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What's all this?

This instructable will explain how to build a fairly basic but working spectrophotometer out of easily sourceable parts.

Of course, this device is nothing compared to a commercial spectrophotometer, but it will allow the builder to understand how such a device works.


Huh? What are you talking about?!

A spectrophotometer measures the amount of light that a certain sample absorbs. 

The basic principle:
  • A beam of light passes through a prism or diffraction grating.
  • Out of the resulting spectrum, a certain (range of) wavelength(s) gets selected by sending the light through a slit.
  • The light passes through the sample and hits a detector.

This way, one can build a "spectrometric fingerprint" of a certain sample that will depend on the molecules in the sample.


Cool, let's do it!

I'd once again like to emphasize that this device is certainly not to be compared to a commercial system, but if you're looking for a fun project that will enable you to learn something more about the workings of a spectrophotometer, this project is for you!

Step 1: Sourcing the Parts

Most parts are fairly easy to obtain.

You will need:

For the stepper circuit:
  • A stepper motor
  • an Arduino (or a stepper driver circuit)
  • 2x 10k resistor
  • 2x 1k resistor
  • L293D H-Bridge
  • 2x NPN transistor (I used the BC547A)
  • A power supply

For the detection and amplification circuit:
  • an op-amp (e.g. TL081)
  • LDR
  • 1x 2k2 resistor
  • 1x 4k7 resistor
  • 1x LED

General parts
  • A base (some kind of board such as a cutting board to mount everything on)
  • Cuvette
  • Diffraction grating, CD or DVD
  • Lightbulb and / or white LED
  • Metal or cardboard plaques
  • Some connectors
  • Some pushbuttons
I would suggest that you use a dc motor rather than a stepper for a more continuous rendering of the test samples spectrum. Limit switches can be used to control its min and max movements.<br> Also running the motor at a slow speed may help better detect narrow peaks.<br> May be adding another amplifier stage would also increase the instruments sensitivity.<br> One can use the sun as a light source to get certain known spectral lines ie.,ne, na, h, for calibration. Or use the sodium lines from phoresece lights, or those from neon lights. Thanks for you constructable. &nbsp; bob
You make some very good points, thanks! I'll look into it.
<p>I have to do more testing but this is quite simple too.</p>
<p>I have built a simple sample passive spectrometer. Using a camper cooler, light meter, lamp with aluminum foil with pinhole. Here are some pictures.</p>
<p>Check out my version! I referenced you in the intro and the closing paragraphs of my instructable called &quot;Shoe Box Spectrophotometer&quot;. </p>
Nice work! Glad I was able to inspire you :)<br><br><br>
<p>Could white light be shone through the sample, and then split into the individual wavelengths? Wouldn't the absorption be the same? This would make the testing much faster and eliminate the need for a motor. Or would it be too hard to calibrate a baseline for each wavelength?</p>
<p>This design would require a photodiode array or linear CCD. Is that the reason why it is not used here? </p>
<p>Why would you not use an RGB LED instead of the stepper motor and diffraction grating? It would be solid state and more controllable.</p>
<p>No LED is a suitable light source for spectrophotometry. LEDs don't have continuous and flat emission spectra. Furthermore, your spectral resolution would only be three.</p>
<p>Couldn't you calibrate with known dye standards?</p>
Hi &amp; hello i am need your help for my project <br>My project is quite simple cause its no need of arduino, but im using op-amp to produce output. <br>1.Is it my circuit for amplification is same as you? And please give me some tips! <br>2. How to measure absorbance (uric acid) using multimeter. If u had any links for my reference, thank you very much <br>
Any suggestion for dark and thick liquids? (if light has problem to pass through the cuevette)
Not all samples are suitable for this type of analysis. But, you can try diluting your liquid by a specific volume. You'll need to take into account solvent miscibility if you don't have or know what the liquid is, probably make a calibration curve, and make an appropriate blank. These are pretty basic techniques of spectrophotometry, so search any unfamiliar terms.
<p>The good thing about dark liquids is you can still use beer's law to calculate concentration and molality and molarity from a sample with dilution.</p>
Thanks a ton. It helped a lot :)
<p>good afternoon;</p><p>Wanted to know if you could help me get me some material type article, slides, application to more site to compare spectra and gives something theoretical. Em spectrophotometer in cell</p><p>Thank you for your attention</p>
<p>good afternoon;</p><p>Wanted to know if you could help me get me some material type article, slides, application to more site to compare spectra and gives something theoretical. </p><p>Thank you for your attention</p>
Can you please explain in detail about TL081?? <br>How will the it affect the output in that case? Is the output amplified by the amplifier..like 5V to 6 or 7 volts?? <br>Kindly Reply <br>Thanks <br>
<p>Sorry for the late reply. Simply put, yes. Take a look at http://en.wikipedia.org/wiki/Operational_amplifier for more info.</p>
<p>I studied Near-IR spectroscopy years ago, and this project is near and dear to my heart. I am a proponent of using the stepper motor. It moves in discrete, reproducible angle steps. If those angles are too great, you would have to use gears to reduce the angles of the steps to the desired amount. Also, clear plastic transmission grating would be good for this project. For very dark liquids, either dilute or decrease the optical path length through the sample (use a thinner cuvette). For calibration would need neon, sodium, or other types of discharge lamps with known wavelengths - or tube type HeNe laser, or some combination thereof. The more standard wavelengths you have, the better you can tweak your calibration.</p>
Thanks for the comment :) <br>I thought the stepper motor would be the simplest way of achieving wavelength reproducibility.<br><br>Unfortunately due to lack of time I never completely finished the project. Maybe somewhere in the far future...
How do I measure the wavelength ?
You'll need to find an absorbance maximum at more than one angle of rotation for known samples that have only one absorbance peak, and use those values of theta to determine the relationship between degree of rotation and wavelength of light.
i want to download the pdf for the same. I tried to register. Not happening. <br>Any suggestion?
how do you calibrate the stepper motor to the wavelength?
Great!!! Simply amazing
Love this project, but why not read the ldr with the arduino, and get it to send the data back to the pc ? That would make it fully automated.
Very good point! <br><br>I haven't had the time so far to improve this project but I'll put it on my to-do list for the (far) future!
You could explore the possibility of using a RGB led as light source (Cost ~ $2.00). By digitally selecting the combination and the strenght of the three fundamental colors you could generate almost a continuous spectrum without the complication of a mechanical device like the motor.
great project!
Thanks!
Great instructable. There are inexpensive diffraction grattings at Edmund Scientific. Not like the ones we use in the laboratory instrumsntation, but quite fair for this kind of project.
Very nice idea. <br>But is a piece of CD the best way of diffusing the light, because the spectrum created isn't perfect...
No, like I mention in step 3 / &quot;splitting the light&quot; a CD isn't ideal. A proper diffraction grating would be better, but is harder to find and much more expensive.

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