The Sentry II Pro motion sensor has four 10mm LEDs that are twice the size of the original Sentry, and this unit has an audible alarm that can be turned on or off with the simple flip of a switch.
 
The LEDs are fast flashing color changing to ensure they are observed when triggered.
The audible sound is easily turned on or off.
The unit is powered by one 9 -volt battery.
The PIR motion sensor can detect movement at distances of 12' or more and has a detection coverage of 105 degrees.
 
Once the Sentry II is powered on, it will flash for several seconds as it takes its initial reading of the area. Once the reading is complete the lights (and optional audible alarm) will turn off until it is triggered. Once triggered the LED lights will begin flashing and changing colors and the audible alarm will sound (if turned on). If no more movement is detected, the unit will turn off after several seconds, resetting until it is triggered again. The Sentry II can be used singularly or in groups for best results. For example, place a few down a hallway to potentially see the direction of movement. These, like the original Sentries, can be placed flat or on their side to cover a specific area. Simply place a Sentry II, flip the switch to turn it on and the device is active. Very easy to use.