Dual optical interrupter switch (has two channels--very unusual and
hard to find!), with low profile. Great for use with small mouse encoder
discs. With some careful alignment of the disk in the slot, you can
build yourself a very inexpensive quadrate encoder that will be the
envy of friends and family.
Comes with basic datasheet info so you know what lead goes where. Add
an inexpensive optical odometer encoder to your robot.