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Design Notes
Operating Modes
- Sleep mode: Battery-powered standby with MCU in deep sleep and accelerometer configured for motion interrupt wake.
- Motion detect mode: Accelerometer interrupt wakes MCU when sustained motion is detected.
- Charging mode: USB-C 5 V input powers charger IC and charges the single-cell battery; system remains powered from charger power-path output.
- Timed heating mode: MCU enables the heater through a MOSFET using open-loop timed bursts with conservative duty cycle.
Initial Heating Strategy
- Use open-loop timed heating only for revision 1.
- Start with conservative fixed heating bursts and cooldown intervals.
- Firmware enforces maximum ON time, minimum cooldown, low-battery lockout, and inactivity timeout.
- Temperature feedback is not required in the first revision; it remains an optional future upgrade if testing indicates it is needed.
Power Architecture
- USB-C sink-only 5 V input.
- Single-cell Li-ion/LiPo battery.
- Charger with power-path management so the system can run while charging.
- 3.3 V regulated rail for MCU and accelerometer.
- Heater powered from system or battery rail through a dedicated MOSFET switch.
Battery Life Direction
- Target battery life: 1.5 to 2 weeks.
- Logic domain current must remain very low in sleep.
- Heater is the dominant load and must use short bursts rather than continuous operation.
- Final heater burst duration and interval will be chosen conservatively to support the runtime target.
First-Pass Schematic Decisions
- Chosen charger: BQ24075 with integrated power-path management.
- Chosen regulator: MCP1700-3302 low-quiescent-current 3.3 V LDO.
- Chosen MCU: STM32L031F4P6 for simple low-power control and I2C support.
- Chosen accelerometer: BMA400 using I2C with INT1 wake output.
- Chosen heater drive: AO3400 logic-level N-MOSFET as a low-side switch.
- USB-C is implemented as sink-only power input with 5.1k pull-down resistors on CC1 and CC2.
- Revision 1 keeps the heater interface intentionally simple and open-loop; exact heater element and thermal optimization remain for later refinement.
Rev 1 Control Platform Scope
- Revision 1 retains the STM32L031F4P6 as the main MCU.
- BLE radio and smartphone app features are explicitly deferred to a future revision.
- No BLE-specific circuitry is included in the current Rev 1 schematic.
Single-Button UI
- Added one momentary pushbutton tied to MCU pin PA3 as the Rev 1 user interface input.
- Button is wired active-low with a 100k pull-up to 3.3 V to keep standby current low.
- Intended Rev 1 behaviors:
- Short press: toggle scent diffusion enable state.
- Long press: switch between simple intensity or timed-diffusion modes in firmware.
- Press during idle: wake or request activity when firmware policy allows.
Interaction With Motion-Controlled Heating
- Motion detection remains the primary automatic trigger for active diffusion behavior.
- The single button adds manual user intent without changing the open-loop timed heater architecture.
- Firmware should require both safety checks and the existing timed-heating limits before any heater activation.
- Manual button actions should never bypass low-battery lockout, inactivity timeout, or cooldown timing.
Future Revision Notes
- Candidate future revision features include BLE connectivity, smartphone app control, richer status indication, and expanded UI state handling.
- Open questions for later revisions:
- Whether a dedicated status LED is needed for button feedback.
- Whether button behavior should be limited to enable or disable only, or also support intensity selection.
- Whether a BLE-enabled MCU should replace the current STM32 in a later connected product version.
Capacitive Touch Button Replacement
- Replaced the original mechanical user switch with a single-channel capacitive touch controller using AT42QT1010-TSHR.
- The AT42QT1010 output is active-high, so an MMBT3904 NPN inverter was added to preserve the original active-low USER_BTN behavior seen by MCU pin PA3.
- Existing resistor R8 remains the 100k pull-up from USER_BTN to 3.3 V, keeping the standby current strategy unchanged.
- Added C13 100nF as the local bypass capacitor between the touch controller VDD and VSS, following the recommended basic circuit.
- Added C14 10nF as the QT1010 charge-transfer sense capacitor between SNS and SNSK.
- Added R11 1k in series with the SNSK path to the external touch electrode for ESD and EMC suppression.
- Added LED1 with R10 1k from 3.3 V for illuminated button feedback.
- U3 SYNC is tied low for fixed mode operation; the actual touch electrode connects externally through R11 and must be implemented in the enclosure or PCB touch pad geometry.
Operating Modes
Initial Heating Strategy
Power Architecture
Battery Life Direction
First-Pass Schematic Decisions
Rev 1 Control Platform Scope
Single-Button UI
Interaction With Motion-Controlled Heating
Future Revision Notes
Capacitive Touch Button Replacement
