picostand

A tiny under-desk USB sensor + a macOS menu-bar app that tracks how much time you actually spend standing at your standing desk, nags you when you don't, and turns it into streaks so you keep going.

The hardware (a Pi Pico + ultrasonic distance sensor) lives under the desk, powered by a USB cable to your monitor. When the monitor is on, the laptop is connected, and the app is running, your activity is logged. Sit and stand transitions are detected from the distance to the floor.

 

git clone https://github.com/ast3150/picostand.git
cd picostand

Why

Standing desks are great. People (me) still don't use them. This solves it with the smallest hardware loop I could think of: ~15 € of parts, no WiFi, no cloud, no batteries. Just a sensor that knows when you're sitting or standing, talking to a small Mac app over USB.

How it works

[ floor ]   ←~~~~~~  ultrasonic ping
   ↑
   |  distance  (633 mm sitting, 937 mm standing on my desk)
   ↓
[ RCWL-9620 ] ── I²C ── [ Pi Pico WH ] ── USB ── [ Mac app ]
                                                       │
                                                       └─ SwiftData
                                                       └─ Notifications
                                                       └─ Menu bar UI

The Pico samples the distance twice a second, applies hysteresis + debounce, and only emits a transition event when it sees three samples in a row clearly in the other zone. The Mac app receives those events as newline-delimited JSON and writes them to a SwiftData store as sit/stand sessions. Stats, streaks, charts, and reminders all derive from that one table.

The Pico has no persistent storage and no clock. The Mac is the source of truth for time; the Pico just knows "you're sitting" or "you're standing". This is on purpose — keeps the firmware tiny (~150 lines) and makes the protocol robust to disconnects.

Hardware

Part	What it does	Approx. price
Raspberry Pi Pico WH	The MCU. WH = with pre-soldered headers, so no soldering required. (Plain Pico W also works.)	~10 €
RCWL-9620 ultrasonic distance sensor (I²C)	Measures distance to the floor. ~2-450 cm range, ±1 cm.	~3 €
4 × female-to-female Dupont jumper wires	Sensor to Pico. Usually come with the sensor.	~1 €
USB-A → micro-USB cable	Pico to monitor's USB output. Must be a data cable, not charge-only.	already had one
Double-sided tape or VHB strips	Stick the Pico + sensor under the desk.	already had some
Optional: small breadboard	Cleaner prototype, easier to undo.	~3 €
Optional: 3D-printed case	Protects the electronics. Search Printables for "RCWL-9620 case" + "Pi Pico case".	filament

_{RCWL-9620 (left, in a printed case) wired to a Pi Pico WH — yellow→GP0 (SDA), white→GP1 (SCL), red→3V3, black→GND.}

Wiring

Connect by label, not pin number — different Pico variants print the labels in different orientations.

Sensor pin	Pico label	Notes
VCC	3V3 (the regulated 3.3 V output — NOT VBUS)	Powers the sensor at 3.3 V so its I²C lines are also 3.3 V, which is safe for the Pico's GPIO.
GND	GND	Any GND pin will do.
SDA	GP0	I²C0 data.
SCL	GP1	I²C0 clock.

Long wires (>30 cm) over USB-powered devices get noisy — keep it short.

Firmware (Pi Pico)

The Pico runs MicroPython. Flash it once, copy main.py, done.

Flash MicroPython

1. Download the RPI_PICO_W UF2 from https://micropython.org/download/RPI_PICO_W/.
2. Hold the BOOTSEL button on the Pico while plugging in USB. A drive named RPI-RP2 mounts on your Mac.
3. Drag the UF2 onto that drive. The Pico reboots and now shows up as a serial device at /dev/tty.usbmodem*.

Install firmware

brew install mpremote   # or: pipx install mpremote
mpremote connect /dev/tty.usbmodem* cp firmware/main.py :main.py
mpremote connect /dev/tty.usbmodem* reset

That's it. The Pico now boots main.py on every power-up and starts streaming events.

Sanity check

# Watch live events for 15 s
pipx run --spec pyserial python firmware/listen.py /dev/tty.usbmodem* 15

You should see something like:

{"t":"hello","fw":"0.1","sit":633,"stand":937}
{"t":"hb","state":"sitting","d":633,"seq":0}
{"t":"tx","state":"standing","d":903,"seq":1}
{"t":"hb","state":"standing","d":937,"seq":1}

Wire protocol

Newline-delimited JSON, both directions, over USB CDC.

Pico → Mac

t	Meaning
hello	Boot announcement with current thresholds.
hb	Heartbeat every ~10 s with current state + last distance.
tx	State transition (sit → stand or vice versa). Includes monotonic seq.
raw	Raw distance reading, only emitted in calibration mode.
cfg_ok	Acknowledgement that thresholds were applied.
cal_ok	Acknowledgement that calibration mode toggled.

Mac → Pico

cmd	Effect
cfg	Set sit/stand thresholds in mm. Not persisted on Pico — Mac re-sends on connect.
cal	Toggle raw-distance mode (used during calibration UI).
ack	Drop events ≤ seq from Pico's in-RAM replay buffer.
replay	Re-emit any buffered transitions. Sent automatically by the app on connect.

The buffer holds the last 50 transitions in RAM only. Since the Pico is powered from a monitor USB port, "Pico powered" implies "monitor on" implies "you're at the desk", so we don't need flash storage.

macOS app

Requires macOS 26 (Tahoe), Xcode 17+, and Tuist for project generation.

brew install --cask tuist     # or: brew install tuist
cd app
tuist install                 # resolve ORSSerialPort dep
tuist generate                # generate Picostand.xcworkspace
open Picostand.xcworkspace

Or build + run from the command line:

cd app
tuist generate --no-open
xcodebuild -workspace Picostand.xcworkspace \
           -scheme Picostand \
           -destination "platform=macOS" \
           -derivedDataPath .build build
open .build/Build/Products/Debug/Picostand.app

What it does

• Menu bar — live state (sitting / standing / blocked), today's standing %, streak count, quick links to the main window and settings.
• Main window — daily hero with progress ring, streak counter, last 7 days bar chart vs. goal line, today's timeline.
• Settings
  • General: daily standing goal (default 2 h).
  • Notifications: enable + minutes-of-sitting before a reminder fires (default 45 min).
  • Desk: connection status, current sensor reading, sit/stand thresholds, Calibrate… sheet, and maintenance buttons (Reset last hour / Reset today / Reset all data).
• Calibration — toggles the Pico into raw mode, shows live cm, capture sit and stand independently. Rejects out-of-range readings (sensor blocked / misaimed).
• Reminders — when you've been sitting longer than your threshold, fires a Notification Center reminder. Suppressed while disconnected or while the sensor is blocked.
• Streaks — a day counts toward your streak only if your total time at the desk was at least as long as your daily goal. Weekends, sick days, and disconnect days are skipped without breaking the streak.
• Sensor blocked handling — if the sensor returns readings outside 300-1500 mm (something in the way, misaligned, garbage), the app closes the open session at the last valid reading and pauses time accumulation. A new session opens automatically when the sensor recovers.

Architecture

The app code lives in app/Picostand/Sources/:

File	Responsibility
App.swift	App entry point, scene composition (MenuBarExtra, Window, Settings), object wiring.
Models/Session.swift	The single SwiftData @Model — (startedAt, endedAt?, state).
Serial/Event.swift	PicoEvent enum + Codable decoder for the JSON line protocol.
Serial/SerialReader.swift	Auto-connects to /dev/tty.usbmodem*, parses lines, surfaces sensorBlocked, emits change callbacks.
Store/SessionStore.swift	Applies events to SwiftData, computes totals / streak / weekly stats.
Store/AppSettings.swift	@AppStorage-backed observable settings.
Store/NotificationScheduler.swift	Per-minute tick that fires the sit-too-long reminder.
UI/MenuBarView.swift	The menu bar popover.
UI/MainWindow.swift	The main stats window.
UI/SettingsView.swift	Settings tabs (General / Notifications / Desk).
UI/CalibrationView.swift	The calibration sheet.
UI/Format.swift	Distance / duration formatters + DeskState icon / label / tint.

The Pico firmware is firmware/main.py. firmware/listen.py is a host debugging tool only.

Calibrating to your desk

The bundled main.py is calibrated to my desk (633 mm sit, 937 mm stand). Yours will differ. Two ways to recalibrate:

1. In the app: Settings → Desk → Calibrate…. Lower the desk, press Capture for sitting, raise it, press Capture for standing, Apply. The new thresholds are persisted on the Mac and pushed to the Pico on every connect.
2. Edit main.py: change SIT_MM and STAND_MM at the top, copy to the Pico via mpremote cp, reset.

Known limitations

• One Mac, one desk. The app talks to whichever Pico shows up as tty.usbmodem* first.
• No persistent storage on the Pico — if it loses power mid-session, the in-RAM replay buffer is gone. The Mac is the system of record.
• The Pico cable taking power only when the monitor is on is the intended "presence" detector. If you sometimes power the Pico separately, the activity log will include time you weren't actually at the desk.
• macOS 26 minimum, because the app uses APIs that ship with it. Older macOS versions would need code adjustments.

Repository layout

firmware/
  main.py             # MicroPython firmware for the Pico
  listen.py           # Host-side serial reader for bench testing

app/
  Project.swift       # Tuist project definition
  Tuist.swift
  Tuist/Package.swift # External SPM dependencies (ORSSerialPort)
  Picostand/
    Sources/          # Swift source (see Architecture above)
    Resources/        # Assets

License

MIT.