Generic · Integrated circuit

HC-SR04 Ultrasonic Rangefinder

Ping-and-listen distance sensor, 2 cm to 4 m. Four pins, 5 V only, and an echo pin that will happily feed 5 V into a 3.3 V board.

Cheat sheet (SVG)

Logic level: 5 V
Package: 4-pin 2.54 mm header

1.Pinout diagram

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Header, front view

HC-SR04

Pin detail

Hover a pin for its mappings and gotchas. Click to keep it here.

Logic level: 5 V
Package: 4-pin 2.54 mm header

Fig. 1. HC-SR04 Ultrasonic Rangefinder, 4-pin 2.54 mm header. Hover or tap a signal to light up every pin that carries it.

2.Watch out

ECHO drives a 5 V pulse. Into an ESP32, a Pico or any 3.3 V board that needs a divider, two resistors is enough. This is the most common way the sensor kills a GPIO.
Trigger with a 10 us high pulse, then time how long ECHO stays high. Distance in cm is that time in microseconds divided by 58, because sound makes the round trip twice.
Below about 2 cm it cannot distinguish the echo from its own transmit burst, so it returns nonsense rather than a small number. Treat anything under the blind zone as invalid.
It needs a reasonably flat target square to the sensor. Soft, angled or narrow objects scatter the ping and read as out of range.
If ECHO never goes low because nothing came back, a naive pulseIn blocks. Always use a timeout.
The HC-SR04P and the RCWL-1601 are pin-compatible and run from 3 to 5.5 V, which removes the level shifting problem entirely. Worth buying instead.
Readings drift with temperature: the speed of sound changes about 0.17 percent per degree C. Over a 30 degree swing that is a few centimetres at 4 m.

3.Level shifting the echo

What has to go around the chip before it will run.

Fig. 2. Level shifting the echo, drawn from Table 1. Dashed parts are optional. Crossing wires are not connected.

Table 1. External parts, and why each one is there.
Part	Between	Why
1 kOhm	ECHO (3) to MCU	In series with the 5 V echo output, the top half of a divider into a 3.3 V input.
2 kOhm	MCU to GND	The lower half. 5 V across 1k and 2k puts 3.3 V on the junction, which is what the input sees.
100 uFoptional	VCC (1) to GND	Across the supply at the module. Each ping pulls a 15 mA burst and a long thin lead sags under it.

TRIG is an input and tolerates a 3.3 V drive, so only ECHO needs the divider. The module has no 3.3 V variant worth trusting: the ones sold as 3 V parts are usually the same board with a different regulator.

4.Full pin reference

Every pin, alias and mapping. This is the table that prints.

Table 2. Pin assignments.

Complete pin reference for HC-SR04 Ultrasonic Rangefinder
Pin	Name	Also called	Functions	Notes
1	VCC5 V	-	Positive supply	5 V. The sensor draws a 15 mA burst on each ping.
2	TRIG	-	Trigger input	10 us high pulse starts a measurement.
3	ECHO	-	Echo output	Outputs 5 V. Divide it down for a 3.3 V board.
4	GND	-	Ground

Primary sources

HC-SR04 datasheet ↗

Found a wrong pin? The whole board is one file, data/boards/hc-sr04.json.

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