  Single-molecule sensitive microscopies and spectroscopies are transforming
biophysics and materials science laboratories. Techniques such as fluorescence
correlation spectroscopy (FCS) and single-molecule sensitive fluorescence
resonance energy transfer (FRET) are now commonly available in research
laboratories but are as yet infrequently available in teaching laboratories. We
describe inexpensive electronics and open-source software that bridges this
gap, making state-of-the-art measurement research capabilities accessible to
undergraduates interested in biophysics. We include a pedagogical discussion of
the intensity correlation function relevant to FCS and its calculation directly
from photon arrival times. We demonstrate the system with a measurement of the
hydrodynamic radius of a protein using FCS that is suitable for an
undergraduate teaching laboratory. The FPGA-based electronics, which are easy
to construct, are suitable for more advanced measurements as well, and several
applications are demonstrated. As implemented, the system has 8 ns timing
resolution, outputs to control up to four laser sources, and inputs for as many
as four photon-counting detectors.
