• Product sits on a bookshelf or similar indoor environment.
• Four off-board 5 V / 1 W adhesive polyimide heater strips heat their target items.
• Heater target behavior: each 1 W heater may operate at 100% duty while its target remains below 38 °C.
• Heating is primarily open-loop because direct measurement of the heated item is impractical.
• PCB-mounted temperature sensors provide safety monitoring near the off-board heaters and ambient room temperature sensing.
• Device is expected to remain installed and reliable for roughly 5–10 years.

• Independently control four heater outputs with PWM or equivalent power modulation.
• Allow quick heater installation using green pluggable screw-terminal connectors for bare wires.
• Monitor four heater-adjacent safety temperature sensors.
• Monitor two ambient temperature sensors placed caddy-corner on the PCB.
• Provide USB-C for both power and ESP32 flashing/debug.
• Work with both simple/dumb 5 V USB-C adapters and USB-C PD laptop chargers.
• Provide a small display, rotary encoder with pushbutton, and front RGB status LED.
• Support WiFi and Bluetooth through the ESP32 module.
• Survive reasonable user/environmental abuse such as common ESD events and power interruptions.

• ESP32-based WiFi/Bluetooth controller.
• 4 low-voltage heater outputs.
• 6 total PCB-mounted temperature sensors:
  • 4 heater safety sensors in a straight line, approximately 55 mm apart.
  • 2 ambient sensors placed caddy-corner and exposed to isolated ambient airflow paths.
• USB-C power + programming connector.
• USB-C PD sink behavior plus fallback/default 5 V sink behavior.
• I2C display, likely small OLED unless cost/availability pushes a better alternative.
• Continuous-turn clicking rotary encoder with pushbutton.
• RGB status indicator.
• Input protection, ESD protection, current limiting/fusing, and power-rail robustness.

Diagram

• Preferred direction: ESP32-S3 module with native USB, WiFi, and Bluetooth.
• Use a module rather than bare ESP32 silicon to reduce RF design and certification risk.
• Preserve required boot/reset strap behavior and provide reliable flashing access over USB-C.
• Keep antenna placement/keepout as a first-class layout constraint.

• Single USB-C port provides power and ESP32 programming/debug.
• Must accept default 5 V from non-PD USB-C supplies.
• Must negotiate or behave correctly when connected to USB-C PD laptop chargers.
• Include USB data-line ESD protection.
• Include input current limiting/fuse protection and transient protection appropriate for consumer USB-powered hardware.

• Four independent 5 V heater channels.
• Each heater is nominally 1 W at 5 V, approximately 200 mA.
• Use low-side logic-level N-MOSFET switching unless a later reason favors a different topology.
• MOSFETs should be conservatively rated so they run comfortably cool at the expected load.
• No heatsinking is allowed; all heat dissipation must be acceptable in a passively cooled enclosure with the heaters also adding heat nearby.
• Heater connectors should be comfortably rated above expected current and suitable for repeated bare-wire installation.

• Four safety sensors are PCB-mounted near the heater exits/locations, approximately 55 mm apart in a straight line.
• Two ambient sensors are PCB-mounted caddy-corner, with enclosure airflow tubes exposing them to room air while isolating them from heater-local air.
• Prefer stable, reputable, commonly available sensors with good long-term drift characteristics.
• Avoid cheap or fragile sensor constructions if they create lifetime drift, adhesive/epoxy degradation, or supply-chain risk.
• Address planning is required if digital I2C sensors are used; use an I2C mux or mixed addressing if needed.

• Small screen on device front; I2C OLED is the initial preferred direction.
• Continuous-turn rotary encoder with tactile clicks and integrated pushbutton for menu navigation.
• Front RGB LED for at-a-glance system status.

• Reasonable ESD protection on USB and externally accessible interfaces.
• Power input fuse/current limit and transient protection.
• Firmware/hardware fail-safe assumptions should prevent heater runaway where practical.
• Consider watchdog behavior, safe boot defaults, and safe state on brownout or reset.

• Primary input: USB-C.
• No battery specified.
• Heater load: 4 × 1 W = 4 W maximum heater power.
• Expected 5 V heater current: 4 W / 5 V = 0.8 A maximum heater load.
• Logic/display/sensor current is expected to be much lower than heater current but must be budgeted during part selection.
• Practical USB-C target: design for at least 5 V / 1.5 A input capability; 5 V / 2 A or 3 A adapters provide comfortable margin.
• PD should improve compatibility with laptop chargers, not require obscure adapters.

Diagram

• Target product retail price: approximately $40.
• PCB and component choices should be affordable for the specification, not premium without reason.
• Prefer common, well-stocked, RoHS/lead-free components.
• Avoid EOL, niche small-run, leaded, or single-source parts.
• Prefer standard packages and parts with reasonable alternates.
• Use generic passives where practical.
• Include useful test points for power rails, USB, I2C, programming, and heater outputs.
• Favor manufacturable footprints and assembly-friendly part choices.

• ESP32 native USB flashing/debug via the same USB-C port used for power.
• PWM-capable GPIO for each heater channel.
• I2C bus for display and temperature sensors.
• GPIO inputs for rotary encoder A/B and pushbutton.
• GPIO/PWM or serial LED interface for RGB status LED.
• Safe heater-off default during boot, reset, firmware update, brownout, and watchdog reset.
• Brownout and watchdog behavior should be part of firmware bring-up.

• Enclosure maximum: 130 mm × 200 mm.
• PCB target maximum: ≤125 mm × 195 mm.
• No heatsinks.
• Passively cooled environment only.
• Current PCB outline target: 124 mm × 190 mm.
• Board coordinate/orientation convention:
  • Back is the side with the USB-C connector.
  • Front is opposite the USB-C connector.
  • Side A is the user-interface side with the OLED and rotary encoder.
  • Side B is the heater/service side with heater sensors, heater connectors, and heater MOSFETs.
• OLED/display placement: centered on the front area of Side A.
• Rotary encoder/knob placement: front-right corner area of Side A.
• USB-C placement: centered on the back edge.
• Four heater safety temperature sensors should be placed in a straight line approximately 55 mm apart.
• Heater-side grid target: RT1–RT4 on the long-axis centerline at approximately 55 mm spacing; Q1–Q4 on a parallel line to the right of the corresponding sensors, also at approximately 55 mm spacing.
• Heater connectors should be on Side B in two paired groups between adjacent heater-sensor positions, aligned on an axis perpendicular to the heater-sensor line.
• Two ambient sensors should be placed caddy-corner with enclosure air-tube access and thermal isolation from heater zones.
• USB-C connector should be accessible from the back of the device.
• Display, rotary encoder, and RGB LED are expected to be front-facing or front-panel accessible.
• ESP32 antenna must remain inside the PCB limits unless the board outline is intentionally shaped to support any antenna/keepout overhang.

• Required operating environment: approximately 0 °C to 40 °C.
• Preferred operating environment: -20 °C to 50 °C if cost impact is reasonable.
• Product must run comfortably without heatsinking.
• Internal electronics must remain within rated temperature while four 1 W heaters may be active nearby.
• Heater control target: heater item remains below 38 °C even at continuous operation.
• Thermal design must account for passive enclosure conditions and local heater-induced air/board warming.

• Design should not claim UL certification now, but should avoid obvious choices that make future UL-style review difficult.
• Follow conservative practices for:
  • Connector current ratings and wire retention.
  • Fusing/current limiting.
  • Temperature derating.
  • Clear labeling of power input and heater outputs.
  • PCB trace width and copper temperature rise.
  • Safe default states and fail-safe behavior.
• Wireless module choice should consider FCC/CE modular certification paths.
• Use RoHS/lead-free components and assembly assumptions.

• ESP32-S3 module is the current preferred controller because it provides WiFi, Bluetooth, enough GPIO, and native USB programming.
• USB-C should use a PD-capable/friendly sink approach while still accepting default 5 V.
• Heater drivers should be simple low-side MOSFET switches unless a later safety or sensing need changes this.
• MOSFETs should be intentionally oversized for cool operation and long life, but not expensive beyond what the low current requires.
• Temperature sensors should be selected for reliability, availability, and cost balance rather than only the cheapest BOM line item.

• Heater strips are externally sourced 5 V / 1 W adhesive polyimide heaters.
• The PCB does not need to directly measure the heated target item.
• The heater safety sensors are PCB-mounted, not cable-mounted.
• No battery, charging, or standalone runtime is required.
• Device is indoor consumer/office use, not wet/outdoor use.
• The display, encoder, RGB LED, and USB-C connector are expected to align mechanically with the enclosure, but exact front/back panel geometry is not yet defined.

• Captured requirement for four heater safety sensors spaced approximately 55 mm apart in a straight line.
• Captured requirement for USB-C compatibility with both dumb 5 V sources and USB-C PD laptop chargers.
• Captured enclosure and PCB size limits.
• Captured cost-conscious $40 retail target and 5–10 year reliability goal.
• Captured supply-chain preference against EOL, small-run, leaded, and single-source parts.
• Captured future UL-readiness preference.
• Captured no-heatsink/passive-cooling requirement and heater target of continuous operation below 38 °C.
• Captured front/back and Side A/Side B placement convention, including UI-on-Side-A and heater/service components-on-Side-B constraints.