Arduino Nano 33 IoT - ACS712 Current Sensor

Hardware Preparation

Or you can buy the following kits:

Overview of ACS712 Current Sensor

The Arduino Nano 33 IoT - Measure Voltage tutorial tells you how many volts a circuit has. The ACS712 answers the other question: how many amps flow through it. The trick is the Hall effect. The load current passes through a thick copper strip inside the chip. That current makes a small magnetic field, and a Hall sensor sitting next to the strip turns the field into a voltage. Since the copper strip never touches the measuring part, the load side is isolated from the Arduino Nano 33 IoT side (2.1 kV RMS). The strip is also only about 1.2 mΩ, so it barely changes the circuit you are measuring.

When no current flows, the OUT pin sits at half of the supply voltage, about 2.5V. Current in one direction pushes OUT above 2.5V, and current in the other direction pulls it below. The output is ratiometric: it follows the real supply voltage, so if the supply is 4.9V instead of 5V, the zero point moves a little too. This is the reason the code measures the zero point itself.

The ACS712 comes in three versions. They look the same, so check the text on the chip or on the shop page:

This tutorial uses the 30A version (chip ACS712ELCTR-30A-T). Its main numbers:

The ACS712 Module Pinout

The module has a 3-pin header on one side and a 2-pin screw terminal on the other side:

• VCC pin: must be connected to 5V. The chip does not work properly on 3.3V.
• GND pin: must be connected to GND (0V) of the Arduino Nano 33 IoT.
• OUT pin: the analog output voltage. On the Arduino Nano 33 IoT, it must go through a voltage divider before it reaches the analog pin (see the wiring section).
• IP+ and IP- screw terminals: the current path. They must be placed in series with the load. In other words, cut one supply wire of the load and put the terminal in between. On some modules these two terminals have no labels. Current that flows from IP+ to IP- reads as a positive value.

There is also a small power LED. It turns on when VCC gets power.

Is the 30A Version Right for Small Currents?

Not really. With the 66 mV per A sensitivity, a tiny current makes a tiny voltage change. On the Arduino Nano 33 IoT, one step of the 12-bit ADC is about 0.8 mV at the pin, which is about 1.2 mV at the sensor output after the divider. That is about 18 mA per step. On top of that, the sensor output has some noise, so a value that jumps between -0.08A and +0.08A with nothing connected is normal. Averaging many readings, as the code does, makes the result much calmer. The 30A module is a good fit for loads from about 0.5A and up: DC motors, 12V lamps, LED strips, heaters and battery chargers. For smaller loads, the 5A version gives better detail.

Wiring Diagram

Two things need care on this board, and both come from the fact that the Arduino Nano 33 IoT is a 3.3V board.

1. The OUT Pin Needs a Voltage Divider

The analog pins of the Arduino Nano 33 IoT accept 0V to 3.3V only, and they are NOT 5V tolerant. The ACS712 output can rise to about 4.5V at full current, which can damage the pin. So we never connect OUT straight to the board. Instead, we put two resistors in between:

• OUT pin → 10kΩ resistor → pin A1 of the Arduino Nano 33 IoT
• Pin A1 → 20kΩ resistor → GND

This divider gives the pin 2/3 of the sensor voltage. The highest output, 4.48V, becomes about 3.0V, which is safe. The zero point of 2.5V becomes about 1.67V. In the code, we multiply the measured voltage by 1.5 to get the real sensor voltage back.

2. Where Does the 5V for the Sensor Come From?

This is the tricky part on the Arduino Nano 33 IoT. The pin marked 5V on the header is really the VUSB pin. It is connected to the 5V of the USB port only through a small solder jumper on the bottom of the board, and this jumper is open from the factory. So, on a new board, the 5V pin gives no voltage at all. You have two choices:

• Option A - close the VUSB jumper: bridge the two pads marked VUSB on the bottom side of the board with a drop of solder. After that, the 5V pin gives the USB 5V whenever the board is powered from USB. The ACS712 only needs about 10 mA, so this is fine for the sensor.
• Option B - no soldering: power the sensor VCC from a separate 5V power adapter, and connect the GND of that adapter to the GND of the Arduino Nano 33 IoT.

Other tutorials for this board, such as the LCD one, talk about the VBUS pin. It is the same idea: that pin only carries USB power, and only when you power the board from USB. Do not use it for motors or other loads that need a lot of current. Here it only powers the small sensor chip.

This image is created using Fritzing. Click to enlarge image

The - wire of the DC device goes back to the - of the 12V adapter. The 12V circuit does not need to share a ground with the Arduino Nano 33 IoT, because the sensor isolates it.

How To Program For ACS712 Current Sensor

• The first step is to set the ADC of the Arduino Nano 33 IoT to 12 bits, so the values go from 0 to 4095 (the default is only 10 bits):

• Turn a raw reading into the real sensor voltage. Divide by 4095, multiply by 3.3V, then multiply by 1.5 to undo the divider:

• Find the zero point in setup(). With the load switched off, average 1000 readings. This is the real "0A" voltage of your sensor, which is often a bit different from exactly 2.5V.
• Calculate the current. Subtract the zero point and divide by the sensitivity (0.066V per A):

• In loop(), average a few hundred readings before each calculation, so the noise becomes smaller.

Arduino Nano 33 IoT Code for ACS712 DC Current

This code calibrates the zero point at start-up, then prints the sensor voltage and the DC current twice per second. Each result is the average of 200 readings.

Detailed Instructions

If you are new to the Arduino Nano 33 IoT, be sure to check out our Getting Started with Arduino Nano 33 IoT tutorial. Then, follow these steps:

• Wire the parts as shown in the diagram. Double-check the 10kΩ / 20kΩ divider: OUT must never touch A1 directly.
• Switch the 12V load OFF (unplug the 12V adapter). The code measures the zero point at start-up, so no current may flow at that moment.
• Connect the Arduino Nano 33 IoT board to your computer using a USB cable.
• Open Arduino IDE on your computer.
• Select the Arduino Nano 33 IoT board and its COM port.
• Copy the code above and paste it into the Arduino IDE.
• Click the Upload button to compile and send the code to the board.
• Open the Serial Monitor and wait for the line "Done. You can switch the load ON now."
• Plug in the 12V adapter to turn the DC device on, and watch the current.

The first three lines with the DC device off show the normal noise: a few hundredths of an amp around zero. In this run, the DC device is a 12V DC motor. With the motor running, the current stays near 0.9A. The 2.24A line is from holding the motor shaft with a finger for a moment: a slowed or stalled motor pulls much more current. If you see negative values with the DC device on, swap the two wires on the screw terminal, or just read the number without the minus sign.

Measuring AC Current

With AC, the current changes direction many times per second (50 or 60 times, depending on your country). The OUT pin swings above and below the zero point all the time, so a simple average would give almost 0A. Instead, we use the RMS (root mean square) value, which is what a normal AC meter shows.

The code reads the sensor as fast as it can for 200 ms. That is 10 full cycles at 50 Hz, or 12 at 60 Hz. For every reading, it takes the difference from the zero point and squares it. Then it takes the square root of the mean of all these squares, and divides it by 0.066:

The wiring to the Arduino Nano 33 IoT stays the same: 5V, GND, and OUT through the 10kΩ / 20kΩ divider to A1. Only the load side changes: the screw terminal goes in series with one wire of the AC device.

Upload it the same way as the DC code, with the AC device switched off at start-up. Then switch the device on and open the Serial Monitor. In this example, a small fan heater runs on its low setting:

With nothing turned on, the RMS value is not exactly zero. The sensor noise is squared too, so a value of about 0.05A to 0.1A is normal. You can ignore readings below that level.

Additional Knowledge

How does the ACS712 compare with the voltage sensor used in the Arduino Nano 33 IoT - Measure Voltage tutorial?

It is evident that the two modules do different jobs and work well together. Read the voltage with one and the current with the other, then multiply them (power = voltage × current) to know how many watts your load uses.

Video Tutorial

Learn More

• Arduino Nano 33 IoT - Measure Voltage
• Arduino Nano 33 IoT - PC817 Optocoupler: Monitor 12V/24V DC Input Signal