Adafruit INA219 Current Sensor Breakout

The INA219 breakout board can be powered by the 5V or 3V pin on your Arduino and communicates via I2C.

To wire up the STEMMA QT version:

Connect board VIN (red wire) to Arduino 5V if you are running a 5V board Arduino (Mega, etc.). If your board is 3V, connect to that instead.
Connect board GND (black wire) to Arduino GND
Connect board SCL (yellow wire) to Arduino SCL
Connect board SDA (blue wire) to Arduino SDA
Connect Vin+ to the positive terminal of the power supply for the circuit under test
Connect Vin- to the positive terminal or lead of the load

To wire up the original version:

Connect to the microcontroller

Connect GND to GND
Connect VCC to 5v
Then connect SDA to SDA (Analog pin 4 on pre-R3 Arduinos)
And connect SCL to SCL (Analog pin 5 on pre-R3 Arduinos)

On R3 and later Arduinos, you can connect to the new dedicated SDA & SCL pins next to the AREF pin. On pre-R3 Megas, SDA & SCL are on pins 20 & 21.

Next we must insert the INA219 current sensor into the circuit we want to measure:

Be careful inserting noisy loads that can cause a sharp current draw, such as DC motors, since they can cause problems on the power lines and may cause the INA219 to reset, etc. When using a DC motor or a similar device, be sure to include a large capacitor to decouple the motor from the power supply and use a snubber diode to protect against inductive spikes.

Connect to the circuit

Connect V+ to the positive terminal of the power supply for the circuit under test.
Connect V- to the positive terminal or lead of the load. This puts the sense resistor in-line with the circuit.
Finally, connect a wire from the negative terminal of the power supply to GND. This allows the sensor to measure the load voltage as well as the load current.

The photo below shows an INA219 breakout board configured to measure the motor supply current on an Adafruit Motor Shield.