Hackaday Prize Entry: A Simple Spectrophotometer

Building on the work of other Citizen Science efforts, [doctek]’s entry for the Hackaday Prize promises to detect pollution, identify chemicals, and perform other analyses with a simple handheld device. It’s a spectrophotometer, and [doctek] is putting some real engineering into this build.

A spectrophotometer is one of the simplest devices able to perform spectroscopy, requiring only a light source, a photoresistor, and some means of producing monochromatic light. By putting a sample in front of the photoresistor, the absorption spectrum of the sample can be measured. With this data, it’s a simple matter to identify the sample.

A light and a photoresistor are simple enough, but as with every precision measurement device, the devil is in the details. [doctec] is using new, low-noise, low-offset opamps, and precision references to get his data. Some of the parts in the schematic were actually designed in this century – a far cry from the ‘plug the photoresistor into the analog input’ projects we see so often.

[doctec] is using a Teensy 3.0 to drive the electronics and collect the data, and he already has the mechanics of this build pretty much figured out. It’s a great project that shows off some engineering skill, making it a great entry for The Hackaday Prize.

23 thoughts on “Hackaday Prize Entry: A Simple Spectrophotometer

  1. I would not use a white led for a light source, the spectral distribution is terrible for this application, huge peaks in blue. A small incandescent lamp would be a lot better. And I am pretty sure the repeatability of the RC servo will cause issues with calibration, better off using a small stepper.

      1. Those are expensive, hard-to-obtain beasties with lower efficiency than standard LEDs. I’m not sure how much of a future they have. The US is planning on imposing strict efficiency rules on LED lighting in a few years that would kill those off.

      1. really you want to get the rate of change of signal and max/min variation as low as possible. if you have to compensate by diddling the results because you are missing a chunk of spectrum your SNR goes out the window.

    1. Even if that were true for every LED does that really make an incandescent lamp the best choice? LEDs are predictable. Good LEDs have datasheets. If I were doing this I would choose a LED whose datasheet includes a nice detailed graph of output vs wavelength. I’m thinking this would be something from Mouser or DigiKey, not EBay. If you know where the peaks and valleys are can’t you account for them in software? Is such information available for any incandescent lamp? I’m guessing not but even if it is available how stable is it? What happens as the filament ages?

      1. LEDs are as predictable as fluorescent lamps. They rely on phosphors which degrade over time which makes the spectrum shift or relative intensities change. If LEDs were a suitable replacement commercial spectrometers would be using them instead of xenon arc or incandescent lamps.

        Incandescent lamps have a very well known output that is stable till they blow.

        1. “Incandescent lamps have a very well known output that is stable till they blow.”

          No, as incandescent filaments and the enclosure gas is contaminated with age, the emitted source spectrum will shift – significantly (depending on the application).

    2. Here is the paper on which I based the decision to use a white LED:
      “White LEDs as broad spectrum light sources for spectrophotometry: Demonstration in the visible spectrum range in a diode-array spectrophotometric detector” by Tomasz Piasecki1, Michael C. Breadmore, and Mirek Macka.

      The actual LED I’m using is Digikey 1125-1199-ND and the lens (for collimating) is 711-1323-ND. The performance of both is well-specified.

      All that said, I do not claim to be an expert on optics or spectrophotometry. This is very much of an experiment for me. I hope to end up with a useful instrument, but I also hope others, more qualified than I, can jump in and contribute.

  2. Nice build, and a useful device even if one can be acquired for 30 some dollars on ebay.

    For benchoff how is two razor blades taped to a box a far cry from analog photoresistors and analog inputs? The separation of the blades is an analogue input and is critical to the functioning of the device.

    1. Wait, $30!? I am only seeing around $500 for a complete lab sample bench-top unit.
      Where are you finding a Spectrophotometer that cheap?
      What are you using for search criteria?
      I am doing a planet rover project with my kids and this would be a great way to add more science sensors, bonus points if it can do a serial connection to an Arduino board. It would be for soil sample analysis in the sample bins along with the microscope camera; maybe on a sensor arm too.

      1. Just watch the auctions, a few days ago a shimadzu with a double cuvet receptacle (one for zeroing) and one for the sample just missing a power cord was up for 30 dollars. Even if they aren’t working it’s pretty typical for lab equipment. If you have ever worked in a lab at some point or the other you have had to fix an analytical machine.

  3. We did had an array of several LEDs at our “bandwidths of interest”
    some LEDs a 360 and,547nm were a bit hard to get hold of , they had really nice metal cans with quartz lenses :P

  4. Great use of an integrated lock-in amplifier there. It’s gross overkill for this application, but it’s not too expensive and probably saves some grief in getting the software filtering going properly. It’s nice to see integrated LIA solutions now: this used to be really expensive.

    The source-side optics of this are a significant limitation to the project though. The LED is physically very large and close to the “grating” (such as it is), so the angular spread of the input light is huge, which seriously degrades the spectral resolution. It would be MUCH better to use the LED without its optic. It will be even better to use collimation optics and an input slit too: a pair +3 lenses from dollar store reading glasses will improve both the spectral resolution and SNR of this device enormously.

    As previously mentioned, the spectral quality of the LED isn’t great. Forget about getting anything useful in the near infrared: there’s no source light there. If all you’re interested in is in the visible range, then it’s not a showstopper. The great honkin’ spectral notch in the blue-green can be corrected for. You could even say the lack of usability in the IR is GOOD, since you won’t be bothered by contamination from the second order light from the grating.

  5. I’d like to try a DIY build of a microcontrollified prism spectrometer covering about 700nm (visible red) down to 200nm (UV) wavelength.
    What type of photodiode would you recommend for this range? May I need multiple photodiodes to cover that range of wavelength?

  6. “… the absorption spectrum of the sample can be measured. With this data, it’s a simple matter to identify the sample.”
    UV/Vis region doesn’t have a lot of information in it so when you are using a compound that does feature the concentration can be measured with little interference. It’s used a lot in dye and transition metal chemistry and in biochemistry. If the compound isn’t known it won’t tell you what it is.

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