Sentry

An autonomous sentry gun subapp that uses the device camera and on-device machine learning to detect people and automatically send a tag from a KTag blaster connected via USB when a person is detected in the crosshair.

How It Works

The back camera streams frames continuously through CameraX.
Each frame is analyzed by MediaPipe Tasks Vision running the EfficientDet-Lite0 object detection model entirely on-device — no network connection required.
When a person is detected with ≥ 50 % confidence, a red bounding box is drawn over them and a continuous 880 Hz tone plays (if enabled).
If the bounding box overlaps the crosshair at the centre of the screen, the sentry fires: it sends a trigger-press command followed by a trigger-release command over USB serial to the connected KTag device.
A cooldown period prevents the sentry from firing again until the timer expires.

Setup

1. Download the detection model

The EfficientDet-Lite0 model is not bundled with the APK and must be placed manually:

subapp-sentry/src/main/assets/efficientdet_lite0.tflite

Download the TFLite (efficientdet/lite0/detection/metadata/1) file from:

https://tfhub.dev/tensorflow/lite-model/efficientdet/lite0/detection/metadata/1

2. Connect a KTag device via USB

Plug a KTag device into the Android device using a USB OTG cable. The app will detect it automatically and show it in the device selector.

3. Grant camera permission

The app requests CAMERA permission on first launch. Deny it and the camera preview will not start.

UI

Element	Description
Camera preview	Full-screen live view from the back camera
Red bounding box	Drawn around any detected person
White crosshair	Fixed plus symbol at the centre of the screen; firing only occurs when it overlaps the bounding box
DETECTED badge	Appears in the top-right corner whenever a person is visible
Device selector	Drop-down listing connected USB serial devices
Connect / Disconnect	Connects to the selected USB device or drops the connection
Status bar	Shows connection state, configured team, and time since last trigger

Settings

Open the settings screen via the gear icon in the top-right corner.

Setting	Options	Default	Description
Tag Team	Blue, Red, Purple	Blue	Team argument sent in the trigger command
Trigger Cooldown	3 s, 5 s, 10 s, 30 s	5 s	Minimum time between consecutive trigger firings
Play Detection Tone	On / Off	On	Plays a continuous 880 Hz tone while a person is visible

USB Serial Protocol

When the sentry fires, it simulates a trigger press by sending two commands in sequence:

KEvent 7 0\n   ← trigger press
KEvent 8 0\n   ← trigger release

The device selector lists all recognised USB serial devices. The app requests USB permission automatically on first connect.

Audio

Sound	Condition	Details
Continuous 880 Hz tone	Person visible in frame	`AudioTrack` sine wave; plays until person leaves frame or "Play Detection Tone" is disabled
Short beep	Sentry fires	200 ms `ToneGenerator` beep

Both sounds play on the media audio stream and respect the device's media volume.

Dependencies

Library	Version	Purpose
CameraX	1.4.0	Camera preview and frame capture
MediaPipe Tasks Vision	0.10.32	On-device object detection
EfficientDet-Lite0	1	Person detection model (Apache-2.0, download separately)
usb-serial-for-android	3.10.0	USB serial communication with KTag hardware

Note: MediaPipe transitively includes Firebase. The AndroidManifest.xml explicitly removes FirebaseInitProvider via tools:node="remove" to prevent any Firebase initialisation.

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