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At the end of my light sensing instructable where I played with the measurement of reducing sun light levels as well as making it into a laser game , I mentioned that I wanted to have a go at making a full spectrometer. This instructable is a step closer.
In the instructable I said I wanted to add a diode array, unfortunately I have not been able to get hold of one, so this will be a scanning spectrometer.
Diode array spectrometers
These work generally by having no moving parts. The light passes through the sample and is split into its separate spectral components using a prism or similar device. The separate wavelengths then fall onto the detector where each absorbance value is measured at the same time.
These work slightly differently in that the light is first split by the prism, it the passes through a narrow slit which helps to reduce the number of wavelengths of light that can fall on the sensor after it has passed through the sample.
In this type of system there is a single detector that has to measure each value in turn and because of this the light splitting device has to rotate to direct the sequence of wavelengths onto the sample and so onto the detector.
The spectrometer in this guide will use a diffraction grating a hologram printed onto a price of glass, I got this from work (a spare from a service visit).
If you don't have access to a gratting, you can make one by cutting a piece of a DVD.
HOW Usefull is this instrument
The usefullness of this instrument is limited only by your imagination examples are as follows:
The above and many more would of course all require calibration of the equipment
|Ten||Light Emitting Diodes|
|One||Light Dependent Resistor 10 K Ohm up to approximately 130 K Ohm|
Batteries : The light source uses 'AAA' x 2 for the LED's you can also make the unit portable by powering the Arduino using a 9 volt battery and of course if you do go portable you will need a battery in the laptop you use to collect your data
The light source for the spectrometer is a battery powered head inspection lamp - I got a cheap one to canablise from the pound shop
The Arduino is an UNO, the LED's are low values and the Pot is one I had laying around.
The LDR is one salvaged from my Cybot which was modified in a previous guide see
I found a good site for calculating the values for resistors - see the first picture on this guide, the link to which is just below here. I used 33 ohm resistors - these where considerably smaller than my first attempt using some a rescued from an old alarm system - they where way too big leaving me wondering why nothing worked when I switched the "finished thing on"?.
There are two bits of software and one add on required for this project to work:
The Arduino IDE is free to download from the following location and comes as either an install of zip file - I like the Zip (easier to use and and be run from a flash drive for portability)
MICROSOFT EXCEL - unfortunately not free - I have office 2013 on a windows 7 computer
Finally the most important bit is the add on - this is for Excel and comes from a site called PRALAX
The file you want is PLX-DAQ and runs as a macro from inside Excel. This is a great little file as it can be used to monitor any serial port and gather the resultant data sent through it. All that is required is some specific lines of code in the Arduino sketch to tell it the send data to specific cells in Excel.
You also need to specify the port speed and ensure that the Arduino is set to the same speed - initially I tried 9600, but although this worked for the test spreadsheet that only sent text to excel from the Arduino, I needed to set it to 128000 for the final project as I would be monitoring the response times for the LED's dependent upon the resistance of the LDR at any given time.