why not a weight activated switch, the switch could be actuated by a metal plate that sits flush with the floor, the plate could be covered with a mat. When someone walks over the plate, the switch gets activated.
You may have to do some modifications to the house, but if the plate and the switch assembly are slim enough then you could use it straight away without making it flush with the floor.
Just my thought
You may have to do some modifications to the house, but if the plate and the switch assembly are slim enough then you could use it straight away without making it flush with the floor.
Just my thought