Public Lab has developed multiple open source Do-It-Yourself spectrometer designs. Choose between the following:

Desktop Spectrometry Kit - a precise and inexpensive webcam-based design built in a conduit box

Foldable Mini-spectrometer - a small paper model which attaches to a smartphone

Both can be made from simple, easy to find materials for very little money. You can also purchase a kit to get started faster at the Public Lab Store

Our third kit uses injection molded plastic parts, which are not as easy to make yourself. But it is more robust and precise than our other devices:

Smartphone Spectrometry Kit - a rigid plastic spectrometer attachment for your smartphone -- robust and very high precision