3 bag
As previous poster, with the inclusion that ours goes to full solid light from flashing light, then cuts power to the motor.
I assume this is a feature to get user's attention, but when it is indicating at a non-full condition, gets frustrating.
Had unit open but cannot trace a sensor without a detailed wiring diagram. Only sensor seems to be an electro-magnetic pickup between the motor flywheen and the brush drive.
Vacuums - Windsor - SR12 Sensor Bagged Upright Vacuum
Answers & Comments
The sensor is a small hole into the vacuum chamber. It moves the alert system as vacuum inside the unit builds up.
The filtration, built into the bag, is getting filled up and creating a higher vacuum inside the unit, giving a false reading. The only ways to eliminate this are to give the bag a shake to clear some of the dirt out of the filter, try to pick up coarser dirt first or buy generic bags-without the filtration.