Megan - For a digital photo sensor, we like the DFRobot IR sensor, it is adjustable to trigger at distances from 3 to 80 cm. Runs on 5VDC for $7.50. Part number SEN0019, DFRobot.com. No affiliation.
We can give you one of ours if you like, so you can try it out. PM me or contact us through
team836@robobees.org.