INA219 Ammeter

Ammeter based on an Arduino UNO and the INA219 Module.

Parts:

Arduino UNO.  The project could be done with any MCU - the only requirement is that it supports an I2C interface. If the MCU did not support internal pull-ups then a pull-up resistor (eg, 10kΩ)would be required for the momentary display switch. Note that this version of the UNO has thru-holes adjacent to each pin, so it would be possible to use the thru-holes to wire the board directly to the other components. However, for this project a prototyping board with header pins was used.

INA219 Current/Voltage measurement module (CJMCU-219).  The module is based on the INA219 IC from TI. It includes the I2C interface with pull-up resistors and a pair of jumpers for choosing an alternate address.    It operates with 3.3V or 5V, but does not have reverse voltage protection.  In the configuration usually supplied (R100 resistor) it can measure up to 3.6A at 26V.

OLED module, 0.96", I2C.  Any equivalent device will work.  The advantage of an I2C display is that it shares the I2C bus already implemented for the current sensor.    0.96" provides an easily readable display with the data on two lines on two screens, selected by a push-button.  To simplify assembly, header pins were soldered to the display module, and a connector used to make the connection to the other components.

Prototyping board, hookup wire.  The board should be sized to be clear of the USB and power connectors at the end of the UNO, to provide maximum options for positioning. 

LED and matching resistor for 5V (optional).

Connectors:
2x2-pin Screw headers, or any other similar type of bare cable connector, or socket for use with adapters.

Push-button SPST (on-off) switch, momentary. This switch is mounted on the faceplate.   It is used to temporarily show the alternate display.

SPDT switch.  This switch is optional - it is used to isolate logic ground from measurement ground (see below).

Schematic

The schematic shows the setup for an Arduino, with Ground, +5V and SDA/SCL for the I2C interface. The INA219 module has IN+ and IN- brought out to one pair of screw terminals.  The other pair is commoned and connected via a switch to logic ground.  Including this switch means that the load ground can be isolated from the logic ground if needed, while still using the screw terminals to make the connection between load and power supply ground.   Note that if the power/load ground is not connected to logic ground then voltage cannot be measured.

The momentary switch is shown connected to A2 - this choice was purely for convenience and any analog or digital pin could be used, provided that the appropriate change is made in code.  This switch changes the displayed values between Load Current/Bus Voltage (the default) and Power/Load Voltage.  Note that load voltage is calculated as bus voltage plus shunt voltage.

The OLED display is wired to the I2C lines in parallel with the INA219 module.  The default address for the display is 0x3C and for the module is 0x40.

The optional power LED is wired across the 5V supply, with a suitable resistor.

Construction

The parts are assembled on a piece of prototyping board.  The prototype was assembled without much regard for space other than matching the dimensions of the UNO.   A UNO protoboard would have worked just as well.   Note that the header pins are modified by pushing the mounting strip to the end of the pins and then soldering from underneath - this places the board as close as possible to the surface of the UNO, minimising the height of the finished assembly.  Also, the proto board can be single-sided and used with the plated side down, so wires can be run on the unplated side and fed through holes to their connecting points.  This makes for a sturdier assembly.  Header pins are used for only one section on the digital pin side of the UNO, as the two sets of headers are not spaced apart with standard 0.1" spacing.  If access to the pins D8 to ARef is required then alternative arrangements must be made.

The INA219 module is then attached with some double-side foam tape, and the wiring run to the proto board. The screw terminals and the push-button switch are mounted to the protoboard - some glue has been added for the screw terminals to ensure the mounting is solid.  The wires for the module IN+ and IN- connections are solid copper, which is actually overkill for the ~3A maximum current, but using solid wire simplified the placement of the module.   Using 90-degree header pins for the display module would have permitted a slightly more compact assembly while still maintaining the convenience of a connector, but in practice the height of the finished unit is set by the screw connectors on the proto board.

Complete

The code uses standard libraries for both the INMA219 and the display, so it is very simple.  The only slightly complex part is formatting the current and voltage readings to provide large lettering on a 128x64 display while still maintaining a suitable level of precision, given that current can range from tenths of a mA to Amps.

Upgrade.  Having completed the project the posssibility of an upgrade emerged.  The SPST switch that is used to isolate the load ground from the logic ground could be upgraded to DPDT.  The second pole could be wired with a centre ground and one lead to a UNO GPIO.  This input could then be tested in code to display only the current when the grounds are not connected (the voltage and power readings fluctuate randomly when the grounds are not connected).  Whether or not this is worthwhile would depend on how frequently measurement are being made that require an isolated ground.  The logic for this additional GPIO input would follow the existing logic for the momentary display switch - configure the GPIO with a pull-up and test for switched to ground when the load ground is isolated. 

Code

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