Ultrasonic Distance Sensor

What it is

A 40 kHz burst goes out, bounces off whatever is in front of the board, and comes back. The time between is proportional to distance: halve it, multiply by the speed of sound, and you have centimetres.

What makes this board different from the sensor it is named after is the chip doing that work. An RCWL-9616 runs from 3 V to 5 V and can hand you the result four different ways, where the classic HC-SR04 is a 5 V part with one interface. The board prints its own supply range over the VCC hole: 3-5V.

There is no regulator and no level shifter here. VCC goes straight to the chip, and both signal pins are chip pins brought out to the header, so the logic level is whatever you feed the supply. On an ESP32 that means the 3V3 rail and four plain wires — no divider.

The four modes

Two solder jumpers on the left edge of the back, J1 and J2, decide which protocol the sensor speaks. Both arrive open, which is GPIO: trigger and echo, the way an HC-SR04 works.

The table is printed on the board itself. J2 is the upper pad.

A third jumper, printed RANGE, sits at the right. It shorts out a 2.2 kΩ resistor between the chip and one transducer, and it ships open. The chip maker's note puts the typical maximum at 3 to 5 m with the resistor and 5 to 6 m without it, for its own transducers; nobody has measured this board either way.

Pinout

Left to right on the back, pins pointing down. GND is the square pad.

Wiring

Four wires. GND to GND, VCC to 3V3 on a 3.3 V board or 5V on an Uno, and the two signals to any two pins. The four pads you can see on the front are the same header holes from the other side, so the order reads backwards there.

Example

GPIO mode, no library. Note the timeout on pulseIn: without it a missing echo blocks the sketch for a full second and then reads as zero centimetres.

#define TRIG 9
#define ECHO 10

void setup() {
  Serial.begin(115200);
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  digitalWrite(TRIG, LOW);
}

void loop() {
  // The supported library holds TRIG high for 500 us, so this does too.
  // Every HC-SR04 example uses 10 us.
  digitalWrite(TRIG, HIGH); delayMicroseconds(500);
  digitalWrite(TRIG, LOW);

  // Time out at ~4 m of round trip. Without a limit, a missing echo blocks
  // for a full second and then reads as a valid measurement of nothing.
  long us = pulseIn(ECHO, HIGH, 25000UL);

  if (us == 0) {
    Serial.println("no echo");        // absorbed, angled away, or out of range
  } else {
    Serial.print((us / 2.0) * 0.0343, 1);   // cm at about 20 C
    Serial.println(" cm");
  }

  delay(100);   // A measurement cycle is 50 ms. Let the last burst die away.
}

from machine import Pin, time_pulse_us
import time

trig = Pin(9, Pin.OUT)
echo = Pin(10, Pin.IN)

while True:
    trig.off(); time.sleep_us(2)
    trig.on();  time.sleep_us(500)
    trig.off()

    us = time_pulse_us(echo, 1, 25000)   # returns negative on timeout
    print("no echo" if us < 0 else "%.1f cm" % ((us / 2) * 0.0343))

    time.sleep_ms(100)

Over a bus

In I2C or UART mode the chip does the timing and the arithmetic, and hands back three bytes holding the distance in micrometres, most significant byte first. Divide by 10 000 for centimetres.

The command is one byte — 0x01 over I2C at address 0x57, or 0xA0 over UART at 9600 baud — then a wait of about 150 ms, then three bytes back. There is no register map and no status flag. Those numbers are the supported library's, written for the RCWL-9610A on the earlier board; the RCWL-9616's maker names the same four modes on the same pins.

Where to start

The handbook below takes the part from the first reading to the four modes and what it can honestly see. If you only read one page, make it the honest range: the 4 m on the box and the 2–3 m you should design to are both true, and the difference is what surfaces do to 40 kHz sound.