Pulse Pal

Pulse Pal is an open and inexpensive (~$210) alternative to pulse generators used in neurophysiology research, and is most often used to create precisely timed light trains in optogenetics assays. Pulse Pal generates four channels of configurable square pulse trains ranging in voltage from +10 to -10V using a bipolar DAC. Two digital trigger channels can be used to start and stop playback. APIs are available in C++, Python and MATLAB, and the hardware designs and firmware are fully open source.

Be sure to check the paper about it and their wiki page.