What you'll learn

Switching an active buzzer with a transistor
Reading a button with a pull-up
Detecting a change in state
Why a GPIO pin should not drive a load directly

Difficulty: Beginner
Time: About 20 minutes
Board: ESP32-DevKitC V4
Parts: 9

How it works

The button on GPIO 4 has a 10k resistor to 3.3 V. The pin reads 1 until you press, then reads 0. An active buzzer has its own tone generator, so it only needs power. A transistor switches that power: GPIO 13 drives its base through a 1k resistor.

The program checks the button every 10 ms. While you hold it, GPIO 13 goes to 1, the transistor turns on and the buzzer sounds. Let go and it goes quiet. The code also prints "ding" when the sound starts and "quiet" when it stops, but only when the state changes.

Parts list

9 parts, plus the jumper wires. They retail for about $10.75; on Mokxi they're free.

1 × ESP32-DevKitC V4
1 × BuzzerIn Starter Kit, Rocket Flight Computer Kit.
1 × Resistor, 1k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × Transistor (NPN or PNP)In LED Cube Kit.
2 × Resistor, 10k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × PushbuttonIn Starter Kit, Christmas Lights Kit, Logic Lab Kit.
1 × Power, 5 V
1 × Power, 3.3 V

Kits that can build it

These 3 starter kits have every part in the circuit. Each kit's page lists what else it can build.

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pin	Connects to
Resistor, 1k Ω pin 1	ESP32-DevKitC V4 pin 13
Resistor, 10k Ω (2) pin 1	ESP32-DevKitC V4 pin 4
Buzzer pin 1	5 V
Transistor (NPN or PNP) pin C	Buzzer pin 2
Transistor (NPN or PNP) pin B	Resistor, 1k Ω pin 2
Transistor (NPN or PNP) pin E	Ground
Resistor, 10k Ω (1) pin 1	3.3 V
Pushbutton pin 1a	Resistor, 10k Ω (1) pin 2
Resistor, 10k Ω (2) pin 2	Resistor, 10k Ω (1) pin 2
Pushbutton pin 2a	Ground

The code

The MicroPython code exactly as the editor opens it. Change a line there and press Run: the board runs it on the official MicroPython firmware, with nothing to compile.

main.py · ESP32-DevKitC V4

# Doorbell: hold the button on GPIO 4 and the active buzzer sounds. The
# buzzer is switched by an S8050 transistor from GPIO 13.
from machine import Pin
import time

buzzer = Pin(13, Pin.OUT, value=0)
button = Pin(4, Pin.IN, Pin.PULL_UP)
was = None

while True:
    ringing = button.value() == 0
    buzzer.value(1 if ringing else 0)
    if ringing != was:
        print("ding" if ringing else "quiet")
        was = ringing
    time.sleep_ms(10)

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

The buzzer never sounds This project
Check the transistor wiring: GPIO 13 through the 1k resistor to the base, the buzzer to the collector, and the emitter to ground.
It sounds without pressing, or never stops This project
Check the 10k resistor from the button line to 3.3 V, and that the button's other side goes to ground.
The buzzer makes a constant tone, or you cannot change its pitch. Buzzer
An active buzzer has its own oscillator, so it only beeps on and off. A passive buzzer needs tone() to set pitch. Check which type you have.
The buzzer is silent or very quiet. Buzzer
Check polarity: the plus mark goes to the pin or supply. For loud buzzers, drive them through a transistor instead of straight from a pin.
The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
The load never turns on, or is always on, because the legs are in the wrong order. Transistor (NPN or PNP)
Check the pinout in the datasheet for your part. A 2N2222 or BC547 can have emitter, base, collector in a different order. Flat side orientation is easy to get wrong.
The base is wired straight to the pin with no resistor, or the motor has no flyback diode. Transistor (NPN or PNP)
Put about 1k between the pin and the base. For motors and relays, add a diode across the coil with the stripe toward the positive side.