A replacement for the Dynamic Lock feature built into Windows, which is unreliable and gives you nothing to set up.
Da BT Dynamic Lock locks your Windows computer when you walk away with your phone — or with another Bluetooth LE device you pick — reliably, and on conditions you choose yourself.
Nothing is paired and nothing is connected: the computer only listens for Bluetooth LE advertisements in the air, and you pick one of the devices it can hear. That leaves two conditions for anything you might want to watch:
- It has to be Bluetooth LE. Classic Bluetooth accessories never show up in the list, no matter how they are paired with Windows.
- It has to keep broadcasting. Most accessories go quiet the moment they connect to something, and a phone on its own is nearly silent — see below.
A beacon, a fitness band or a watch that advertises continuously works without anything installed. A phone does not, which is what the phone app is for.
Windows locks when it stops hearing your phone. The catch is that Android barely broadcasts anything on its own — measured on a stock phone lying still: 1.6 advertisements per minute, with gaps up to 244 seconds. That is far too little to decide anything.
So this project has two parts:
| Part | What it does |
|---|---|
| Windows app | listens for the phone, locks the screen when it goes quiet |
| Android app | broadcasts a small Bluetooth LE beacon, restarts itself after reboot or aeroplane mode |
The phone app turns those 1.6 signals per minute into a steady stream, which makes the decision trustworthy. It costs almost nothing in battery: an advertisement is a short radio signal, not a data transfer, and normal daily use shows no drain worth mentioning.
The phone app is deliberately tiny, and it cannot reach the network. It
has no INTERNET permission at all, so there is nowhere for anything to be
sent even in principle — the only permissions it asks for are Bluetooth
advertising, running in the foreground, starting after a reboot and showing
its notification, and you can check that yourself in the app's permission
list. The whole thing is a handful of Java files in phone/src/ with no
third-party libraries, so there is not much to audit.
It can only lock, never unlock. Windows does not allow third-party programs to unlock a session — you come back and sign in with your PIN or fingerprint as usual.
The real advantage here is that the app shows you the signal instead of hiding it, and then lets you set the rules. The window carries a live chart of the strength your own phone and your own room actually produce, and every threshold below is something you set against what you can see there. Nobody else decides for you what "away" means.
You choose which event locks the screen:
- Signal disappears (the default). A phone in a pocket is weak enough that a wall cuts it off completely, so the edge of the room becomes the edge of the signal. Robust, and it needs no tuning.
- Signal drops below a strength you pick (−100 to −60 dBm). The phone counts as "at the desk" only while it stays above that level — this is how you draw the boundary tighter than a wall, for an open-plan room or a desk near a doorway.
Either way the lock happens only after a delay you choose (12–120 seconds of not counting), so a brief dropout never locks your screen.
Why the app smooths the signal. A single reading jumps by several dB with the phone lying perfectly still, and the absolute values shift with the phone, the pocket, a body in the way, and whatever else shares the Bluetooth radio — an active mouse alone swallows a large share of the packets. So the threshold is compared against the median of a short window, not the last reading. For the same reason the app never converts dBm into metres: the difference between "at the desk" and "across the room" is smaller than the signal's own fluctuation, so any distance in metres would be a guess dressed up as a number.
The chart also tells the two failure modes apart: a curve that sits low means
a weak signal, one that is high but full of gaps means interference. Our own
measurements behind the defaults are in docs/DESIGN.md —
yours will differ, which is exactly why the settings exist.
The app runs as an icon near the clock. Clicking it opens a window with a live signal chart and all the settings:
- which device to watch — picked from whatever is currently heard
- when to lock — 12 to 120 seconds of silence
- range (sensitivity) — optionally require a minimum signal strength
- countdown before locking (information only, it cannot be cancelled), how high up the screen it appears, and whether it shows on every monitor or on the primary one alone
- do not lock while typing or moving the mouse (off by default — anyone at the desk could otherwise postpone locking)
- warning when the phone disappears for good, so a silent failure does not go unnoticed
- do not lock on saved networks (off by default) — for wherever locking is not needed. A network counts only when both its name and the access point's MAC address match, so a hotspot named after one of yours cannot switch the guarding off; a mesh needs each access point saved
- pause for a chosen period, then resume by itself
- Czech / English interface
The chart is there to tell two different problems apart: a curve that sits low means a weak signal (distance, pocket, body), while a curve that is high but full of gaps means interference — usually a Bluetooth mouse.
Windows — run DaBTDynamicLock-setup.exe from the latest release. It
offers a Start menu shortcut, a desktop shortcut and automatic start after
sign-in. The program goes to C:\Program Files\Da BT Dynamic Lock; settings
and history live in %APPDATA%\Da BT Dynamic Lock, per user.
Android — install DaBTDynamicLock.apk from the same release, open it,
tap Turn broadcasting on, and allow the Nearby devices and Notifications
permissions. Set the battery mode to unrestricted and enable autostart,
otherwise the system will eventually stop the service.
Full instructions: English manual · česky
windows\ Python 3.14 + bleak, pystray, Pillow, tkinter
phone\ Java, no Gradle (aapt2 + javac + d8 + apksigner)
installer\ PyInstaller
powershell -ExecutionPolicy Bypass -File installer\build_installer.ps1 # setup.exe
powershell -ExecutionPolicy Bypass -File phone\build.ps1 # APKThe Android build needs no Android Studio and no Gradle, but the tools
(a JDK and the Android SDK build-tools) have to be present in an
_android-build/ folder next to the project folder, one level above
it — they are ~700 MB of downloads that nothing at run time depends on, so
they are deliberately kept out of the project. The script does not
download them, it stops with an error when they are missing. Once they are
there, the build works offline.
Tests, to be run before any release:
py tests\test_logic.py the locking decision
py tests\test_window.py window behaviour, device list, language switch
py tests\test_installer.py a full install and uninstall cycle
py tests\preview.py renders the window to PNG so the look can be checked
py tests\preview.py installer the same for every installer window
py tools\check_docs.py checks the documentation against the code
- Locking only. Unlocking is not possible for third-party software.
- No distance in metres. You can require a minimum signal strength, but not "lock at three metres" — signal strength cannot be turned into a trustworthy distance. That is a limit of the radio, not of this program.
- The Windows binary is unsigned. SmartScreen will warn on first run; a code-signing certificate costs more per year than this project is worth.
- The phone app must keep running. Aggressive battery optimisation (Xiaomi, Samsung, Huawei) will stop it unless you allow it to run freely.
Apache License 2.0 — see LICENSE.