Alright, I just got in the HC-SR04.  And, as I feared, I'm not getting enough range.  I used the example you package with your NewPing library ("NewPingExample") with MAX_DISTANCE changed to 5000 and an if statement so that it'd only display the range if it was greater than the range that was displayed before (so I didn't just have tons of data constantly shooting across the serial monitor).  The max range I got (and only if I stood still in front of the sensor so that it could measure the range several times) was about 150cm.  I need the sensor to go off with a person walking toward it at normal (or slightly slower than normal) walking speed at about twice that range.
So should I try a different sensor?  Do I need an amplifier?

Thanks.

Does anyone have any experience they'd like to share before I buy the MB1040 LV-MaxSonar-EZ4, then?  From what I can tell it supposedly detects large objects (hopefully a person is large enough) up to 6.45m away over a width of about 0.6m from the center of the beam.  Does this seem about right?

I'm looking for a distance sensor that can reliably measure if someone is within about 10 ft of it.  A digital proximity sensor might work too, if I can pretty reliably change its max distance by playing with the input voltage.  Cheap is the name of the game here, but it does need to work.  I've been looking at the HC-SR04, but a couple of commenters said it only worked for them up to ~60 cm, while others said it worked up and beyond 10ft.  Any suggestions?  -- I'm looking at ultrasound because I will be using it at school where there are several Wi-Fi hotspots and a ham radio club, I don't know how much noise this'll cause an IR sensor.

I will be using this in a lab where the space directly in front of the sensor will be clear for at least 20 ft, but there will be some tables to the side.  I was thinking maybe I could put some kind of tube around the sensor to narrow it's detection width.  Something like a toilet paper roll or a disassembled flashlight tube.  Do you think this'll work or will it be likely to always go off because it's reading the tube?

Thanks, that's what I wanted to know.

I'm working on my first project with arduino, and I just need to know if I can connect the arduino to a wall adapter and a computer at the same time.  Will they both be supplying power (i.e. overpowering the arduino)?  I ask because I plan to send data to the arduino from my computer as my computer reads bluetooth from my phone (still haven't figured out how to do that yet), but then use that info to power a little DC motor via a transistor.  I don't get the 5V needed for the transistor from just my computer, but I can with the wall adapter.  So will connecting them both at the same time fry my arduino?  (Wall adapter is regulated 9V, 1A)