Active transponders are what I’m thinking of. I don’t think they need to be as bulky as EZ Pass.
Don’t know of anyone selling active RFid transponders in small quantities. FCC cert costs dictate going for big markets. But I’m not in that part of the biz; maybe there is.
you could probably roll your own with SFE 315Mhz xmtrs. The transmitters don’t require modulation and could be used for a intermittent interrogation request via a low d/c cw burst.
you wouldn’t even have to use the SFE rx in the “RFid card”. just a SA612 mixer hooked up to ant. an crude LO osc and a bpf on your IF. Put a detector on the bpf output and if it breaks a threshold, have a cheap PIC ucontroller pump out a serial msg into another xmtr as a reply. Of course, the interrogator needs a rx to decode the transmission. And another Ucontroller to decode it and act on the rev’d code.
Mount some trip-wire IR-beam(or similar) on each side of the door(or door-frame). When both have been triggered, someone has passed through. Which way is desided by which one is triggered first. When one is triggered, activate a Bluetooth scan and accept the closest device as the user which passed through.
Could also trigger some security camera to trigger a snapshot just in case.
There will be some margin for error since range to a bluetooth device is not certain just by jusing RSSI after a sweep. So if there are several persons within range there could be a small amount of chance that their logged instead.
I was actually reading this thread to see if anyone would suggest Open Beacon - it seems, on the surface, to be perfect for this type of use but just wondering if anyone had messed with it.