The Sentry Ring

Difficulty

Advanced

Kit Cost

$45

Build Time

3-5 Weekends

Competition

AI Demo · Utility · Freestyle

The Build

Upgrade any porch camera or doorbell with AI-powered package defense. Person detection triggers recording and alert. If a person enters the package zone and picks up a package, a servo-controlled water nozzle delivers a precision blast. This is machine learning meets practical deterrent — non-lethal, legal (on your property), and deeply satisfying.

The system uses a Raspberry Pi 4 running MobileNet SSD for real-time person detection, a pan servo for target tracking, and a solenoid-controlled water valve for delivery. All controlled via a phone app with arm/disarm scheduling, live video feed, and event logging. Water only — no projectiles, no pepper spray, no legal gray areas.

Mount a Pi Camera or USB webcam with a clear view of your porch and package drop zone. Position it high enough to avoid tampering but low enough for good facial detail. Wide-angle lens (120-160 degrees) covers more area but reduces resolution at distance.

Test the view before permanent mounting. Mark the package zone on the ground (tape or chalk). Verify the camera can see the entire zone plus approach paths. Lighting matters — install under an overhang or add an IR illuminator for night operation.

MobileNet SSD runs on the Pi 4 at 10-15 FPS — fast enough for real-time detection. The model detects people, packages, animals, and vehicles. You care about people and packages. Everything else is ignored.

Define a region of interest (ROI) where packages typically sit. When a person enters the frame AND enters the package ROI, start tracking. If the person picks up the package (package bounding box disappears while person is still in ROI), trigger the water system.

// Detection logic (simplified)
if (person_in_frame && person_in_package_zone) {
  track_target(person_bbox);
// ... full detection pipeline in project files
  

Pan servo mounts on a bracket attached to your camera housing or porch rail. The water nozzle attaches to the servo arm. When a person is detected, calculate the horizontal position of their bounding box and steer the servo to aim at them.

Servo range: 0-180 degrees. Map camera frame X-coordinate to servo angle. The kit includes a servo with metal gears (critical — plastic gears strip under water pressure). Calibrate the mapping so center-frame equals center-servo-position.

// Servo aiming (simplified)
target_angle = map(person_bbox.center_x, 0, frame_width, 0, 180);
pan_servo.write(target_angle);
// ... full tracking controller in project files
  

12V solenoid valve controls water flow from a garden hose. The valve sits inline between the hose and the nozzle. When the Pi sends a signal, a MOSFET relay activates the valve and water flows for 1-2 seconds (configurable burst duration).

Nozzle choice matters. Adjustable spray nozzles let you tune from mist to jet. For competition, use a focused stream — more range, less collateral spray. Mount the nozzle on the servo arm with zip ties or a 3D-printed bracket.