INA219 Current Sensor and MicroPython

More Micropython programmers — and especially beginners — should know about Awesome MicroPython. It’s a community-curated list of remarkably decent MicroPython libraries, frameworks, software and resources. If you need to interface to a sensor, look there first.

For example, take the INA219 High Side DC Current Sensor. It’s an I²C sensor able to measure up to 26 V, ±3.2 A. It does this by measuring the voltage across a 0.1 ohm precision shunt resistor with its built-in 12-bit ADC. I got a customer return from the store that was cosmetically damaged but still usable, so I thought I’d try it with the simplest module I could find in Awesome MicroPython and see how well it worked.

I guess I needed a test circuit too. Using all of what was immediately handy — a resistor I found on the bench and measured at 150.2 ohm — I came up with this barely useful circuit:

simple circle with 3.3 V DC supply ad two resistors of 150.2 ohms and 0.1 ohms in series
Should indicate a current of 3.3 / (150.2 + 0.1) = 21.96 mA

The INA219 would be happier with a much higher current to measure, but I didn’t have anything handy that could do that.

Looking in Awesome MicroPython’s Current section, I found robert-hh/INA219: INA219 Micropython driver. It doesn’t have much (okay, any) documentation, but it’s a very small module and the code is easy enough to follow. I put the ina219.py module file into the /lib folder of a WeAct Studio RP2040 board, and wrote the following code:

# INA219 demo - uses https://github.com/robert-hh/INA219

from machine import Pin, I2C
import ina219

i = I2C(0, scl=Pin(5), sda=Pin(4))
print("I2C Bus Scan: ", i.scan(), "\n")

sensor = ina219.INA219(i)
sensor.set_calibration_16V_400mA()

# my test circuit is 3V3 supply through 150.2 ohm resistor
r_1 = 150.2
r_s = 0.1  # shunt resistor on INA219 board

# current is returned in milliamps
print("Current       / mA: %8.3f" % (sensor.current))
# shunt_voltage is returned in volts
print("Shunt voltage / mV: %8.3f" % (sensor.shunt_voltage * 1000))
# estimate supply voltage from known resistance * sensed current
print("3V3 (sensed)  / mV: %8.3f" % ((r_1 + r_s) * sensor.current))

with everything wired up like this (Blue = SDA, Yellow = SCL):

breadboard with RP2040 pico board and INA219 sensor board benath it, and the 150 ohm wired as a circuit on the side
all of the wires

Running it produced this:

I2C Bus Scan:  [64] 

Current       / mA:   22.100
Shunt voltage / mV:    2.210
3V3 (sensed)  / mV: 3321.630

So it’s showing just over 22 mA: pretty close to what I calculated!

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