Public Lab Research note


Homemade spectrometer

by david_uwi | September 30, 2016 12:35 | 1,293 views | 12 comments | #13512 | 1,293 views | 12 comments | #13512 30 Sep 12:35

Read more: publiclab.org/n/13512


380-930 nm resolution about 4 nm. Cost < 50GBP Fully self contained (battery powered). With color display (240x320) and SD card slot. Image shows spectrum of a didymium filter (wavelenght calibration) P1030132.jpg


12 Comments

Very nice, but more detail is needed.

stef

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It uses a TSL1402 diode array (I got mine from digikey) it has 256 pixels. A holgraphic diffraction grating (1000 lines) plastic lenses x2 (3 cm focal length) (both form Greenweld). Microcontroller PIC16F1788 the light source is a torch bulb. P1030133.jpg

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Here's a real spectrum which I stored on the SD card. It is of potassium permanganate. Because there is a very strong absorbance 500-580 nm and the sample is transparent elsewhere it is a tough spectrum to obtain experimentally due to random scattering off the diffraction grating. There is some internal compensation for the scattering effect but due to limited processing power (the 16F1788 has an 8-bit processor) the correction is far from perfect. It seems I can only upload JPG images so the spectrum has some JPG artifacts as well which were not present in the original GIF format.

PMAN256B.jpg

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Hi, this looks really cool! Are you interested in posting build instructions so others can follow your steps?

I'm impressed at the cost -- what's driving the display?

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The display uses SPI communication (it has an internal ILI9341 driver). The PIC16F1788 has a hardware SPI module. These type of TFT displays can be picked up for <£10 , but they refresh rather slowly due to the slow serial communication, but this is not a problem for this application. The processor was programmed using GCBasic. Apart from the programming it's fairly straightforward stuff. Bulb - slit - followed by a lens placed at 1 focal length should give a parallel beam then through the cuvette and onto the diffraction grating. Then a lens to focus the beam onto the diode array (approx 1 focal length between lens and diode array). Getting the SD card to work was really difficult as there is little (and conflicting) information on the internet.

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Hi @david_uwi -- i'd like to help you format your notes into instructions that someone else could follow, would this be of interest?

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Yes I have no problem with that. It is however more involved than other projects I see on this site. It needs a lot of fine soldering (maybe a PCB could be fabricated to make this easier). It also requires a microprocessor and the knowledge and equipment to program it (I can make my software available). And of course some optical alignment is needed to get the light to hit the diode array (preferably in focus) and give the required wavelength range.

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@david_uwi That's fine, some of our tools require more technical understanding; the wheestat also requires programming as well as soldering (to my understanding) but if it works, it works. Hopefully over time we as a community can simplify the build, or at the very least learn from it by knowing how its made!

Thank you so much for both posting this and joining our conversation!

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is this something that a Raspberry or Arduino could be use to substitute for the processor - I ask due to the following that both processors have. also there "shields" that can be used to where only minimal amount of work is required to have a working circuit.

would you have a schematic that you could post?

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Hi @david_uw, I am really impressed with your work :-) Very nice. I would like to build your spectrometer. Would you share your codes and assembling instructions ? Did you already considered the possibility to connect your device through Bluetooth in order to use smartphone screen ? This is the very first time I post here.. I hope I did it right. Thank you.

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Update: The diode array has gone from obsolete to unobtainable in a matter of a few months. I am testing an ILI1402 (made by ic Haus) which is supposed to be a direct replacement but without the small gap between the two 128 element dies, but it operates at 5V so I am evaluating various step-up converters. I am going to replace the colour display with a B/W reflective LCD as it will be much faster and will take much less current. I am also looking into using a flash memory chip to store the data then transfer out with a USB connection as the SD card has given nothing but problems. I am also tinkering with the optics to increase resolution and am probably going to add a UV LED along with incandescent bulb to improve the sensitivity below 400 nm.

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Finally got something working using the ic-Haus LFL1402 diode array. https://www.ichaus.de/iC-LFL1402 I normally use 3 V (2xAA bateries) so I had to use a step-up for the diode array (NCP1402 50T1). I am using a bigger display (3" 400x240) it is also faster as it uses parallel communication It was obtained from DX.com (open smart). Below is the test spectrum of didymium glass

spectrum_display.png and this is the menu screen

menu_display.png

Notice the green asterix by the USB transfer indicating that the USB cable is attached. I have decided to use usb rather than SD cards this time (as SD cards turned out to be very difficult due to the number of different types). The spectrometer can store up to 24 spectra in flash which can be transfered by the usb link (using a FT201X I2C to USB chip). The optics are a bit more involved as there is collimation onto the sample holder, followed by focusing onto a slit then further collimation onto the diffraction grating then re-focused onto the diode array. It gets much better throughput, but is very tricky to allign. IMG_0636a.jpg

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