Bluetooth Sensor

Bluetooth must stay on

This sensor scans over the phone's Bluetooth radio. If Bluetooth is turned off, it cannot scan for the beacon at all — it cannot tell whether the beacon has moved into or out of range, and Sentinel Circle reports the sensor as not working rather than guessing. Keep Bluetooth switched on for the whole time monitoring runs.

Optional: Keep Screen On (kiosk mode)

On a phone dedicated to monitoring and kept plugged in, turning on Keep Screen On (Application Settings) keeps the screen lit while Sentinel Circle is visible and gives faster Bluetooth recovery. Its "Return to the app automatically" sub-option brings the app back 30 seconds after someone leaves it — see the app's "Coming Back To The Screen" help page. Keep Screen On is off by default; your first Start Monitoring with this sensor asks whether to switch it on.

Fires an alert when a specific Bluetooth device — like a small beacon on a keychain — goes out of range. Ideal for a beacon or wearable the person always carries.

Details

How it works

Sentinel Circle keeps one continuous Bluetooth scan running, and listens in on it for your device. If the device hasn't been heard for about 45 seconds, that starts the out-of-range check rather than finishing it: a Bluetooth scan can go quiet because the device left, or because the phone's Bluetooth stack stopped delivering results. Sentinel Circle then runs a short listening test to prove the scan is still healthy and your device really is missing, which takes about another 15 seconds. So the earliest an absence can be confirmed is roughly a minute after the last sighting.

If Bluetooth is switched off

Switching Bluetooth off does not raise an alert. With the radio off Sentinel Circle has no information at all: it cannot tell "the beacon is out of range" from "I cannot listen". It holds the last state it was sure of, and neither an alert nor an all-clear follows until Bluetooth is back on.

The sensor's row on the main screen reports the fault instead. It also means switching Bluetooth off is not a way to test the sensor — use the tin trick below, or carry the beacon out of range.

Which devices work: fixed address only

This sensor only works with a device that keeps one fixed Bluetooth address. Sentinel Circle follows a device by that address. Phones, watches and earbuds change theirs every few minutes for privacy, and so do trackers built for Apple's Find My or Samsung's SmartThings Find. A sensor tied to a device like that works for a little while and then silently stops finding it. The Select Beacon screen says this at the top.

	Examples
Usually works	Dedicated Bluetooth beacons (iBeacon or Eddystone) and keychain beacons — a beacon's address is set at manufacture and cannot change. Many keyfinder and luggage tags. Some heart-rate chest straps.
May work — try it	A car. If the car's Bluetooth shows up in Select Beacon while the car is on, it can work. Many cars only use classic Bluetooth, which this sensor cannot see, so it may not appear at all.
Does not work	Phones, watches, earbuds. AirTag and other Apple Find My trackers, Samsung SmartTag, and similar. Beacons set to Eddystone-EID (their identifier rotates by design).

These are categories, not guarantees: makers change firmware, and some trackers are reported to behave differently between models (Tile trackers, for instance, are reported to broadcast a fixed address, but one user saw it change). Always check a device before relying on it: pick it with the tin test below, then watch the status line under the beacon fields on the sensor's edit screen. If the line says "Not seen" while the device is right next to the phone, its address has changed and it will not work.

How to find a beacon to buy

Search the web (Google, Bing, DuckDuckGo) for one of these phrases, keeping the quotation marks:

• BLE beacon "fixed MAC address"
• iBeacon keychain "static MAC address"

Shop and datasheet pages that mention a fixed or static MAC address are the ones to look at. Check that the product page itself says the address is fixed (a beacon's address is normally printed on the case or in its setup app), avoid anything described as "privacy", "rotating" or "Find My compatible", and confirm it with the tin test when it arrives. A search result can still be wrong, so the tin test and the status line are the real check.

Setting up a new beacon (first use)

Taking a beacon out of its box is usually not enough — most new beacons have to be switched on first, and until they are, Select Beacon will not list them. Beacons differ a lot, so the maker's instructions (on the box, in the leaflet, or on the product page) are the final word. These are the things worth trying, roughly in this order:

1. Remove the pull tab or film. Many beacons ship with a small plastic strip between the battery and its contact so the battery does not drain on the shelf. Pull it out. If the battery has sat in a warehouse for a long time, fit a fresh one (most keychain beacons use a CR2032 coin cell).
2. Press the button. Press it once, or hold it for a few seconds (the manual says which). Some beacons are asleep from the factory and only start advertising after a press, and some only wake when shaken. A flashing light usually means it is now on.
3. Activate it in the maker's app, if it has one. Some beacons must be added to, or activated in, the maker's own app before they start working — and some are configured there (name, signal strength, how often they advertise). Install that app, add the beacon, finish its setup, then close the app. Many beacons stop advertising while a phone or app is connected to them, so leaving the maker's app connected can make the beacon vanish from Select Beacon.
4. Give it a moment, and bring it close. Hold the beacon right next to the phone. Some beacons slow down their signal when nothing is nearby and speed up again once a phone scans for them, so wait a minute and tap Start again.
5. Restart it. Take the battery out for a few seconds and put it back, or use the beacon's reset button or sequence if it has one (check the manual — a reset can also clear its settings).
6. Remove an old pairing. If you paired the beacon with this phone before, remove it in Android's Bluetooth settings, then scan again. A stale pairing can stop a beacon from advertising normally.

If it still does not appear, check it with a second app. A free Bluetooth scanner such as nRF Connect shows every Bluetooth LE device the phone can hear. If the beacon is not in that list either, it is not advertising (flat battery, still asleep, or already connected to something else), and Sentinel Circle cannot see it either. If it shows there but not in Select Beacon, check that it is a fixed-address device (see above): a beacon that changes its address will show a different address each time.

Once a beacon has been activated you normally do not need to repeat any of this. A new battery may need the button press again.

How to choose a beacon

Tap "Select Beacon" to scan for nearby Bluetooth devices. You'll see both paired devices and nearby beacons — pick the one you want to monitor from the list. You can't type in the beacon's name or address by hand — you always pick it from the Select Beacon scan.

The one screenshot of this screen in this manual (further down) has the device names and addresses blanked out: a real scan lists whatever devices are nearby when it is taken — your neighbor's earbuds, your own phone, a car's hands-free kit. What follows describes the process step by step, closely enough that you should recognize each part when you open it.

The Select Beacon screen, step by step

The screen walks through three steps, shown as a short instruction line above the device list that changes as you go:

1. Start. Tap Start and the app scans once for every Bluetooth device it can currently see — both devices your phone is already paired with, and unpaired nearby devices. Everything found is listed, strongest signal first, with a signal-strength bar next to each entry (a solid bar for a strong signal, a fading one as it weakens). Paired devices are labeled as such. This scan also becomes the "before" picture the next step compares against.
2. Find what changed (the Compare step). Do something to the device you want to identify — walk it out of the room, turn it off, or use the tin trick below — then tap "Find what changed." The app scans again.
3. Result. The list re-sorts to put what changed at the top: devices that vanished entirely come first, then devices whose signal dropped the most, then any device that's newly appeared. Devices whose signal didn't meaningfully change sit at the bottom. If the thing you moved was your beacon, it should be sitting at (or very near) the top. Tap it, then tap "Use This Beacon" to save it.

You can back out of the flow at any point — tapping the instruction step again after Result resets back to Start, in case the scan needs repeating.

Devices that change their address. A device whose Bluetooth address is known to rotate periodically (most phones and watches — see the note on rotating addresses below) is listed in its own group at the bottom, Devices that change address, closed until you tap it (it opens by itself when those are the only devices found). Each one is marked Changes address. They are kept out of the tin test on purpose: their address changes between the two scans, so they would show up as "gone" and "new" and hide the beacon you are looking for. They never appear on the result view.

Not sure which device is your beacon?

If there are lots of Bluetooth devices nearby, it can be hard to tell which one is yours from the Start list alone — several may have generic or blank names. This is exactly what the Compare/Result steps above are for. A reliable way to trigger a clean, deliberate change:

1. Find a metal tin with a lid — a biscuit or cookie tin works well.
2. Line the bottom with some paper or cloth.
3. Open the Select Beacon screen with your beacon next to the phone, and tap Start.
4. Put your beacon inside the tin and close the lid — metal blocks the signal.
5. Tap "Find what changed."
6. The device that vanished or dropped the most is your beacon — tap it to select it.
7. Take it out of the tin and tap "Use This Beacon."

Double-check your choice: after saving, your beacon will show up on the main screen's Sensors view. Try the tin trick again — this time take it to another room — and the signal should disappear completely, setting off your configured alerts.

Configuration

• Select Beacon — scans for and lists nearby Bluetooth devices.
• Beacon Name — the selected device's name (set automatically).
• Beacon Address — the selected device's unique Bluetooth ID (set automatically).
• Debounce Settings — Debouncing waits and double-checks before alerting; No Debouncing alerts as soon as Sentinel Circle can trust the reading, which for Bluetooth is still about a minute after the beacon actually goes (see above). See Debounce.
• Alert delay — the time from the beacon last being heard to the alert (default: 90 seconds, only shown with Debouncing on). It is an end-to-end budget, not an extra wait: the minute Sentinel Circle needs to be sure is counted inside the number you set, so setting 30 seconds still gives you an alert at about a minute.

Requires

• Bluetooth permission. On Android 12 and newer this is the "nearby devices" (Bluetooth scan) permission; below Android 12 Android requires location permission for Bluetooth scanning instead, and on Android 11 and below the location switch must also be on.
• A Bluetooth device the person carries with them.

A note on rotating addresses

Many phones and watches change their Bluetooth address every few minutes for privacy. A sensor pinned to an address like that stops working within about a quarter of an hour, so the device picker lists those devices separately, in the Devices that change address group, and keeps them out of the tin test. Pick a dedicated beacon or tag instead, which keeps one fixed address. A sensor tied to a device from that group can stop finding it, and nothing will tell you why.

If a device you configured earlier starts rotating its address, Sentinel Circle warns you about that sensor on the Sensors screen.

Trigger direction

By default this sensor alerts when the device disappears (Sentinel Circle Mode). It can be switched to alert when the device appears instead (Sentinel Gate Mode) — see Trigger Direction.