• Installed inside a small portable smart-film sample kit.
• Powered from a common USB battery bank or 5 V USB adapter.
• No external power button required; the demo starts automatically when USB power is connected.
• High-voltage controls are not user-accessible.
• Prototype should support voltage/frequency testing before locking the production configuration.

• Accept 5 V DC through USB-C without USB Power Delivery negotiation.
• Present itself as a USB-C sink using CC pull-down resistors.
• Generate a controlled bipolar AC waveform for PDLC film.
• Default to 1.0 s film energized / clear and 1.0 s film de-energized / frosted.
• Recover automatically after power loss or brownout.
• Avoid long-term DC bias across the film.
• Include a passive output discharge resistor and optional active discharge path.
• Include a configurable USB battery-bank keep-alive load.
• Indicate status using one LED, with an option to disable it.

• USB-C 5 V input, target input current up to about 2 A.
• Input fuse/resettable fuse, reverse-current protection, TVS/transient protection, filtering, and local decoupling.
• 3.3 V logic rail for MCU and control circuitry.
• MCU-controlled high-voltage enable, H-bridge timing, keep-alive load, status LED, configuration jumpers, watchdog, and safe reset behavior.
• Regulated high-voltage DC bus generated from 5 V by a compact high-frequency isolated or safely separated converter.
• High-voltage H-bridge output to create a bipolar square wave.
• Selectable output: 60 / 70 / 80 VAC RMS, optional development-only 90 VAC RMS.
• Selectable output frequency: 60 / 75 / 100 Hz.
• Safe high-voltage discharge paths and protected test points.

Diagram

• USB-C receptacle with 5.1 kΩ pull-down resistors on CC1 and CC2.
• Input protection sized for up to approximately 2 A available input current.
• Reverse-current protection should use a low-loss ideal-diode/load-switch approach rather than a simple series diode if voltage headroom allows.
• Add USB input TVS, bulk capacitance, ceramic decoupling, and EMI filtering.

• 3.3 V regulator for MCU, status LED, configuration straps, and low-voltage control logic.
• Brownout behavior must hold the HV converter and H-bridge off until firmware is initialized.

• Small low-cost MCU with internal oscillator, watchdog, brownout detection, enough GPIO for H-bridge control, HV enable/fault, keep-alive switch, active discharge, status LED, and configuration pins.
• Compact programming pads, not exposed as user controls.

• MCU-switched resistor load on the 5 V rail.
• Initial target: 75–150 mA pulse for 100–250 ms every several seconds.
• Include solder jumper to enable/disable.
• Load resistor must be pulse-power and temperature rated.

• Preferred: high-frequency flyback or equivalent transformer-based converter from protected 5 V to regulated high-voltage DC bus.
• Must include primary current limiting, snubber/clamp, bus feedback/regulation, overvoltage clamp/shutdown, and discharge resistor.
• Converter must not allow uncontrolled no-load bus rise when film is disconnected.

• Low-power HV H-bridge creates a bipolar square-wave output at 60/75/100 Hz.
• Firmware must enforce dead time and never permit sustained DC across the film.
• MOSFET gates must have pull-down/off-state resistors so reset defaults to off.

• Output connector or pads must not be exposed pin headers.
• Passive discharge across output: initial range 100 kΩ to 220 kΩ, preferably two series resistors for voltage sharing.
• Optional active switched discharge path only during the frosted/off phase; it must be interlocked so it cannot short the energized H-bridge.

• USB-C input connector.
• Two smart-film HV output terminals or compact locking connector.
• MCU programming pads.
• Test points: GND, protected 5 V, HV DC bus, output waveform. HV test points must be physically isolated from USB, mounting holes, and user-accessible edges.
• Solder jumpers/configuration pads for voltage, frequency, mode, keep-alive enable, LED enable, and active discharge option.

• Source: standard 5 V USB battery bank; no USB PD negotiation.
• Normal target input power: under 5 W.
• Maximum expected input current capability: approximately 2 A.
• The power bank may shut down at low current, so the design includes a pulsed keep-alive load.

• Compact PCB target: about 35 mm × 50 mm if creepage/clearance and manufacturability allow.
• At least 2 layers; 4 layers may be recommended if it improves EMI/noise containment or compactness.
• Prioritize common, available components and manageable packages for repeatable assembly.
• Include test points for production bring-up, but keep HV test points isolated and clearly marked.

• Safe startup: all HV outputs off until initialization completes.
• Wait for HV bus stabilization before energizing film.
• Default automatic cycle: 1 s on, 1 s off, repeat indefinitely.
• Optional continuous clear mode and 3 s cycle timing.
• Generate 60/75/100 Hz H-bridge drive with controlled dead time and symmetric duty.
• Watchdog and brownout handling must shut down HV safely.
• Read solder-jumper configuration at boot.
• Control keep-alive load pulses.
• Control optional active film discharge only during the off phase.
• Provide normal and fault LED patterns.

• Separate regions: USB/low-voltage, flyback switching, HV DC bus, AC output.
• Maintain conservative spacing for all HV nodes; add slots where useful.
• Silkscreen high-voltage boundary and warnings.
• Keep HV away from USB connector, programming pads, mounting holes, and board edges that may be touched.
• Avoid exposed high-voltage pin headers.

• Default waveform: bipolar square wave unless film testing indicates sine/modified waveform is needed.
• Default production setting: 70 VAC RMS at 75 Hz.
• Use firmware-controlled automatic cycling rather than a physical user button.
• Use fixed configuration jumpers/pads instead of a user potentiometer.
• Include both passive output discharge and optional active discharge to improve frosted-state return time.
• Treat 90 VAC RMS as development-only, not production default.

• PDLC film area is up to about 1 ft².
• The film behaves primarily as a capacitive load.
• The output is not directly user-touchable in the final sample kit.
• USB source provides 5 V and can support normal load under 5 W.
• Final film capacitance and voltage limit will be verified with the actual film supplier/sample.

• Starts automatically when USB power is applied.
• Clear/frosted cycle timing is 1.0 s / 1.0 s by default.
• H-bridge output has no meaningful DC bias over time.
• Output RMS settings measure close to 60/70/80 VAC options under intended film load.
• HV bus remains regulated and safe under film disconnected/no-load condition.
• Film returns to frosted state quickly enough with the selected discharge option.
• Input current remains compatible with common USB banks.
• Keep-alive prevents target USB power banks from shutting down.
• No accessible HV controls or exposed HV headers.

• Initial specification created from pasted user requirements on 2026-07-14.
• Project renamed from random draft name to USB PDLC Film Controller.