the sensor into water and wait for the measured value to rise/sink to the temperature of the surrounding medium. The value was noted down when the measurement was stable for at least five seconds.

Thermistor calibration slush

We used a slush (finely crushed ice with a few tablespoons of water) to measure at 0°C. We used boiling water (roiling boil, cut off heat, let settle for roil to stop) to measure at 100°C.

Self-heating

While doing the measurement in slush, self-heating of the thermistor became a problem. Since its resistance reduces proportionally to decreasing temperature, the thermistor would let more current pass and thus self-heat. The measured voltage-drop at the thermistor would vary by about 2% while being inside the slush. This problem can easily be overcome by not constantly applying Vcc to the thermistor, but only for the short time of measurement. Thus we connected the Vcc pin of the sensor to a digital output pin of the micro controller. We chose a period of 5ms for measurement and a period of 100ms pause. That was sufficient to get a stable measurement.

Gaussian elimination

The results of our measurements were:

• for Tntc = 0°C, Rntc = 14457 ohms
• for Tntc = 25°C, Rntc = 4700 ohms
• for Tntc = 100°C, Rntc = 384 ohms

The above figures - each put into the Steinhart-Hart equation - left me with a set of three linear equations with three unknowns. There's a neat mathematical method to solve this this and, of course, there is a Wikipedia article on it: Gaussian elimination. Really neat: Gaussian elimination computes a matrix decomposition. You could use pen and paper to do it manually, or simply use your computer. If appropriate math software is unavailable, use this online calculator to solve linear equation systems. I found a few on the web, the one I linked to was the only one providing proper results. I ended up with the following coefficients:

• A: 0.0011657006
• B: 0.0002508247
• C: 0.0000001077

Result

The above coefficients, set into the Steinhart-Hart equation, let me compute a look-up table to put into the micro controller. For each value the micro controller could get from the ADC, I provided an according temperature value inside the boundaries from -40°C upto 120°C.

Thermistor graph – resistance/temperature

Links

• Resource about  NTC thermistor calculations on Sourceforge: http://thermistor.sourceforge.net/
• http://mathworld.wolfram.com/GaussianElimination.html
• http://easycalculation.com/matrix/gaussian-elimination.php
• http://www.sosmath.com/matrix/system1/system1.html

MegaBitMeter Scale Preview

The ideas for both MegaBitMeter and TorrentMeter were conceived in different places, yet at about the same time. @pocsascha got inspired to create his MegaBitMeter while working on energy data at ICMP5. @skytee reflected on "the Internet being a series of tubes", yet certainly all crafted from copper and brass and thus the need for an appropriate "pressure gauge". Halfway through implementing our ideas, we met and exchanged ideas on how to make scales and drive meters. Now, after getting all the requests for a kit, we make it available. This is a prototype:

MegaBitMeter prototype

The bandwidth meter DIY kit will have the metal stand you see and the above light blue scale. Soon you can roll your own! If you attend 27c3 or BerlinSides this December, you can get one from us in person. As soon as the shop is up, we'll post an update on Twitter.

I needed a whiteboard and had this spare table top: frosted glass plate with an aluminum frame. While putting it up onto the wall with two strong hooks, I thought it would make a great diffuser for a lamp. Let it be a white board and a source of indirect light in arbitrary colors. And that's the idea of my FnordBoard.

FnordBoard Parts and Assembly

To make a backlit whiteboard, a FnordlichtMini is a perfect match. That's an inexpensive Open Source RGB color changing LED hardware, based on an Atmel AVR ATmega8. Several of these can be chained up and individually controlled. Since my whiteboard should be backlit, one or more of those shall be mounted on the bottom side of the table top. It should shine right across the glass plate, so a lot of light would be diffused. The table top is actually an IKEA Vika Lauri. The bottom side is frosted, the top side is easy to write on with a whiteboard marker.

Frosted glass table top with aluminum frame

The frame is held together by each a metal plate and four rivets in each corner. The bump in the middle of the plate has an M10 thread to attach a leg onto.

FnordBoard parts

How to mount it now? I had some of these white plastic caps that come e.g. with IKEA Expedit book cases . (One is supposed to affix these shelves with a metal brackets to the wall, covering each bracket with a plastic cap.) Conveniently, one of these RGB color changing lamps just fits onto one of those white plastic caps. Even more conveniently, when sliding a plastic cap under one of the metal plates holding the table top frame together, it just fits under the threaded hole so you can bolt it down.

FnordBoard FnordlichtMini mount

I did have some spare M10x40 bolts (from Sultan legs), yet the head would've sticked way out. Thus I cut off the bolt with a saw, so it would be flush with the metal bump, about 22mm (7/8th in) long. To make it a proper headless bolt, there shall be a slot in one end so it can be turned with a screw driver. It should look like this:

Headless Bolt

With the plastic cap bolted down, a FnordlichtMini can be attached to it. You might want to use a bolt for this, since the PCBs have mounting holes. Last step: add power cabling. That can be drawn along inside the mortise of the aluminum frame. Supply voltage is 12V DC.

FnordBoard Assembly

Attach to wall and enjoy!

FnordBoard color changing RGB backlit whiteboard

Fnordlicht and FnordlichtMini

The Fnordlicht project started out as a kit used for teaching young people electronics and programming. Several of these can be chained up to a bus using inexpensive ribbon cable. They will take commands via a serial interface attached to the first of the chain. Check out the protocol: they can each display an individual color and change color quite rapidly. @fbz made an incredible DJ desk installation including 40 of these, controlling each one separately by music via processing. Check out fd0's Fnordlicht page for more.

TorrentMeter Mk. 2

I got inspired by reading Tom Igoe's article in Make Magazine issue 11, about an antique gauge displaying air quality data from the web. It hit a nerve: friends and I had once built a steam powered teletype. Since we've got fiber optical intertubes at home, I wanted a big brass gauge telling me how big my pipe was. And that's the idea of my TorrentMeter.

TorrentMeter Mk. 1

TorrentMeter Mk. 1

The TorrentMeter Mk. 1 is a prototype. It's made from a a custom made voltmeter, scale 0-150, 150=5V. Thus it's rather easy to use a PWM output pin of an arduino to drive it. The gauge draws only a few mA, thus damaging the micro controller by dragging too much current out of the output pins was no issue. Unfortunately, neither is a micro controller's pin output necessarily 0V for low nor 5V for high. Thus a simple switching transistor came to help.

TorrentMeter Mk. 1 schematics

Depending on your setup you might want to protect the transistor output from current flowing back from the gauge coil by adding a suppressor diode to the output.

Software

So far a gauge is connected to a micro controller, Arduino that is. That sure is overkill, but it is readily available. Getting the gauge to actually display the desired information? In my case I want to simply have my router post the current incoming bits per second (the torrent)  to the Arduino. Check out my repository on github! There is a perl script, grepping the desired information from my Linux router's /proc, and a loop that's started on boot. That loop calls the script periodically and pipes the output to the serial port emulated by the FTDI chip on the Arduino. The Arduino converts it into a pulse width. If that was too fast for you, you might want to read the above mentioned Make Magazine article. For the Arduino software, just combine what you can learn from the Arduino AnalogInSerial and Dimmer examples.

TorrentMeter Mk. 2

I got a 1908 voltmeter in a brass housing, about 20cm (8in) in diameter. It had a resistor made from a wire wick. The gauge's internal resistance without that is now about 67 ohms, eats up 150mA when the handle is at full scale, and wants about 10V. That's too much to be driven directly by a micro controller. A switching amplifier, made from a couple parts, shall help.

TorrentMeter Mk. 2 schematics

Notice the two parts. They have different power sources but common ground. The left side is just a pull-down resistor, a power LED and a switching transistor. Its output is connected to the right part, which is the actual switching amplifier. From a software perspective it works similar to TorrentMeter Mk.1. Yet, my antique gauge had an aperiodic scale. Thus I had to come up with a calibration array that would map input values to the appropriate pulse width.

The Scale

Look at the aperiodic scale that my antique voltmeter came with: Volts as unit, manufacturer logo (Siemens-Halske) and down below it says  "aperiodic, DC".

TorrentMeter Mk. 2 - The Scale

I needed a linear scale from 0 through 100, saying Mbps, thus making it a TorrentMeter.   I chose the logo of Dingfabrik as manufacturer logo, since that is where @PylonC and I disassembled the voltmeter when it arrived in the mail. And I needed it to look as old as the original, so I stained it. To do so, I just took a clean baking tray, filled it with black tea and slowly shoved in a sheet of paper sideways. That's rather important to avoid having bubbles under the sheet, thus creating an even stain. I let it sit for a couple of minutes and let it dry in the oven at 50°C (120 F). Ready to be printed on! The scale was drawn with Inkscape, see repository on github for files.

Staining paper, Picard stlye. Tea, Earl Grey, hot!

By the time I was done with TorrentMeter Mk. 1 and thinking about how to make the scale of TorrentMeter Mk. 2, @saschaludwig contacted me. We live only about half an hour from each other. He helped me to draft the Mk. 2 scale and I showed him how the TorrentMeter was driven. He was working on his own version: Megabitmeter, a modern gauge with a laser cut and laser edged scale, qualified for series production. And boy, I can't wait for its release!

I do a lot of projects. I am around a lot of projects. I never talk about them. While elaborately and efficiently documenting my professional projects, I neglected to document my private ones.

I shall change that.