Here's an online tutorial for connecting the HS04 Ultrasonic Sensor to the Micro:Bit

[MrDMurray]

Use this instead of a laser trip wire because it won't need a long wire across the floor and up to the other side of the door to mount an LDR receiver that would be needed to pick up the laser beam. This just measures the distance to whatever is front of it. Then when someone walks in front of it, it'll change and you can code it to register that as someone walking by.