White’s Electronics Bullseye 2 - Manual (Page 3)

Batteries
To open battery door, placing your thumb on the recess area
on battery door, press down and sliding it out. Put in fresh 9V
battery (alkalines recommended) observe polarity. (If battery is
put in wrong orientation, battery door will not be able to close.)
Low battery alert:
When battery is low, unit will vibrate or beep continuously
Battery life: up to 20 hours.
Selecting Audio or Vibration Alert:
Open battery door, remove battery, slide switch to desired
alert type. Put battery back and close battery door
Sensitivity adjustment:
Rotate sensitivity adjustment knob clockwise to increase
sensitivity or counter-clockwise to decrease sensitivity. For
maximum sensitivity, press and "hold" the ON button and at the
same time rotate sensitivity knob clockwise until the unit is
vibrating or beeping continuously, then rotate counter-clockwise
to the point the unit just stop vibrating or beeping.
(If sensitivity setting exceed its maximum allowable range, the
unit will beep or vibrate all the time when ON button is pressed)