Simple but very fun idea! For a sudoku novice like me beating even the easiest board was pretty hard at just 5 minutes. The sound effects and blink animation made it feel quite tense but the lack of pay-off when you run out of time was a bit disappointing.
I think you should consider increasing the time a bit but subtracting some of it whenever you input a wrong number - right now there doesn’t really seem to be any real punishment for doing so which makes trying random numbers a valid strategy.