This week I finally received all the parts I ordered and was able to build and test the sensor. Below is a picture of the photodiode circuit.

It works quite well. Under blue illumination the photodiode creates a slight voltage. However, under red illumination the increase in voltage is 10 times more then under blue illumination. The use of a red filter may not be necessary because the red LEDs are not sensitive to blue light.

            Next I need to start testing the response of the LEDs to dim red light, like that produced by fluorescence. This will tell me how many red LEDs I should use in parallel (I am thinking four), and how much resistance to put in the above circuit.

            Lastly, I’ll be taking a trip to home depot soon to look for a PVC housing. I need something that can be hold the Arduino and the LEDs. Acrylic can be glued to the PVC to provide a clear window for light transfer. Another benefit of using PVC is that heating it in the oven makes it very soft. So I can reshape PVC tubes if need be.

I also received the data logger and SD card this week. It took a lot of time to figure out how the data logger works, but I now have it writing values to a text file.