Prox Sensor help?

Wondering if anyone can lend a hand. I am building a proximity kill switch of sorts on the Arduino. Basically, I need to see a wristband within 10 feet of the Arduino. RFID is seen within an inch, and bluetooth is working within 25 feet, but I need something in between. I have an engine running when wristband is detected but when worker leaves 5 foot "geofence" I need engine to shut down. The engine is not stationary either. Anyone have an idea that might help me out? Appreciate it.

That kind of measuring is often requested and most answers are “No”.
Give the machine operator a machine operating key that is chained to him by a 5 feet chain……

In a controlled environment, you might be able to use ultrasonics.

Base unit transmits a broad spread IR pulse and then triggers (after a calculated microseconds delay) a ultrasonic range detector that has the transmitter deactived.

Remote unit monitors IR and upon “seeing” the pulse, activates same brand ultrasonic range detector.

Base unit hears remote unit and thinks it’s echo from itself and gives range reading.

In my mind that would work, but noise and infrared interference at the site might make it difficult.
Plus now you need to make a tether on the remote so the worker doesn’t simply leave the remote and walk away.

I'd add an "activity sensor" in the remote so it stops pinging when it doesn't detect normal hand movements.

You can adjust the power and sensitivity of the Bluetooth receiver. But Bluetooth and all other wireless networking systems are specifically designed to get some part of the message through in the worst transmission conditions. You may find there's a piece of metal in the lunch room that reflects the signal perfectly so your carefully tuned 10-foot range is extended significantly.

Ultrasonic seems like the best, simplest solution.

What would a towel or other shielding stuff hung over the sensor do?...……. Traindrivers have used the floor sweeping brush to jam the Dead Man Switch.....

You might try running some experiments with the Bluetooth RSSI; as you move away the RSSI will drop. With some filtering/averaging you might be able to use that. 5 ft isn't very far tho...