Presence Sensor

LD2410 presence sensor

Optional hardware accessory: an HLK-LD2410 (or LD2410B / LD2410C) 24 GHz radar presence sensor, wired to the kiosk via USB-UART. The sensor reports presence + distance to a body in front of it; Kinboard uses this to:

• POST presence state to /api/presence so the dashboard knows whether anyone's nearby
• Turn the display off after N seconds of nobody-detected
• Wake the display the moment someone walks up

Vastly nicer than a fixed sleep timeout. The display stays on while you're cooking and off while you're not.

Hardware

• Sensor: HLK-LD2410, LD2410B, or LD2410C — €5-€15 from AliExpress / Banggood / Amazon. The B and C variants have additional features (Bluetooth config, slightly tighter detection patterns) but the base LD2410 is fine.
• USB-UART adapter: any FTDI FT232 or CH340-based 3.3V USB-to-serial dongle. €3-€10. Make sure it's 3.3V — the LD2410 doesn't tolerate 5V.
• Wiring: 4 pins on the LD2410 (GND, +5V, TX, RX) → 4 pins on the USB-UART (GND, VCC, RX, TX — note the swap, TX-to-RX and vice versa).

TODO: photo of the LD2410 wired to a USB-UART adapter

Place the sensor pointing at the area in front of the kiosk display, with line-of-sight (it sees through wood and drywall, not metal). Mount it 1-2m above the floor at chest height.

Software (Windows)

The Windows path uses the presence-sensor.py script in the repo root. It opens the FTDI port, reads LD2410 reports, POSTs presence to /api/presence, and controls Windows display power via SendMessage(SC_MONITORPOWER).

Install Python deps

& "C:\Program Files\Python312\python.exe" -m pip install aio-ld2410 aiohttp pyserial

Configure

Edit the script's top-of-file constants or set env vars:

BAUD_RATE = 256000          # LD2410's default baud rate
DISPLAY_OFF_DELAY = 30      # seconds of no-presence before screen off
WATCHDOG_TIMEOUT = 30       # restart serial connection if no data this long
API_URL = os.environ.get("KINBOARD_URL", "http://localhost:3001").rstrip("/") + "/api/presence"

The find_serial_port() helper auto-picks the FTDI port; if you have multiple USB-serial adapters, hardcode port = "COM3" (or whatever).

Run

The script takes the device ID as its first argument:

& "C:\Program Files\Python312\python.exe" C:\presence-sensor.py 680d36e4-a08e-4d69-bca9-4339fffcf017

Find the device ID:

1. Open Kinboard → Settings → Devices
2. Toggle Presence sensor on for this device
3. The 8-character truncated UUID appears below — click the copy button to get the full ID

Run hidden (so the user never sees a console)

Wrap the launch in a small VBS:

Set WshShell = CreateObject("WScript.Shell")
WshShell.Run """C:\Program Files\Python312\python.exe"" C:\presence-sensor.py YOUR_DEVICE_ID", 0, False

Save as C:\run-presence-hidden.vbs. Window-style 0 = hidden.

Auto-start at logon

Register a Scheduled Task:

• Trigger: At logon of the kiosk user
• Action: wscript.exe with arguments C:\run-presence-hidden.vbs
• General → Run as user, Run only when user is logged on, Run with highest privileges
• Settings → Restart on failure (1 min, 3 attempts)

This is what the Kiosk-Windows-11-Mele-4C reference deployment uses.

Software (Linux)

Same presence-sensor.py script works on Linux with two changes:

1. The display-power code path uses SendMessage(SC_MONITORPOWER) — that's Windows-only. On Linux you need to swap to your DPMS backend (wlopm, xset, or hyprctl).
2. The serial port name will be /dev/ttyUSB0 (or similar) instead of COM3.

Edit the set_display_power() function:

def set_display_power(on):
    cmd = ["wlopm", "--on" if on else "--off", "*"]
    subprocess.run(cmd, check=False)

Then run via systemd:

# ~/.config/systemd/user/presence-sensor.service
[Unit]
Description=LD2410 presence sensor for Kinboard
After=graphical-session.target

[Service]
ExecStart=/usr/bin/python3 /home/kiosk/presence-sensor.py YOUR_DEVICE_ID
Restart=on-failure
RestartSec=5

[Install]
WantedBy=default.target

Enable: systemctl --user enable --now presence-sensor.service.

How the data flows

LD2410 sensor (24 GHz radar)
    │
    │ serial frames @ 256000 baud
    ▼
USB-UART adapter (FTDI / CH340)
    │
    │ /dev/ttyUSB0 or COM3
    ▼
presence-sensor.py
    ├──► POST {presence, distance, device_id} → /api/presence
    │       └─► Kinboard webapp updates `devices.last_presence` in DB
    │             └─► Realtime → other clients see the state change
    │
    └──► On no-presence > DISPLAY_OFF_DELAY:
            SendMessage(SC_MONITORPOWER, 2)   (Windows: turn display off)
            Or wlopm/xset on Linux

Reading the dashboard

When the sensor is connected, /settings/devices shows a green dot next to "Presence sensor" with the current distance. Useful for debugging — wave at the sensor and the distance updates within ~1s.

Configuring sensitivity (optional)

The LD2410's default sensitivity is good enough for most living-room scenarios. If you need to tune it:

• B-variant has a Bluetooth config app on Android — pair to the sensor and adjust per-zone sensitivity / max range
• All variants accept config commands over the serial port; the aio-ld2410 library exposes them. See its docs for the API.

Common adjustments:

• Max distance: default 6m, drop to 3-4m for a kitchen-counter mount to avoid triggering on people in the next room
• Static / moving target threshold: default works; raise if you get false positives from a HVAC vent

Why radar, not PIR?

PIR (passive infrared) sensors detect movement, not presence. If you stand still while reading a recipe, a PIR thinks you've left and turns the screen off. The LD2410's mmWave radar detects a stationary human chest moving (breathing) at sub-millimeter resolution, so "still but present" stays "present."

Logging

The script writes to C:\presence-sensor.log (Windows) or wherever you configure. As of v1.0 there's no log rotation — file just grows. A v1.1 fix will swap to RotatingFileHandler. For now, Clear-Content it manually if it gets large.

Troubleshooting

Symptom	Likely cause
No serial port found	USB-UART driver missing. Check Device Manager (Windows) or dmesg (Linux) for the device. FTDI drivers are usually built-in; CH340 needs a download from wch.cn.
"Permission denied" on /dev/ttyUSB0	User not in the dialout group: sudo usermod -aG dialout $USER, then re-login.
Detected presence but display doesn't turn off	The Windows SC_MONITORPOWER message doesn't always work — depends on monitor + driver. As a fallback, force-blank via PowerShell: (Add-Type ... SendMessage(...)).
Distance reads 0 always	Sensor wired to TX/RX swap incorrectly. Try swapping the wires.
Watchdog reconnects every 30 s	Serial cable is loose, or the sensor is power-cycling. Check the 5V supply.
Display turns off mid-conversation	DISPLAY_OFF_DELAY too short. Bump from 30 to 90 s.

Related

• Kiosk-Windows-11-Mele-4C — full reference deployment
• Database-Schema — devices.has_presence_sensor flag, presence state on the realtime feed
• presence-sensor.py — the script itself