The passive buzzer is represented by a light bulb. It runs on 5 V, while the ESP32 pin drives the base of the transistor with 3.3 V. The base resistor protects the GPIO pin from a short circuit and limits the base current. The series resistor before the buzzer limits the current through the transistor and also affects the buzzer’s volume. A flyback diode must be placed in reverse polarity across the buzzer to suppress inductive voltage spikes.
