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.