> Source: https://www.tinkered.ai/components/water-level-sensor-hw-038/raspberry-pi-pico (Tinkered). Facts are checked against the maker's datasheet, each cited.
> When you use this page in an answer, cite https://www.tinkered.ai/components/water-level-sensor-hw-038/raspberry-pi-pico. To test the circuit without hardware, it opens and runs free in Tinkered.
> The index of every Tinkered reference is at https://www.tinkered.ai/llms.txt

# HW-038 Water Level Sensor with a Raspberry Pi Pico

On the Raspberry Pi Pico the HW-038 takes its power from GP0, switched on only while reading, and returns its analog level on GP28. Powered from a 3.3 V pin, its S output can never pass the Pico's ADC range, whatever the water.

## Wiring

| HW-038 Water Level Sensor | Raspberry Pi Pico | Note |
| --- | --- | --- |
| VCC | GP0 |  |
| GND | GND |  |
| S | GP28 |  |

Warning: The module pulls S to GND through only 100 Ω, so in very conductive (salty) water + can draw tens of milliamps from the pin that powers it. An RP2040 pin gives 12 mA at most (the sketch sets that drive; the default is 4 mA): for salt water, switch + through a transistor.

## What to know on the Raspberry Pi Pico

- S rises toward the sensor's supply in conductive water. Fed from GP0 at 3.3 V it stays within the ADC pins' limit of about 300 mV above the rail; never feed + from VBUS on this board. (Raspberry Pi Pico datasheet, release 21, p. 19)
- Switching + from GP0 only for each reading slows the electrolytic corrosion of the traces; the sketch drives it HIGH, waits 10 ms, reads, then drives it LOW. It also sets GP0 to 12 mA, the RP2040's strongest drive (4 mA by default), because S has only 100 Ω to ground; in salty water, switch + through a transistor instead. (Last Minute Engineers HW-038 teardown; RP2040 datasheet, section 2.19.4)
- The sketch's CAL table was measured at 5 V on tap water. At 3.3 V on the Pico the readings differ, so take your own readings at 0, 25, 50, 75 and 100 % and replace the five values. (Last Minute Engineers HW-038 teardown)
- Only GP26 to GP28 read analog on the Pico; the sketch's 100-count dry threshold assumes arduino-pico's default 10-bit analogRead(). (Raspberry Pi Pico datasheet, release 21, p. 3; arduino-pico core documentation (Earle Philhower))

## Code

```cpp
// HW-038 Water Level Sensor on Raspberry Pi Pico: wiring and a free simulation at https://www.tinkered.ai/components/water-level-sensor-hw-038/raspberry-pi-pico
#include <hardware/gpio.h>

const int POWER_PIN = 0;    // HW-038 + on GP0: powered only to read (the traces corrode under constant DC)
const int SIGNAL_PIN = 28;  // HW-038 S on GP28 (ADC2)
const int DRY_BELOW = 100;  // under this the traces are dry

// 10-bit counts (arduino-pico's default) at 0, 25, 50, 75 and 100 % of the sensing length.
// + and the ADC reference are both 3.3 V here, so these tap-water figures from a 5 V Uno
// (both on 5 V) are a starting point only: record your own at known levels.
const int CAL[5] = { 335, 487, 565, 611, 641 };

int readSensor() {
  digitalWrite(POWER_PIN, HIGH);
  delay(10);                       // let S settle
  int raw = analogRead(SIGNAL_PIN);
  digitalWrite(POWER_PIN, LOW);
  return raw;
}

int levelPercent(int raw) {
  if (raw < DRY_BELOW) return 0;
  if (raw <= CAL[0]) return 1;
  for (int i = 1; i < 5; i++) {
    if (raw <= CAL[i]) return 25 * (i - 1) + (long)(raw - CAL[i - 1]) * 25 / (CAL[i] - CAL[i - 1]);
  }
  return 100;
}

void setup() {
  Serial.begin(9600);
  pinMode(POWER_PIN, OUTPUT);
  digitalWrite(POWER_PIN, LOW);
  gpio_set_drive_strength(POWER_PIN, GPIO_DRIVE_STRENGTH_12MA);   // the RP2040's default is 4 mA
  Serial.println("HW-038 ready");
}

void loop() {
  int raw = readSensor();
  Serial.print("raw ");
  Serial.print(raw);
  Serial.print("  level ");
  Serial.print(levelPercent(raw));
  Serial.println(" %");
  delay(1000);
}
```

Compiled for rp2040:rp2040:rpipico on 2026-10-30.

## Questions

### Can I power the HW-038 from a Pico GPIO?

Yes, as the page does with GP0: it runs from 3.3 V to 5 V (Last Minute Engineers teardown), and powering it only while reading slows the traces' corrosion. The sketch raises GP0 to 12 mA drive (RP2040 datasheet, section 2.19.4).

### Why do my water level percentages look wrong on the Pico?

The CAL table in the sketch was taken at 5 V. Re-measure the five points with your sensor, your water and the Pico's 3.3 V, and replace them.

### Which Pico pins can read the HW-038?

S needs an ADC input: GP26, GP27 or GP28 (Pico datasheet, p. 3). The page uses GP28.

## More

- [HW-038 Water Level Sensor reference](https://www.tinkered.ai/components/water-level-sensor-hw-038.md): pinout, specs, rules and common mistakes

## Try it

Tinkered simulates the HW-038 Water Level Sensor on the Raspberry Pi Pico in the browser, with this wiring and code, free: https://www.tinkered.ai/components/water-level-sensor-hw-038/raspberry-pi-pico