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Electronics interview questions

Electronics questions a Embedded / hardware can be asked, from first job to senior. Open one to see the strongest answer and why it works.

  1. An LED runs from a 3.3 V pin and its datasheet says it drops about 2 V at the current you want. How do you size the series resistor?

    The strongest answer: The resistor takes the leftover 1.3 V, so its value is 1.3 volts divided by the current I want; then I round up to a standard value and check the pin can source that current

    Why it works: Right — subtract the drop, apply Ohm’s law, then sanity-check the pin’s current limit, which is the step people skip.

    • The resistor sees supply minus the LED drop.
    • R is volts across it over current.
    • Round up to a standard value.
    • Check the pin’s current limit.

  2. A reset pin “randomly” browns the board. First hardware question?

    The strongest answer: Is it floating — pull-up/down, solder, and the reset source — I do not start by rewriting the scheduler

    Why it works: Exactly. Floating resets are electrical. Mutexes do not solder a resistor.

    • Check float vs pull.
    • Look at solder and the reset source.
    • Do not start in the scheduler.
    • Do not leave reset floating.

  3. The board “reboots when the motor starts.” First electrical thought?

    The strongest answer: Supply sag / brownout — I look at rails, decoupling, and the brownout detector, not a new state machine first

    Why it works: Exactly. Motors kick rails. Disabling BOD is how you run on garbage and corrupt flash.

    • Suspect rail sag with motors.
    • Check decoupling and BOD.
    • Do not start in the state machine.
    • Do not disable brownout detect.

  4. In reset, before your code runs, a GPIO drives a heater FET. What should you have designed?

    The strongest answer: A hardware default that is safe — pull that keeps the FET off — software is late to the party

    Why it works: Exactly. Power-up is hardware. Faster C does not beat the pad default.

    • Safe hardware default on actuators.
    • Software starts too late.
    • Do not race main() against a FET.
    • Do not default heaters on.

  5. A 5 V sensor output goes straight into a 3.3 V MCU pin. What is your concern, and what do you do?

    The strongest answer: Unless that pin is rated 5 V tolerant, 5 V pushes current into its protection diode and damages it; I check the datasheet, then add a level shifter, or a divider if the signal is slow

    Why it works: Right — tolerance is a per-pin property in the datasheet, and the other direction matters too: 3.3 V may not reach a 5 V input’s high threshold.

    • Check pin voltage tolerance in the datasheet.
    • 5 V into a 3.3 V pin needs shifting.
    • A divider only suits slow signals.
    • Check the 5 V input’s high threshold too.

  6. A low-side MOSFET switching a motor gets hot, even though the datasheet promises a very low on-resistance.

    What do you look at?

    The strongest answer: The gate drive. On-resistance is quoted at a gate voltage, so 3.3 V into a part that wants 10 V leaves it half on; I check that rating, the drive level and how fast the edges are

    Why it works: Exactly — and slow edges heat it too, because it spends longer in the linear region on every switch.

    • On-resistance is quoted at a gate voltage.
    • Match the FET to your drive level.
    • Keep edges fast enough to leave linear.
    • Pulldown on the gate for reset.

  7. ADC readings wander with the rail. What did the design forget?

    The strongest answer: A reference and a layout that respects it — I do not “calibrate in software” a floating AVDD and call it done

    Why it works: Exactly. A reference. Bits and averages of a wandering rail are prettier wander.

    • Give the ADC a real reference.
    • Software cannot invent a rail.
    • More bits ≠ a reference.
    • Do not average a sag and ship it.