Distance
Interfacing HC-SR04 Distance with Raspberry Pi
What is the HC-SR04?
The HC-SR04 measures distance by timing how long it takes a 40kHz ultrasonic pulse to bounce off an object and return. You send it a short trigger pulse, it fires eight ultrasonic bursts, and the ECHO pin stays high for exactly as long as the round trip took — convert that time to distance and you're done. No library is strictly required since the whole protocol is just two GPIO pins and a stopwatch.
It's a great first sensor for obstacle avoidance, parking sensors, liquid-level gauges, and non-contact proximity switches. The catch that trips people up most is voltage: the sensor runs on 5V and its ECHO pin outputs a 5V pulse, which will drive a 3.3V-only board's GPIO out of spec. Every board below except the 5V-native Arduino Uno needs a simple voltage divider on ECHO — see the per-board wiring notes.
Specifications
- Operating voltage
- 5 V DC
- Operating current
- ~15 mA
- Measuring range
- 2cm to 400cm
- Accuracy
- ±3mm
- Measuring angle
- ~15° effective beam angle
- Ultrasonic frequency
- 40 kHz
- Trigger pulse width
- 10 µs TTL pulse to start a measurement
- Interface
- Digital trigger/echo pulse pair — no library or bus protocol required
Pinout
| Pin | Name | Description |
|---|---|---|
| 1 | VCC | Power, 5V DC |
| 2 | TRIG | Trigger input — send a 10µs HIGH pulse to start a measurement |
| 3 | ECHO | Echo output — goes HIGH for the duration of the ultrasonic round trip. Outputs at VCC level (5V) regardless of board voltage. |
| 4 | GND | Ground |
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TRIG is a standard input and is safe to drive from a 3.3V GPIO — most boards read a 3.3V HIGH as a valid trigger. ECHO is the pin that causes problems: it always swings up to the sensor's 5V supply rail, so wiring it straight into a 3.3V-only GPIO (ESP32, Raspberry Pi, STM32) risks damaging that pin over time. Use a simple resistor voltage divider on ECHO — 1kΩ in series with the signal and 2kΩ from the tap point to GND works well — to bring the HIGH level down to a safe ~3.3V before it reaches a 3.3V board.
Variants
- HC-SR04
- ~$1–3
- HY-SRF05
- ~$2–4
- JSN-SR04T
- ~$3–6
- US-100
- ~$3–5
The plain HC-SR04 is the default choice for indoor, dry, breadboard-range projects. If the sensor needs to live outdoors or in a tank, use the waterproof JSN-SR04T instead — same trigger/echo timing concept, but check its longer minimum range before swapping it in. If you want native 3.3V operation without a voltage divider, the US-100 is the more forgiving pick.
Setting Up HC-SR04 with Raspberry Pi
Library
gpiozero by Raspberry Pi Foundation
Preinstalled on Raspberry Pi OS. If missing: sudo apt install python3-gpiozero
Wiring
| Component pin | Raspberry Pi pin | Note |
|---|---|---|
| VCC | Pin 2 (5V) | The sensor needs a real 5V supply to hit its rated range — do not power it from 3.3V. |
| TRIG | GPIO23 (Pin 16) | Safe to drive directly — Pi GPIOs output 3.3V, which the sensor reads fine as a trigger HIGH. |
| ECHO | GPIO24 (Pin 18) | REQUIRED: add a voltage divider (e.g. 1kΩ in series, 2kΩ from the tap to GND) before this pin. Pi GPIOs are NOT 5V-tolerant and the raw ECHO signal can damage them. |
| GND | Pin 6 (GND) | Common ground with the 5V supply is required for the divider math to work. |
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- gpiozero's DistanceSensor already handles the trigger pulse, echo timing, and averaging internally — no manual pulseIn-style timing code needed on this board.
- The Pi's GPIOs are strictly 3.3V — the voltage divider on ECHO in the wiring above is not optional, unlike the Arduino Uno where the sensor and board share a 5V logic level.
max_distanceis in meters and defaults to 1m in gpiozero; set it to the sensor's real 4m range (as above) if you need longer readings, or tighten it if you only care about close objects and want faster timeouts.
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