COVID took your sense of smell. This open-source sensor does what your nose can't.
One-time build cost ยท No subscriptions ยท No consumables ยท Open source
An NYU/NIH RECOVER study (2025, n=3,535) found that 80% of COVID patients who reported smell changes still scored abnormally low on clinical scent tests two years later. Of those, 23% were severely impaired or had lost smell entirely.
With over 150 million confirmed US COVID infections and roughly 5% experiencing persistent olfactory dysfunction, that's approximately 7.5 million Americans with meaningfully impaired smell โ many of them parents of young children.
Babies can't tell you they need a change. Your nose was the alert system. When that system goes down, diaper changes get delayed. Delayed changes cause diaper dermatitis, affecting up to 50% of infants. The solution isn't a smarter diaper. It's a smarter nose.
Baby feces emits a cocktail of volatile organic compounds โ hydrogen sulfide, skatole, indole, ammonia, methyl mercaptan. The Sensirion SGP40 metal-oxide sensor is tuned to detect exactly these sulfur and nitrogen compounds.
The sensor feeds readings through Sensirion's open-source VOCindex algorithm, compensated for temperature and humidity via the SHT41. Clean air reads ~100. A diaper event spikes to 200โ400+ within 15โ30 seconds. The device uses a 3-zone state machine with hysteresis to avoid false triggers:
Three consecutive readings in a zone required before state transition. Five-minute cooldown after alert to suppress noise during the diaper change itself. Configurable thresholds via BLE โ tune from your phone without reflashing.
| Part | Description | Price |
|---|---|---|
| Seeed XIAO nRF52840 | MCU + BLE 5.0 + USB-C + onboard LiPo charging. 21ร17mm. | $9.90 |
| Sensirion SGP40 Adafruit breakout |
MEMS VOC sensor. IยฒC, 0x59. Highly sensitive to HโS, skatole, mercaptans. | $9.95 |
| Sensirion SHT41 Adafruit breakout |
Temp + humidity compensation for SGP40's VOCindex algorithm. ยฑ0.2ยฐC. | $5.95 |
| 500mAh LiPo 3.7V JST-PH |
~200โ330 hours runtime. Charges via XIAO's USB-C port. | $7.95 |
| Piezo buzzer + RGB LED | Local audio/visual alert. 440Hz tone on detection. | $2.00 |
| Resistors + button | 3ร 100ฮฉ for LED current limiting + tactile pushbutton for baseline reset. | $0.55 |
Every consumer smart diaper product has either been discontinued, is limited to a single market, or requires expensive consumable sensors that cost more per month than the diapers themselves.
| Feature | PoopSense | Lumi by Pampers | Monit | Opro9 |
|---|---|---|---|---|
| Status | Active / Open Source | Discontinued DEAD | Korea only | Discontinued DEAD |
| Detection | VOC (gas) โ actual poop chemistry | Moisture (wetness only) | VOC + moisture | Moisture |
| Upfront cost | ~$36 | $350 (camera + sensors) | $79 | $30 |
| Ongoing cost | $0 | $30โ50/mo (special diapers) | $15/mo (sensor pads) | $20/mo (sensor strips) |
| Year 1 total cost | $36 | $710โ950 | $259 | $270 |
| Requires special diapers? | No | Yes | Yes (sensor pads) | Yes (sensor strips) |
| Open source? | Yes โ full BOM, firmware, enclosure | No | No | No |
| Battery life | ~200โ330 hours | N/A (wired camera) | ~72 hours | ~48 hours |
This isn't just a baby product. VOC-based soiling detection applies anywhere someone can't tell when a change is needed.
The real play: The baby market is the wedge. The elderly care market โ $2.9B in the US alone, growing 11.5% CAGR โ is the scale opportunity. Every nursing home in America has the same problem: delayed changes cause skin breakdown, pressure ulcers, and infections. A $35 sensor per bed that alerts staff in real time isn't a gadget. It's a clinical tool.
All from Adafruit + Seeed Studio. XIAO nRF52840, SGP40 breakout, SHT41 breakout, 500mAh LiPo. Ships in 2-3 days.
Four wires: 3V3, GND, SDA, SCL. If using Stemma QT / Qwiic cables, it's plug-and-play โ no soldering. SGP40 at 0x59, SHT41 at 0x44, no address conflicts.
Arduino IDE + Seeed nRF52 board package + Sensirion SGP40 library. Upload, open serial monitor, confirm VOCindex readings in the 80-120 range for clean air.
Run for 24-48 hours in the nursery to establish baseline. Introduce a soiled diaper at detection distance (20-40 cm). Note the spike โ adjust thresholds via BLE if needed.
3D-print a 45ร35ร20mm case with ventilation slot over the sensor. Velcro to crib rail or changing table. Keep 20-40 cm from diaper area. SGP40 is sensitive enough โ direct contact not needed.
500mAh LiPo, USB-C charging via XIAO onboard charger. Average current draw ~1.5mA. 200โ330 hours per charge depending on BLE connection frequency and alert activity.
BLE 5.0. Non-connectable advertisements at 500ms interval broadcast VOC index, zone, and battery level. No pairing required โ any BLE scanner app works. GATT service available for configuration.
Sensirion SGP40 MEMS metal-oxide. VOCindex 0โ500, temperature/humidity compensated. 1Hz sample rate. Response time to fecal VOCs: 15โ30 seconds at 20-40cm.
Approximately 45 ร 35 ร 20mm with 3D-printed enclosure. Ventilation slot with acoustic fabric protects sensor while allowing airflow. Velcro or clip mount.