Final answer:
The error in the student's sentence is a run-on or comma splice. The sentence should be corrected with either a semicolon or the addition of a coordinating conjunction such as 'and'.
Step-by-step explanation:
The sentence given by the student contains a run-on error, also known as a comma splice, where two independent clauses are incorrectly joined by a comma without a coordinating conjunction. The corrected sentence should include a conjunction like "and" to connect the clauses properly or use a semicolon to separate them if no conjunction is added. Here is the corrected sentence:
Before I go to sleep, I set my alarms; I use two: one digital clock and my smartphone.
Alternatively, adding a coordinating conjunction would look like this:
Before I go to sleep, I set my alarms, and I use two: one digital clock and my smartphone.