This standalone sensor allows to measure the optical density (OD) aka turbidity through a standardized cuvette. 1-Channel for different wavelengths use multiple sensors with different 5mm LEDs. The design fits nicely a TIL-99 phototransistor.
see more on: http://hackteria.org/?p=2194
The layers were made using 3mm MDF and laser cut. The settings for rastering need to be optimized to have the right thickness to perfectly fit the LED in between. Same for the depth of the pin-header row.
The sensor can be read using an arduino or a BabyMidiGnusbuino. Various tests have been run on the BabyMidiGnusbuino (a 2$ variant arduino compatible a85 platform), using pure data and MIDI to talk to the sensor. Especially Kafi-Schnapps is very useful for calibration and getting the detection limits of the sensor.
see details on: http://hackteria.org/wiki/index.php/DIY_spectrophotometer