The amazon device is well designed. There is a switch to start the motor (1) when the pencil is inserted, and also an end switch (2) to stop it when the pencil tip reaches the bottom. A kind of logic makes you have to remove the pencil to reset.

The ridiculously powerful replacement motor is a brushless inrunner (2435 4800KV). It fits almost exactly in the space of the original motor. Of course an additional case has to be printed to hold the ESC and the battery. The batteries are 2x 18650 Li-ion rated for high current discharge. I didn’t properly measured the output current, however even at half of their claimed 20A, we should have a decent amount of power to sharp a pencil.

To drive the electronic speed control module, we need a servo PWM signal. I choose a small PIC16F15313 microcontroller, and used 2 of its timers to implement a 50Hz signal, with a pulse length from 1 to 2ms. Conveniently, the ESC is providing the 5v, no need for extra voltage regulator.

The microcontroller also replicates the sharpener logic by reading the 2 switches. And generates ramp up and ramp down signal for the motor for a smooth and satisfying operation.

Files for the PIC and the case are here : https://github.com/pierre-muth/pencilSharpener