SharkFin

Here is the schematic for a new helmet mounted break light I've been working on. The idea is to measure acceleration and rotation enough to detect breaking from any head angle. Working on the filter now. The prototype is based on the Boarduino from ladyada.net with two breakout boards available from pololu.com.

Improved brake light

So the brake light was acting a little flaky, turning on over bumps at random. Before, I was using a moving average filter, with ad-hoc checks. I decided to try out a IIR filter because they can produce good results with few taps. This is my first implementation of such a filter.

Infinite impulse response or IIR filters combine measured (feed forward) data and filtered (feed back) data. They get their name because a single non-zero input sample followed and preceded by all zeros can cause the filter to oscillate indefinitely as opposed to a finite impulse response or FIR filter which will return to zero after at most N measurements where N is the number of filter coefficients.

The problem with the brake light was that high frequency jitters were tripping the braking threshold. The only information I really car about is sustained braking, which is low frequency information. I needed a low pass filter, a filter that passes low frequency information and attenuates high frequency information. The Butterworth filter is a low pass IIR filter that is working great for this application.

The filter coefficients are determined by the sample interval, the cutoff frequency, and the number of filter coefficients to use. I chose 2 Hz to be the cutoff frequency. Any lower frequency would delay brake indication, any higher frequency could let in unwanted jitter. I wanted to keep the number of filter coefficients small so that the micro could keep up, It is having no trouble with that; I could probably get away with more filter coefficients, but that delays response time as well.

I was pleased to see how well this filter performed!
I captured this acceleration data riding up and down the street without braking and with. In each image, the raw data is pictured in blue, the filtered data is in yellow, and braking threshold (on the top plot) is indicated in red. As you can see, the raw data is very noisy. The noise is actually bumps on the side of the road, so don't blame the pololu 3-axis accelometer ($17.95!!) breakout I'm using. I purposely aimed for them to make sure the threshold was not set to high which could set of the brake lights every time you hit a bump. With the unfiltered data, it is impossible to set a consistent threshold for turning on the brake lights. The filtered data works though!

The above image just was captured with out braking. I just rode up and down the street without touching the brakes. The threshold was set near the minimum filtered acceleration in the X direction.

This data was collected getting up some speed and braking to various degrees. The X direction is in the velocity direction. Deceleration is indicated by lower X values. You can see the braking go below the threshold even for mild braking.

The Y and Z axis will be used for a future helmet mounted version.

Here is a snap of my data capture setup, thanks to the wave shield from ladyada, and battery and dual proto board from liquidware.com








Here are the filter coefficients, they should work with any IIR filter implantation. The complete BikeLight5 code can be found here.

// feed forward coefficients
double ff[] = {
1.93545410516e-06,
5.80636231547e-06,
5.80636231547e-06,
1.93545410516e-06
};
// feed back
double fb[] = {
1.0,
-2.94973583971,
2.90072698836,
-0.950975665016
};

Arduino on a bread board.

I've got a couple of ideas for AVR chips freed from the arduino board. Below is my scratch built* arduino. Thanks to LadyAda for posting the schematic for the BoardDuino. Schematic to come.



* By "scratch built" I mean, assembled from the contents of the boardduino kit.

Reuben Margolin

Watch this video of a Genius at work.
http://blog.makezine.com/archive/2009/01/maker_profile_kinetic_wave_sculptur.html

Wave Shield almost there.

Lady Ada pointed out R1 was missing, now the SD works! The D2A seems to be the next hurtle.

Arduino Wave Shield HELP!

I am having trouble with the arduino wave shield by ladyada. I really only need to get the SD card reader up and running. After assembling and programming I get the error messages:

-- Card init. failed! (most of the time)
-- Couldn't open filesys (once in a while).

The 2 GB card is formated as FAT16, with 8.3 filenames. Any suggestions welcome.

MeggyJr Build.

Evil Mad Scientists

Elizabeth and I wanted an excuse to try out the sweet MeggyJr kit from the Evil Mad Scientists Labrotories (EMSL). She has invented a game called Light and Heavy that MeggyJr will be perfect for.

The kit is fantastic, by far the best electronics kit I have ever built. Extreme care has been taken to maximize your probability of a successful build. The detailed color instructions are included with the kit so you won't have to print those out when the kit arrives. A careful read through before you start might save you hours in the long run, so just do it even if you think you already "get it". Every couple of steps, EMSL include full color pictures of the board so far, which adds reassurance.

If you happen to miss something in the instructions, additional hints are stencilled on the board itself "J1 Other Side" for instance.

The parts are easy to find in ZipLock bags with the step number clearly printed on the outside. And the parts themselves are well chosen and of high quality and should stand up to some hard gaming.

For future designs, I'd like to see sockets for the ICs, they get soldered directly to the board. Also, the way the screen sits the electrolytic capacitor is a bit awkward. They were obviously trying to keep MeggyJr as thin as possible.

When we powered her up for the first time, MeggyJr loaded Attack of the Killer Tomotoes, but the blue LEDs on a single row did not function. Careful inspection of the LED headers revealed scant solder on a couple of the pins which was easily remedied.

That said, how could I have possibly soldered the TTL programming interface in the wrong way? Anyway I did, and to make matters worse, my iron got switched off about the time I discovered the renegade part. So I ripped out all of the vias (through hole copper) along with the part (I know).

So I'm disappointed, but know EMSL were kind enough to provide two programming methods. So I figure, I'll test out the ISP programming header first. This works! and we can compile games in arduino and burn the games using avrdude by hand. As a fall back, it's great.
(NOTE FROM EMSL: First, for the record of folks reading this in the future: DO NOT go deleting the bootloader just because you don't know what else to do.)

So, relieved that all is not lost, I return my attention to the TTL header. Testing the connections reveals that all but pin 5 are not functional. I add jumpers based on the most excellent schematic EMSL have provided.


Now that all connections are in order, I need to replace the bootloader on the processor. I had no idea this would be so difficult! One issue is that the arduino IDE does not automatically work with mega-isp, an awesome adruino script that allows your arduino to operate as an avrisp, and the only way I have to upload the bootloader. It took a while to get the right combination of lock and fuse bits but I finally got avrdude to do the dead. Elizabeth and I literally jumped up and down when we loaded MeggyJr_FroggyJr via the TTL port. Here is the script that finally worked.

#!/bin/sh
OPTS="-c avrisp -p m168 -P /dev/ttyUSB0 -b 19200"
avrdude $OPTS -U efuse:w:0x00:m -U hfuse:w:0xdd:m -U lfuse:w:0xff:m
avrdude $OPTS -U lock:w:0x3f:m
avrdude $OPTS -U flash:w:ATmegaBOOT_168_diecimila.hex:i
avrdude $OPTS -U lock:w:0x0f:m # this was the culprit

COMPLETE!

Let me know if you have any questions!

Bikelight Schematic

Here is a sketch of the schematic. It is composed of three pieces: the SPI interface, the accelerometer circuit and the turn signal circuit. The pin names used in the code are indicated on the right.

Amy's Bikelight

Stay tuned. The brake light accelerometer was hindered by a bug in the arduino pulseIn code, which has been fixed. I also added LED feedback for the turn signal switch.

Emily has taken a couple of videos, that we will post soon.

Amy's New Light Arduino Source Code

The code for Amy's bikelight can be downloaded here.

Amy's Bike Light Installed

Here it is! Installed.

Amy's new Bike Light!

This bike light is made from an arduino, and two LED matrices from SparkFun. An accelerometer controls the break lights and a switch controls the turn signals. More details and a schematic later.

Cycling Shoe Covers

Emily's feet got cold on our ride the other day so I whipped these together for her. I have to make sure she is ready for the "Cycle of Change" ride for Obama's inauguration. The top is a windproof stretchable fleece. The straps are from an old helmet. They worked great!

arduino Battery Charger

I was a bit disappointed when I found out my new $150 bike light could not charge itself. Apparently you are supposed to unplug it when it gets warm to the touch, but not to leave it plugged into long because this will ruin the battery! This seemed like a problem for the arduino!

The plan was to use the power from charger itself to power the arduino. This caused a voltage drop of 3 volts, meaning longer charging times. Since I need this daily for my commute, this would not do.

The next version used two power supplies, one for the arduino, one for the charger. This is a bit ugly, but works fine. Both wall warts are plugged into a three way to make it easier to unplug.

The real hurdle was controlling the higher charging voltage with the 3V arduino. The opto-isolator I used initially would not allow enough current to charge the battery. Next was a home-brew isolator that consisted of a photo-resistor and a led taped together. Still not enough current. I finally picked up a 1 A relay from RadioShack. This worked!

The final design uses two LM35 temperature sensors I found on ebay, to compare ambient temperature to the battery temperature. When the temperature delta exceeds 15 degrees F for 10 minutes the charge is considered complete. If at any time the battery temperature exceeds 95 degrees F, power is removed from the battery and no restart occurs.

Here is the schematic and source code. Be sure to wire up the LM35 temperature sensors according to their documentation; if they heat up you have it backwards!



Source Code

Buckyball Adjacency Matrix

Needed this to create a problem on http://puzzlerace.appspot.com/ and couldn't find it on the web. Here it is (it's all there just copy and paste to your editor).

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300 MPG Aptera Hybrid

I need this car! Check out this Popular Mechanics review: .

Quantum Erasure

Please have a look at Emily's slides from the Kids Inquiry Conference. SensL has offered to loan us some single photon detection sensors for our next experiment!

Results

Two Fives

1. Find the length of the hypotenuse of a the triangle. (1 Point)

Instant Warnings

Perhaps it is possible to create an instant global (or national) warning network, in case of pending doom (think tsunami). Messages would initiate from a trusted source and would propogate much like an email virus but with source authentication at each transfer. A client program would monitor a designated socket with low duty cycle, when a warning is first received and authenticated, it would log the receipt and propogate the message forward. Subsequent receipts of the same warning would be ignored.

If OLPC is successful, then this platform could be used to support this warning network.