Data Logging with SD Cards - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 306

LoggerHardware
All you need to do is to add an analog distance sensor to your existing setup.
If you're using the same setup as me, you do not even need a breadboard; just
connect the proper wires to power, ground, and A0 (for the analog signal output
from the sensor). With everything attached, it should look like Figure 13-22.
Figure 13-22: Entrance logger hardware
For this to actually work well, you want to mount the IR distance sensor and
Arduino on a wall so that the IR beam cuts horizontally across the door. This
way, anybody walking through the door must pass in front of the IR distance
sensor. Don't affix anything to your wall until you've written the software in
the next step and uploaded it. I suggest using easily removable painters tape to
hold it to your wall so that you don't damage anything. Once set up, the system
should look something like Figure 13-23.