ESP8266 - ACS712 Current Sensor

Hardware Preparation

Or you can buy the following kits:

Overview of ACS712 Current Sensor

The ACS712 tells the ESP8266 how much current flows through a wire. Inside the chip, the current passes through a thick copper path. This current makes a small magnetic field, and a Hall sensor placed next to the copper turns that field into a voltage. The load side and the ESP8266 side never touch electrically (the chip is isolated up to 2.1 kV RMS), and the copper path is only about 1.2 mΩ, so putting the sensor in a wire hardly changes your circuit. It works for both DC and AC current.

The module comes in three versions. They look the same, so read the text on the chip. This tutorial uses the 30A version.

The output is "ratiometric". It means the output moves together with the real supply voltage of the module. If VCC is a little lower than 5V, the zero point is also a little lower than 2.5V. This is why our code measures the zero point itself, instead of assuming 2.5V.

The ACS712 module has a 3-pin header and a 2-pin screw terminal:

• VCC pin: must be connected to 5V. On the NodeMCU, this is the VIN pin (some boards print VU).
• GND pin: must be connected to GND (0V) of the ESP8266.
• OUT pin: the analog output. It must go to A0 through a voltage divider, never directly.
• IP+ and IP- screw terminals: must be placed in series with the load. Cut one supply wire of the load and put the terminal in the gap. Current that flows from IP+ to IP- gives a positive value. On some modules, these two screws have no label.

The module also has a red power LED that turns on when VCC is connected.

How the ACS712 Output Turns into Amps

With no current, OUT sits at the zero voltage (near 2.5V). Current in one direction pushes OUT up, and current in the other direction pulls it down, by 66 mV for each amp. So the formula is:

For example, if Vzero is 2.40V and Vout is 2.47V, the current is 0.07 / 0.066 ≈ 1.06A.

Why the ESP8266 Needs a Voltage Divider and Averaging

The A0 pin of the NodeMCU board accepts 0V to 3.3V and gives a 10-bit value from 0 to 1023. The ACS712 OUT pin can rise to about 4.5V, which is too high. Two resistors fix this: OUT → 10kΩ → A0 → 20kΩ → GND. The A0 pin then sees only 2/3 of the OUT voltage (4.5V becomes 3.0V), and the code multiplies the value back by 1.5.

One A0 step is about 3.2 mV. After the divider, it is about 4.8 mV on OUT, which equals about 73 mA. So the 30A sensor is a coarse tool. It is good for loads of about 0.5A and up, such as motors, lamps, LED strips, heaters and chargers, but not for a single small LED. On top of that, the ESP8266 ADC is quite noisy, and the WiFi radio adds more noise. A jump of ±50 to ±100 mA at 0A is normal. That is why the code reads A0 hundreds of times and uses the average. On the ESP8266, averaging is not optional, it is the main trick that makes the readings usable.

Wiring Diagram

The ACS712 gets 5V from the VIN pin of the NodeMCU, and its OUT pin reaches A0 through the 10kΩ/20kΩ divider. The 12V DC device (such as a DC motor, LED strip or lamp), the 12V adapter and the screw terminal of the ACS712 form a separate loop.

This image is created using Fritzing. Click to enlarge image

See more in ESP8266's pinout and how to supply power to the ESP8266 and other components.

How To Program For ACS712 Current Sensor

• The first step is to define the A0 pin and the numbers needed for the conversion:

• Read A0 many times, average the result, and change it to the voltage on the OUT pin:

• In setup(), while the load is still OFF, save the average voltage as the zero point (1000 readings).
• In loop(), apply the formula:

ESP8266 Code for ACS712 - Measure DC Current

The code below finds the zero point at startup, then prints the OUT voltage and the current every 500 ms. Each printed value is the average of 300 readings.

Detailed Instructions

To get started with ESP8266 on Arduino IDE, follow these steps:

• Check out the how to setup environment for ESP8266 on Arduino IDE tutorial if this is your first time using ESP8266.
• Wire the components as shown in the diagram. Leave the 12V adapter unplugged for now.
• Connect the ESP8266 board to your computer using a USB cable.
• Open Arduino IDE on your computer.
• Choose the correct ESP8266 board, such as (e.g. NodeMCU 1.0 (ESP-12E Module)), and its respective COM port.
• Copy the code above and open it with the Arduino IDE. No library is needed.
• Click the Upload button in the Arduino IDE to compile and upload the code to the ESP8266.
• Open the Serial Monitor. Wait until the zero voltage is printed. The load must be OFF during this time.
• Now plug in the 12V adapter so the DC device turns on, and watch the current.
• If the DC device is a motor, hold its shaft lightly with your fingers. The motor works harder and the current goes up.

This run uses a 12V DC motor as the DC device. The first lines show the load OFF. The small values around 0.05A are noise, not real current. The middle lines are the motor running freely. The higher lines appear when the shaft is held.

Measuring AC Current

AC current changes direction many times every second (50 or 60 times, depending on your country). So the OUT pin swings above and below the zero point, and a simple average would give almost 0A. Instead, we measure the RMS value, which is the number a multimeter shows for AC.

The code samples A0 as fast as it can during a 200 ms window. This covers 10 full cycles at 50 Hz, or 12 cycles at 60 Hz. For each sample, it takes the difference from the zero voltage and squares it. Then:

Upload it in the same way, with the load OFF at startup. Then switch on an AC device, for example a small heater:

With no load, the RMS value is not exactly 0A. This is the ADC noise again. Values below about 0.1A should be treated as 0A.

Video Tutorial

Additional Knowledge

A few things affect the accuracy of the ESP8266 + ACS712 setup:

• VIN is not exactly 5V. When the NodeMCU runs from USB, VIN often reads a little lower (for example around 4.7V), because of a protection diode on the board. The zero calibration fixes the offset. The sensitivity also drops a little, to about 0.066 × (VIN / 5). If you measure VIN with a multimeter, you can put that value into SENSITIVITY.
• The NodeMCU board has its own resistor divider on A0 (it is how A0 accepts 3.3V). It loads our 20kΩ resistor a little, so the readings can be about 2% low. If you compare with a multimeter, you can raise DIVIDER_FACTOR slightly, for example to 1.53.
• For small currents (below 5A), a 5A version of the ACS712 gives about 3 times more signal per amp.

Let's compare the ACS712 with the voltage sensor used in the ESP8266 - Measure Voltage tutorial.

It is evident that the two modules work as a pair: the voltage sensor tells you the voltage, the ACS712 tells you the current, and by multiplying them you can calculate the power of a DC load in watts.

Learn More

• ESP8266 - Measure Voltage
• ESP8266 - PC817 Optocoupler: Monitor 12V/24V DC Input Signal