What the interviewer wants
- Hardware/firmware block diagram and task structure
- Power budget for one year
- Authentication of unlock commands and the motor state machine
Constraints
- Nordic-class BLE SoC (Cortex-M4F), 512 KB flash, 64 KB RAM
- 4 × AA alkaline (about 2,500 mAh usable)
- Typical use: 10 unlocks a day, each followed by an automatic re-lock; keypad wakes on touch
- Motor draws 600 mA for 0.5 s per operation; stall current 1.5 A
- Mechanical key override exists
If you ask the interviewer
- Can the lock reach the internet? Only through the paired phone. No Wi-Fi.
- What if batteries die? The mechanical key works. Warn users well before that.
- Who can unlock? The owner and up to 20 guests, who may have time-limited access.
How it's graded
The rubric a senior interviewer would use. Each item is scored 0 to 3 and weighted.
| Power budget | weight 3 |
| Unlock authentication | weight 3 |
| Motor control and safety | weight 3 |
| Firmware structure | weight 2 |
| Updates and lifecycle | weight 1 |
| Validation | weight 2 |
How to structure your answer
- Requirements and assumptions: the numbers that drive the design.
- Architecture: blocks, tasks, data flow, memory.
- Key decisions and trade-offs, with the alternatives you rejected.
- Failure modes and recovery.
- Testing and validation.
More design rounds
Write your design in five sections and get it graded against this rubric, with a follow-up question from the AI interviewer.
Answer this round