Arduino, a LED, and two photodiodes

Greetings, I’m a chemistry teacher and would like to build a sensor for students to do measurements on fluorescent compounds.

While I have planned on learning more about Arduino I have not gotten around to doing it yet, so I am not completely certain that Arduino can help me with this project. I hope somebody here can give me some pointers.

The outline of what I want is a follows: A diode for exciting the fluorescent compounds, and two photodiodes for measuring two different fluorescent compounds (two different wavelengths). One compound changes its fluorescence based on pH and the other doesn’t, and I ultimately need to measure the ratio between their fluorescence.

I am planning on using a 20mA 3.2V LED to excite the compounds, and using optical filters to adjust what wavelengths reach the photodiodes. Once everything is together it would be ideal if the numbers could be sent live to a computer via USB, but right now I need to know that I can make the sensor electronics work.

I have tried to read up on light sensors, and my impression is that I need to use photodiodes as the light intensities won’t be very high – the highest intensity I home to measure would be visible to the naked eye if there are no other light sources, but not much more than that. My first problem is picking the photodiodes, I was thinking something like:

http://www.mouser.dk/Search/ProductDetail.aspx?R=SFH_2430-Z

It is flat (makes it easy to place optical filters on it) and has a peak wavelength placed nicely in the middle of the visible spectrum. I assume I need some kind of amplification circuit, but I am not much of an electronics guy.

So my questions are as follows:

  • Can I have an Arduino run a LED and record signals from two photodiodes at the same time? A time resolution of about 1 second would be acceptable, but 10-100 ms would be neat. I assume this is doable.

  • Will the photodiode I found work well with Arduino, or can anyone suggest a replacement?

  • What kind of amplification circuit would work well with my photodiode (or its replacement)? I assume that I need an amplification circuit for each photodiode.

Ideally the circuit should be somewhat inexpensive, as the cheaper it is the more units we can afford and the more students can get one. It may be basic questions, but I’m hoping someone can give me some pointers so I can put something together for the students and focus on learning the Arduino elements relevant to this project first.

  1. That’s indeed doable.

  2. That’s where the trouble starts. Leds emit a very narrow band of wavelenghts and you should check if the compounds get excited by that at all. And kind of the same for photodiodes. The peak might be in the middle but it might also be very narrow. That SFH 2430-Z for example isn’t very sensitive in the low end (blue end). So do you know what wavelengths excite the compounds and what they emit?

  3. And I think a simple opamp circuit can do the amplification :slight_smile: This was the first I could find.

http://www.ti.com/lit/an/sboa061/sboa061.pdf

Google on photodiode amplifier will find many examples.

You will need to experiment with your apparatus to find the optimum gain though.

EDIT

If it is pureley the ratio of the two measurements you need, feeding the outputs of the two photodiode amplifiers into a differential amplifier may be a good idea.

You can also get devices like this. http://docs-europe.electrocomponents.com/webdocs/0892/0900766b80892a99.pdf

Where the amplifier is built in and can be read directly by the arduino a to d convertor.

Many thanks for the replies, sorry for the slow response - the Light-to-Voltage Converter looks like a good fit, seems like it will simplify my circuit which is much appreciated with me being an Arduino novice!

I will order some different parts and see what I can come up with.

Look for digital light sensor, BH1750, ebay and other sell them build as a small module. Using such sensor offers 16-bits resolution (arduino has only 10) and they factory calibrated.

Find the physics department or physics teacher and ask them to teach you about wavelengths and how light behaves.

As far as this fluorescent litmus getup, what's wrong with having a numbered scale and having students qualitatively pick which the result resembles just like plain ol paper litmus strips? I don't understand the need for trying to meter something so sloppy. Adding unnecessary complexity that distracts instead of enhances.

A diode for exciting the fluorescent compounds, and two photodiodes for measuring two different fluorescent compounds (two different wavelengths). One compound changes its fluorescence based on pH and the other doesn't, and I ultimately need to measure the ratio between their fluorescence.

What [u]are[/u] the excitation and emission wavelength maxima of the compounds? Please post the compound names and/or images of the spectra if you have them.

What do you mean by "changes its fluorescence"? Is the fluorescence quenched or does the excitation or emission maximum shift?