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Lucid Software·Product Manager·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Got a product design question at Lucid Software that was basically a hardware-constrained UI problem. Short prompt, not much context given, but there was a lot to unpack once you started thinking about it.

Questions Asked (1)

Q1

Design a system for reloading bus transit cards using an ATM-style interface with an LCD screen that displays 5 lines of text, each with a physical button on the left side.

System DesignProduct Sense & IdeationTechnical Trade-offs
Author's notes

The hardware constraint is what makes this interesting and also what I kind of fumbled at first.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the user, context, and constraints (e.g., kiosk vs. ATM, payment methods, security). Then outline the end-to-end user flow and system components, emphasizing the unique 5-line/5-button interface and how it maps to user needs. Finally, discuss trade-offs (e.g., simplicity vs. security, offline vs. online) and success metrics.

Pro tip: Treat the 5-line/5-button interface as a feature, not a limitation—design for clarity and speed, and consider accessibility (e.g., audio prompts, tactile buttons) to stand out.

1. Clarify Requirements and Constraints

Ask questions to understand the target users, environment (e.g., indoor/outdoor), payment methods, security needs, and integration with existing transit systems. Define success metrics (e.g., transaction time, error rate).

2. Design the User Flow

Map the end-to-end journey: card insertion, amount selection, payment, confirmation, and receipt. Optimize for the 5-line/5-button interface by grouping options logically and minimizing steps.

3. Define System Components and Interactions

Outline hardware (card reader, cash/coin acceptor, card dispenser, printer) and software (UI, payment processing, backend integration). Describe how the LCD and buttons interact with the system state.

4. Address Technical Trade-offs and Edge Cases

Discuss trade-offs like online vs. offline operation, security (encryption, fraud prevention), and error handling (e.g., card jam, payment failure). Consider scalability and maintenance.

5. Prioritize Features and Define MVP

Prioritize features based on user needs and business goals (e.g., reload only vs. new card issuance). Define an MVP and a roadmap for future enhancements (e.g., mobile integration, loyalty programs).

Key Points to Mention

  • User-centered design: ensure the 5-line/5-button interface is intuitive, with clear labels and feedback (e.g., beeps, lights).
  • Security: PCI compliance for payment, encryption of card data, and physical tamper resistance.
  • Reliability: handle network outages with offline mode, and ensure robust error recovery.
  • Accessibility: include audio instructions, braille, and adjustable screen contrast for visually impaired users.
  • Integration: APIs to connect with transit backend for balance updates and transaction logging.
  • Metrics: define KPIs like average transaction time, success rate, and user satisfaction to measure success.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.