Instructables
Picture of Simple Arduino Laserharp
Here I will describe a way to make a simple Arduino laserharp, a musical instrument that plays music when you block a beam of laser light from hitting a phototransistor. This project was done as a part of the PHYC 308L class at the University of New Mexico, and my detailed notes outlining my process can be found on my Open Science Notebook.

The final product is pictured above.

 
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Step 1: Parts

Picture of Parts
The parts that you will need for this project are:

Arduino Uno                                                                   $34.99 from Radioshack
Breadboard                                                                    $8.99 from Radioshack
Laserpointer                                                                   $6.99 from Amazon
Diffraction Grating                                                         $8.00 from Amazon
Speaker                                                                          $3.59 from Radioshack
100  resistor
4.7 k resistors (3x)
NTE3034 Phototransistor Detector (3x)                  about $2.50 each
Some cardboard and tape

Step 2: Circuit Assembly

Picture of Circuit Assembly
Assembled the circuit as shown below by connecting it to the Arduino.

Step 3: Construction

Picture of Construction
Next, carefully align the laser with the diffraction grating and the phototransistors. To do this, cut pieces of cardboard to lift the laser pointer to the appropriate level, and tape the diffraction grating to one side. Then adjust its location, and the locations of the phototransistors until the beam strikes each of the phototransistors. At this point, I tape everything into place on the table so that it would not be easily knocked around. The final set up is pictured above.