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# CaptainKirk

Member Since: August 31, 2008

Country: United States

• For what it's worth, I've been using this with a Pixel2 from Rabid Prototypes, it's very impressive.

One thing that others might find helpful is the ability to measure the actual wavelength of an LED or what have you, even if it falls between the channel bins. This worked well for me:

First, you need to be sure that there is a single dominant wavelength. This can be done by comparing the maximum value of any channel, and ascertaining that there is no more than one other value greater than about 10% of that, and that it is from a channel adjacent to the maximal channel. We'll base the wavelength on the ratio of these two readings (or just the one reading if it falls at the center of a channel)

The relative response of each channel can be approximated as a Gaussian function with the following parameters: (curve fitted from spec sheet)

response = exp(-1 • (nm - center)^2 / (2 • 9^2))

The spacing (delta) between channels varies between 25 and 50 nm. This method will only work with spacing less than about 30, maybe 35 nm as there needs to be some response overlap. Fortunately this covers the visible spectrum pretty well.

Save the readings in an array as follows:

Denote the bin number with the maximum reading as peakBin.

We also use an array storing the actual bin frequencies:

frequency[0] = 760 frequency[1] = 730 frequency[2] = 705 ...

If the secondary peak wavelength is above (shorter wavelength) than the dominant peak, c = 1, otherwise c = -1.

Finally, note the nm spacing between these two bins and calculate the following two values

a = 0.0026808 • delta^2 b = 186.51 / delta

These were determined by plotting the ratios of calculated channel response with different spacing and curve fitting the result.

Now, given appropriate readings (one or two adjacent channels reading much higher than all the others) we can approximate the actual wavelength:

wavelength = frequency[peakBin] - c * (log10(spectrum[peakBin + c] / spectrum[peakBin]) + a) * b

Of course the accuracy is limited by the tolerance of the bin centers (±10 nm).

I used this to measure a low pressure sodium vapor lamp and read 589 nm, dead on the D-line doublet.

I also measured a series of LEDs with the following results

Min Measured Max 730 742 740 nm 650 657 660 nm 620 629 630 nm 600 599 610 nm 585 595 595 nm 520 521 530 nm 495 498 500 nm 460 472 470 nm 455 452 460 nm 445 449 450 nm 420 426 430 nm

I hope this is helpful to someone.

• Awesome! I'm making a little analyzer with this and a Pixel 2 from Rabid Prototypes. Data looks much better now, thanks!

• When I tried checking out the triad, I get the data from only the AS72652 chip repeated three times. Blocking out the other two sensors doesn’t change anything, but blocking the x52 chip makes all the readings go to zero.

And if you look at your Banana Scan data in Google docs, the columns of data for 435, 585, and 760 nm are identical, down to the hundredth place! The only columns that don’t follow this pattern have 7921.65 in one place and 921.65 in another, my guess is a typo.

This is really cool hardware, but I think this is a bug somewhere...

• It's a peak detector, along with the capacitors and the 100k resistor. The output will generally follow the amplitude envelop of the audio signal.

• For anyone who's interested, I made a laser cut acrylic case for mine and uploaded the design to Dropbox. Not waterproof or anything fancy, it just made it easier for me to mount on things (I use it for time-lapse videos mostly).

It's a stack-up of 5 layers. In the DXF file, the two mostly solid pieces on the right are the top and bottom, thickness can be whatever you want, I used 2 mm plexiglas. The pieces on the left are 3 mm plexiglas. Cut green first, red second and blue last. It should be fairly self-evident how to assemble, but a couple things are tricky.

Start with the base, put three 2-56 5/16 inch screws in the holes that will line up with the holes on the HackHD board.

Place three of the acrylic cut washers over the three HackHD mounting screws. (There's a spare...)

On top of the base, place the piece with the full HackHD outlined hole.

Next position the HackHD camera. I found the holes to be pretty tight. Put nuts on the screws and gently tighten. I had to grind one side of the nut next to the connector to fit.

Next place the piece with the two hexagonal holes. Place a couple of 3 mm nuts in the holes. (Sorry for mixing metric and English, it's what I had on hand.)

Next the similar piece that doesn't have the hexagonal holes, and finally the top.

Insert three 5/8 inch 2-56 screws in the other 2 mm diameter holes from the rear and gently tighten with nuts.

The remaining piece can fit over the screw heads on the back, and with a couple of 3 mm screws of appropriate length the assemble can be mounted on a bracket or what have you. The spacing of these screws is 40 mm.

• Tantalums can also explode with high dI/dt, even if the voltage rating is never approached. Battery power can be especially bad for that. Happened on a product I was working on, fortunately we had some prototypes go and redesigned it before it was released. Here's a paper from AVX with plenty of information... http://www.avxtantalum.com/pdf/SURGTANT.PDF

• I've done this with a different brand of the same stuff. I didn't get super details like alginate or plaster of paris would give, but it was much easier, and it was definitely recognizable as a face.

Heat it up, roll it out to about 1/8 inch thick and about 10 inches in diameter, lean back and gently mold it to your face like a big pancake. Let it cool. Cold water helps if you're up to that.

You might want to make a small hole over the mouth area, but I found the fit loose enough that wasn't necessary. And of course in an emergency you can just peel it off and start over.

• Point well taken. I mostly speaking tonguke in cheek, and thinking back to Polaroid's PolaPulse batteries. They were 6 volt flat battery packs designed to be built into film packages. Having a large surface area, they could put out a lot of current, and stories circulated that two in series could start a car.
For these batteries, they should be pretty rugged, but you're right, don't try this unless it's the only way to start your truck while you're fleeing the zombie hordes.

• Yes it would. Hmm, two in series might be able to jump-start a car.

• Without searching through all the other batteries, there are various reasons why the energy density varies. The batteries here can sustain a 30C discharge. Typical Lithium batteries are rated at far lower discharge rates, on the order of 1/2C to 2C. There is a compromise in order to do that safely, and energy density usually takes a hit.

No public wish lists :(

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