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?