Smart Stove Safety Knob
A fail-safe retrofit device that automatically shuts off unattended stovetops. A flat spiral clock spring stores mechanical return energy when turned ON; 12 radial ridges on the stationary inner casing retain the torque passively with zero electrical power. When the AM312 PIR sensor detects no presence and the user fails to respond via the BLE companion app, dual horizontal micro servos disengage the 3 locking legs via a 2.5 mm axial lift, and the spring drives the valve back to OFF. Gas & electric compatible, tool-free install, sub-$50 BOM.
Product Architecture & SolidWorks CAD Drawings
Non-Intrusive Retrofit Enclosure
Engineered with a low-profile coaxial form factor (Ø86 mm prototype, 32.5 mm height) that slips directly over existing stove valve stems without modifying the appliance or voiding home insurance warranties.
System State Machine
Complete embedded decision flow. The firmware guarantees zero false shutoffs during attended cooking, and zero missed shutoffs when genuinely unattended.
Hall switch = OPEN
ESP32 hardware cutoff
Standby: 0 µA (Zero draw)
Hall = CLOSED → Wake
PIR continuous polling
Inactivity timer running
Spring wound + 12 ridges hold
Push notification via BLE
30 s grace countdown
Escalating phone alarm
PIR still active
PWM pulse to dual servos
2.5 mm axial lift
Spring drives shaft → OFF
Execution: <3 seconds
0 → 1 · Rotation Trigger
Cook rotates knob away from OFF position. Embedded magnet moves away from lateral reed switch, closing circuit. ESP32 boots instantly. Spring winds, 12 ridges lock each 30°.
1 → 2 · Inactivity Expiry
PIR reports no adult motion within 3–6 m cone for Tinactivity (default 5 min). BLE push notification transmitted to companion app. 30 s grace period begins.
2 → 3 · Grace Expiry
User fails to respond within 30 seconds. ESP32 activates dual horizontal servos. 2.5 mm vertical lift disengages 3 legs from ridges. Spring snaps valve to OFF.
↩ Safe Return Paths (Preventing Nuisance Alarms):
1 → 1: Any motion detected by PIR resets inactivity timer to 0.
2 → 1: User taps “Keep Cooking” in mobile app → resets timer immediately.
2 → 1: Cook walks back in front of stove during 30s grace → PIR auto-clears alert.
2 → 3: User taps “Shut Off Now” → instant remote mechanical actuation.
🍲 Slow Cooking Mode
For long stews or simmering, cook sets an app timer (e.g. 45 min). PIR inactivity checks are suspended during this window, resuming standard 5-min monitoring upon timer expiration.
🔋 Dead Battery Fail-Safe
Return energy is purely mechanical (spring wound during manual turn-on). A depleted battery cannot prevent physical shutoff or slip the knob into an open flame hazard.
🚫 Anti-False-Positive Filter
PIR Fresnel lens tuned specifically for adult torso motion at stove height. In laboratory usability trials (6 users × 10 min cooking), zero false shutoffs occurred.
📱 Instant Remote Shutoff
User can manually trigger shutoff from anywhere in BLE range (~10 m). Measured latency between tapping “Shut Off” and valve return completion averaged 8.2 seconds.
Companion App · BLE Spatial Monitor
In Session 1 usability testing, traditional list-based navigation caused 50% wrong-knob selections. The redesign adopted a spatial 2×2 grid mirroring physical stovetop geography, achieving 100% correct identification in under 2 seconds.
Tap burner to pair via BLE
Functional Specifications (Report 3):
- F1: Spatial 2×2 grid matching burner arrangement
- F2: BLE pairing per UID, one-to-one device bonding
- F3: Color feedback: grey=off, animated red=active flame
- F4: Per-burner countdown presets (15, 30, 60 min)
- F5: Escalating alerts: silent push → audible siren
- F6: Modal override: “Keep Cooking” or “Shut Off”
- F7: Auto-shutoff signal if unacknowledged
- F9: Remote manual cutoff from anywhere in home
Flat Spiral Spring & Ratchet Engineering
Why a Flat Spiral (Clock) Spring — Not a Torsion Wire?
❌ Conventional Round-Wire Torsion Spring:
Bending stress scales directly with rotation angle (σ ∝ θ). At 315° of knob rotation, a d=1.4 mm steel wire experiences ~3,800 MPa stress against 1,170 MPa yield limit. Yields permanently on first use.
✅ Flat Spiral (Clock) Spring:
Stress is governed by strip thickness and minimum mandrel radius (σ = E·t / Rmin), independent of winding angle θ. Produces constant return torque (0.325 N·m) across full 0°–350° stroke.
Strip Geometry Selection · Tnom = (σallow · b · t²) / 6
σallow = 1,300 MPa (AISI 1095) · Target: SF ≥ 1.5 vs measured peak resistance Tpeak = 0.201 N·m
| Width b (mm) | Thickness t (mm) | Tnom (N·m) | Safety Factor | Design Verdict |
|---|---|---|---|---|
| 6 | 0.4 | 0.208 | 1.03 | Low margin |
| 6 | 0.5 | 0.325 | 1.62 | SELECTED ◄ |
| 6 | 0.6 | 0.468 | 2.33 | Pass (oversized) |
| 8 | 0.5 | 0.433 | 2.16 | Pass |
| 10 | 0.5 | 0.542 | 2.69 | Pass (oversized) |
12 Radial Ridges (Variant A)
Integrated into bottom case stationary wall. Ramp angle (42°) permits effortless clockwise turning; steep tooth face locks against reverse torque. Zero pawl, zero moving latches.
2.5 mm Axial Lift Stroke
Dual horizontal SG90 micro servos thrust upward simultaneously. Lifts the 3 locking legs over ridge height. Spring instantly drives knob counter-clockwise to OFF in <3 s.
Modular 3-Profile Adapter
Accommodates D-Shaft, Double-D, and Round/Splined stems (4–8 mm). Setscrew collar locks onto stem without marring. Slip-on retrofit tested on 20 commercial ranges.
Laboratory Test Results
| Validation Test | Sample / Trials | Measured Performance | Observations & Engineering Notes |
|---|---|---|---|
| Servo Disengagement (45°–350°) | 150 | 148/150 (98.7%) | 100% success after symmetric horn alignment |
| Spring Return Torque (0°–360°) | 50 | 0.318–0.337 N·m | Mean 0.328 N·m, all > 0.305 N·m requirement |
| Shaft Compatibility (3 Types) | 50 | 50/50 (100%) | D-Shaft, Double-D, Round/Splined (4–8 mm) |
| Thermal Stability (Sustained) | 134°C stove panel | < 0.5 mm deflection | Worst-case top-mount electric range |
| Ridge Retention Endurance | 500 cycles | < 0.1 mm wear | Zero crack initiation at ridge root fillet |
| BLE Pairing Latency | 6 users | 42 s mean | From app launch to confirmed device handshake |
| Tool-Free Installation | 6 users | 4.6 min mean | Range 3.4–5.9 min, zero hand tools required |
Bill of Materials & Scale Economics
| Component Item | Engineering Specification | Prototype Cost |
|---|---|---|
| ESP32-C3 Microcontroller | RISC-V 32-bit core, BLE 4.2, 4 MB flash, deep-sleep <5 µA | $5.41 |
| PIR Sensor (AM312) | Miniature cylindrical, 3–6 m range, 100–120° FOV, 3.3 V, <20 µA | $1.63 |
| Hall Effect Sensor (A3144) | Lateral mount switch, 6 mm NdFeB magnet, open-drain gating | $0.38 |
| Micro Servo SG90 ×2 | Horizontal mount, 4.8 V, 1.8 kg·cm, 2.5 mm axial thrust | $4.34 |
| Flat Spiral Spring | AISI 1095 hardened steel, 6×0.5 mm strip, T_nom = 0.325 N·m | $3.26 |
| Custom PCB (Circular Ø68 mm) | 2-layer FR4, 1.6 mm, HASL, Gerber-fabricated EasyEDA | $1.00 |
| Wiring + Connectors | JST-PH 2.0 mm, 22 AWG silicone wiring, heat-shrink | $0.04 |
| Fasteners & Consumables | M2 standoffs, Sn63/Pb37 solder, flux, structural adhesive pad | $10.00 |
| NdFeB Permanent Magnet | 6 mm disc, N52 grade, embedded in bottom case lateral pocket | $0.06 |
| Li-Ion Battery + USB-C | 3.7 V 1200 mAh LiPo rechargeable, TP4056 charge controller | ~$1.00 |
Production Cost at Scale
Competitive Landscape
| Product Name | Detection Method | Actuation Principle | Gas Stove | Tool-Free | Unit Price |
|---|---|---|---|---|---|
| SmartKnob (Ours) | PIR (AM312) + Inactivity Timer | Mechanical spring return (fail-safe) | ✓ | ✓ | < $50 |
| Ome Smart Knob | Knob position timer only | Motor-driven rotation (active) | ✓ | ✓ | ~$129 |
| Smarturns | PIR + external hub | Alert only (no physical shutoff) | ✓ | ✓ | ~$245 |
| iGuardStove | PIR + inline timer | Electrical power cutoff only | ✗ | ✗ | ~$199 |
| FireAvert | Smoke alarm sound trigger | Electrical power cutoff only | ✗ | ✗ | ~$498 |
12 Iterations → V12 Final Prototype
First Functional Spring
Rotational spring mechanism validated. Identified yield failure in conventional torsion wire; pivoted to flat spiral clock-spring.
Solenoid → Dual Servo Pivot
Solenoid current spike and axial stroke insufficient under high torque. Replaced with dual horizontal SG90 micro servos.
Internal Ridge Profile (Variant A)
Eliminated separate pawl. Integrated 12 radial ramp ridges into bottom case inner wall with 3 locking legs on top case shaft.
Snap-Fit & Coaxial Integration
Final snap-fit latches permitting 2.5 mm axial travel with hard retention stop. Coaxial runout ≤0.1 mm across full 350° rotation.
Project Outcome & Commercialization Roadmap
Validated Physical Hardware
V12 prototype fulfills all 6 mechanical & safety requirements. 800-trial torque verification, 98.7% release reliability, 134°C thermal survival, and 500-cycle ridge endurance.
User-Validated Spatial UI
Session 2 usability achieved 100% correct burner identification within 2 seconds. Zero false-positive shutoffs. 42-second initial BLE bonding.
Regulatory & Production Path
Patent-eligible combination claims for mechanical-electromechanical failsafe. Next phase: UL 60730 / IEC certification, hardened steel tooling, and 1,000-unit pilot batch.