ACS712 current sensor with ESP32 DevKit V1 — divided output, DC load

The ACS712 measures current with a Hall-effect sensor: the current passes through a copper path in the chip, and the output voltage moves away from 2.5 V in proportion. It is a common choice for solar, battery and motor projects.

On the ESP32 DevKit V1 the module needs 5 V on VIN, and its output must go through a 10 kΩ / 20 kΩ divider into D35 because it swings above 3.3 V. The sketch measures the zero point at boot, averages 200 calibrated readings, and prints current and power for a 12 V DC load.

Wiring

Board quirks

• OUT idles at VCC/2 (2.5 V) and swings up to about 4.5 V at full current — above the ESP32’s 3.3 V limit. The 10 kΩ / 20 kΩ divider keeps the whole swing between about 0.3 V and 3.0 V, inside the ADC’s accurate window.
• The zero point moves with the 5 V supply (the sensor is ratiometric) and with temperature, so the sketch measures it at boot. Start it with the load switched off.
• A 5 A ACS712 gives 185 mV/A; after the divider that is about 123 mV/A at the pin, and the ESP32 ADC is noisy by ±10–20 mV. Expect about ±0.1 A resolution, even with the 200-sample average. For small currents (under 1 A) use an INA219 on I2C instead.
• D35 is on ADC1 and input-only — right for an analog sensor and unaffected by Wi-Fi, so you can add a power dashboard later.
• This guide is low-voltage DC only. Putting the ACS712’s screw terminals in a 230 V circuit is electrician-only work: the cheap modules have small gaps between the mains and logic tracks. For mains current use a non-invasive CT clamp (SCT-013) or a PZEM-004T installed by an electrician.

Code

// ACS712 current sensor (5 A version) on an ESP32 DevKit V1, measuring a 12 V DC load.
// VCC → VIN (5 V), GND → GND, OUT → 10 kΩ → D35 (GPIO35), 20 kΩ from D35 to GND.
// The divider turns OUT (2.5 V ± 185 mV/A) into 1.67 V ± 123 mV/A at the pin.
// Start it with NO current flowing — the zero point is measured at boot.

constexpr uint8_t CUR_PIN = 35;           // ADC1_CH7, input-only — works with Wi-Fi on
constexpr float DIVIDER = 1.5f;           // (10 kΩ + 20 kΩ) / 20 kΩ
constexpr float MV_PER_A = 185.0f;        // 5 A: 185, 20 A: 100, 30 A: 66 (read the chip marking)

float zeroMv = 2500;  // OUT at 0 A, measured in setup()

float readOutMv(int samples) {
  uint32_t sum = 0;
  for (int i = 0; i < samples; i++) sum += analogReadMilliVolts(CUR_PIN);
  return sum / (float)samples * DIVIDER;   // millivolts at the module's OUT pin
}

void setup() {
  Serial.begin(115200);
  delay(500);
  zeroMv = readOutMv(500);
  Serial.printf("ACS712 on GPIO35 — zero at %.0f mV (keep the load off during start-up)\n", zeroMv);
}

void loop() {
  float mv = readOutMv(200);               // averaging tames the ESP32 ADC noise
  float amps = (mv - zeroMv) / MV_PER_A;
  if (fabsf(amps) < 0.05f) amps = 0;       // below the noise floor of a 5 A sensor
  Serial.printf("OUT %.0f mV  Current %.2f A  (≈ %.1f W at 12 V)\n", mv, amps, amps * 12.0f);
  delay(1000);
}

// ACS712 current sensor (5 A version) on an ESP32 DevKit V1, measuring a 12 V DC load.
// VCC → VIN (5 V), GND → GND, OUT → 10 kΩ → D35 (GPIO35), 20 kΩ from D35 to GND.
// The divider turns OUT (2.5 V ± 185 mV/A) into 1.67 V ± 123 mV/A at the pin.
// Start it with NO current flowing — the zero point is measured at boot.

constexpr uint8_t CUR_PIN = 35;           // ADC1_CH7, input-only — works with Wi-Fi on
constexpr float DIVIDER = 1.5f;           // (10 kΩ + 20 kΩ) / 20 kΩ
constexpr float MV_PER_A = 185.0f;        // 5 A: 185, 20 A: 100, 30 A: 66 (read the chip marking)

float zeroMv = 2500;  // OUT at 0 A, measured in setup()

float readOutMv(int samples) {
  uint32_t sum = 0;
  for (int i = 0; i < samples; i++) sum += analogReadMilliVolts(CUR_PIN);
  return sum / (float)samples * DIVIDER;   // millivolts at the module's OUT pin
}

void setup() {
  Serial.begin(115200);
  delay(500);
  zeroMv = readOutMv(500);
  Serial.printf("ACS712 on GPIO35 — zero at %.0f mV (keep the load off during start-up)\n", zeroMv);
}

void loop() {
  float mv = readOutMv(200);               // averaging tames the ESP32 ADC noise
  float amps = (mv - zeroMv) / MV_PER_A;
  if (fabsf(amps) < 0.05f) amps = 0;       // below the noise floor of a 5 A sensor
  Serial.printf("OUT %.0f mV  Current %.2f A  (≈ %.1f W at 12 V)\n", mv, amps, amps * 12.0f);
  delay(1000);
}

Expected serial output

ACS712 on GPIO35 — zero at 2497 mV (keep the load off during start-up)
OUT 2499 mV  Current 0.00 A  (≈ 0.0 W at 12 V)
OUT 2721 mV  Current 1.21 A  (≈ 14.5 W at 12 V)
OUT 2885 mV  Current 2.10 A  (≈ 25.2 W at 12 V)

ACS712 on GPIO35 — zero at 2497 mV (keep the load off during start-up)
OUT 2499 mV  Current 0.00 A  (≈ 0.0 W at 12 V)
OUT 2721 mV  Current 1.21 A  (≈ 14.5 W at 12 V)
OUT 2885 mV  Current 2.10 A  (≈ 25.2 W at 12 V)

Common errors