Ultrasonic Proximity Buzzer

This project builds a proximity alarm that detects nearby objects and sounds a buzzer when they get too close. An HC-SR04 ultrasonic distance sensor measures the distance to objects in front of it, and an Arduino Uno processes the measurements to trigger a piezo buzzer whenever something enters the alarm zone.

The guide includes a complete wiring diagram showing how to connect the distance sensor and buzzer to the Arduino, a full parts list, and ready-to-upload firmware that lets you adjust the alarm distance from 2 to 400 centimeters. Assembly takes just a few minutes and requires no soldering if using a breadboard.

Wiring diagram

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Gather all the parts

Qty	Component
1	HC-SR04 HC-SR04 ultrasonic distance sensor module with separate 40 kHz transmitter and receiver transducers, measuring 2 to 400 cm. A 10 us pulse on TRIG starts a measurement and ECHO stays high for the echo's round-trip time. This HC-SR04+ board runs from 3.3 V or 5 V.
1	Piezo Buzzer Small passive piezo buzzer (TDK PS1240) for beeps, tones and alerts. It has no built-in oscillator, so drive one lead with a 3-30 V square wave from a microcontroller pin using tone() or PWM and connect the other lead to ground. It is loudest around 4 kHz and works from about 2 to 10 kHz; driving both leads in antiphase roughly doubles the volume.

Qty

Component

HC-SR04

HC-SR04 ultrasonic distance sensor module with separate 40 kHz transmitter and receiver transducers, measuring 2 to 400 cm. A 10 us pulse on TRIG starts a measurement and ECHO stays high for the echo's round-trip time. This HC-SR04+ board runs from 3.3 V or 5 V.

Piezo Buzzer

Small passive piezo buzzer (TDK PS1240) for beeps, tones and alerts. It has no built-in oscillator, so drive one lead with a 3-30 V square wave from a microcontroller pin using tone() or PWM and connect the other lead to ground. It is loudest around 4 kHz and works from about 2 to 10 kHz; driving both leads in antiphase roughly doubles the volume.

Assemble it in 3 steps

1. Connect the distance sensor

Unplug USB first. Connect HC-SR04 VCC to Nano 5V and GND to GND. Connect TRIG to D9 (starts a measurement) and ECHO to D10 (returns the distance pulse). Read the printed pin labels on the sensor.

Reversing VCC and GND can damage the sensor.

2. Connect the piezo buzzer

Use a small passive piezo buzzer such as the PS1240, not a loud magnetic buzzer. Connect one lead to D3 (sound signal) and the other to GND.

3. Position the sensor

Point the two round sensor faces toward the area to monitor. The alarm sounds for objects between 2 and 100 cm away and stops when no valid nearby echo is returned. Power the Nano through its USB connector.

Soft, small or angled objects may not return a reliable echo. This is not a safety alarm.

Review all connections

1. Connections between "sonar" and "Arduino"

Function	sonar	Arduino
`power`	VCC	5V
`ground`	GND	GND
`digital`	TRIG	GPIO 9
`digital`	ECHO	GPIO 10

2. Connections between "buzzer" and "Arduino"

Function	buzzer	Arduino
`pwm`	SIG	GPIO 3
`ground`	GND	GND

Deploy the firmware

main.cppOpen in Schematik

#include <Arduino.h>

const uint8_t TRIG_PIN = 9;
const uint8_t ECHO_PIN = 10;
const uint8_t BUZZER_PIN = 3;
// Change this to choose the alarm distance (2 to 400 cm).
const unsigned int ALARM_DISTANCE_CM =110;
bool buzzing = false;

void setup() {
  pinMode(TRIG_PIN, OUTPUT);
  digitalWrite(TRIG_PIN, LOW);
  pinMode(ECHO_PIN, INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  digitalWrite(BUZZER_PIN, LOW);
}

void loop() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  const unsigned long duration = pulseIn(ECHO_PIN, HIGH, 30000UL);
  // Zero means no echo, not an object at zero distance.
  const float distanceCm = duration / 58.0f;
  const bool detected = duration > 0 && distanceCm >= 2.0f &&
                        distanceCm <= ALARM_DISTANCE_CM;
  if (detected != buzzing) {
    buzzing = detected;
    if (buzzing) tone(BUZZER_PIN, 4000);
    else noTone(BUZZER_PIN);
  }
  delay(65); // Leave at least 60 ms between measurements.
}