What You'll Build
A proximity alarm that mimics the parking sensors built into modern cars. The HC-SR04 emits an ultrasonic pulse and measures the echo time to calculate distance. Your Arduino maps that distance to a buzzer tone — slow beeps at 50 cm, rapid beeps at 15 cm, and a solid tone inside 5 cm.
The whole circuit needs no libraries and uses just six wires. It's a reliable first project that teaches pulse timing, pulseIn(), and analog-to-digital mapping in about 40 lines of C++.
💡 Tip
The HC-SR04 runs on 5V and its ECHO pin outputs 5V logic. The Arduino Uno's digital pins are 5V-tolerant, so you can connect ECHO directly. On a 3.3V board (Nano 33, Pico) you'd need a voltage divider.
Components
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Wiring
Keep the connections simple:
| HC-SR04 Pin | Arduino Pin |
|---|---|
| VCC | 5V |
| TRIG | D9 |
| ECHO | D10 |
| GND | GND |
Buzzer + → D8. LED anode → D7 through a 220 Ω resistor to GND.
Live simulation
Steps
- 1Assemble the circuit on a breadboard following the wiring table above
- 2Open Arduino IDE 2, select board: Arduino Uno, and the correct COM port
- 3Paste the sketch and click Upload
- 4Open Serial Monitor at 9600 baud to watch live distance readings
- 5Move your hand toward the sensor — the buzzer pattern should change at each threshold
- 6Tweak WARN_CM / CLOSE_CM defines to match your use case
Going Further
- Add a 16×2 I2C LCD to display the numeric distance instead of (or alongside) the Serial Monitor
- Replace the active buzzer with a passive one and use the
tone()frequency to create a musical scale — lower pitch for far, higher for near - Mount the sensor in a 3D-printed bracket and power from a 9V battery for a portable device
- Combine two HC-SR04s (front and rear) on D9/D10 and D6/D7 for a dual-axis parking assistant
