Re: [DIYbio] Re: spectralworkbench.org diy spectrophotometer

You could build a probe out of some plastic optical fiber. The laser goes into one bit of fiber, and exits into the sample. A millimeter from the exit is another fiber end, conducting the light up to the photosensor. You dunk the probe into the beaker of bugs and broth, and let a microcontroller monitor the optical density and look for logarithmic change, whereupon it beeps or lights an LED. You can run the laser (or a red LED) from different output pins on the microprocessor, each one giving you 10 times more light than the last one, through the use of different resistors attached to each one. A microcontroller like the Ti430 Launchpad (cost is $4.30 for the USB enabled development board) has a 10 bit analog to digital converter, and 16 output pins, giving you a possible accuracy of 26 bits, assuming you can control the brightness of the LED using the 16 bits and get all of the range. That gets you to OD 7, where 99.99999% of the light is blocked.

But if you only need OD 2 or 3, the 10 bit ADC and maybe three output pins will easily get you there. And it could send the data back to your laptop computer for graphing or recording by printing to the USB port in humanly readable ASCII.

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