This instructables show how to make a simple power meter using 3 components: ATtiny85, INA219 and OLED module.

It can continuous measure the voltage(V), current(mA) and accumulated power usage(mWh). And also plot a simple graph to visualise the figures.

Step 1: Why?

While developing IoT device, power consumption does matter, especially if it cannot wired. A simple multi-meter is not enough for continuous monitoring the voltage and current at the same time. And also the accumulated power consumption (mWh) is very important for deciding how big a battery required in the IoT project.

So I use the components in hand to create this ATtinyPowerMeter.

Step 2: Preparation

The power meter should not drain too much power itself, it is the major criteria to choose the components:

MCU

ATtiny85 only draw less than 1 mA while running at 1 MHz and 8 KB program flash can well fit a power meter program.

Power Monitor Module

INA219 module is the only module I can found that can monitor voltage and current at the same time (leave comment to me if you known other modules). It draws around 1 mA power.

Display Module

SSD1306 OLED display module drain few mA for display, it is the major power consumption. It should be better if using mono LCD display, but I only have this I2C display module in hand, so I will use it this time.

Battery

According to the data sheet, the above 3 components can operate at 3 V to 5.5 V, 2 rechargeable AAA battery is not enough, 3 AAA battery is better, 1 Lipo battery also ok. But in this project I am testing a new type battery, Lithium iron phosphate (LiFePO4 or LFP) battery. It is the same as AAA battery in size, but can provide 3.2 V 600 mAh. It should be good enough for most electronic project but I am still testing it.

Others

A 380 hole breadboard, some breadboard wire, a breadboard friendly battery holder, a 3 pins screw terminal block and a switch. And also a ISP for programming the ATtiny85.

Step 3: Download Source Code

Download source from GitHub:

https://github.com/moononournation/ATtinyPowerMete...

If you are not familiar GitHub, simple press "Clone or download" button and then "Download ZIP".

Step 4: Program ATtiny85

Use Arduino compile and program the source to ATtiny85.

You may find more details in these instructables:

https://www.instructables.com/id/Programming-the-A...

https://www.instructables.com/id/How-to-Program-AT...

https://www.instructables.com/id/How-to-program-th...

https://www.instructables.com/id/Programming-the-A...

https://www.instructables.com/id/Programming-an-At...

Step 5: Circuit Design

Here is the connection summary:

Battery +ve -> switch -> ATtiny85 pin 8, INA219 module Vcc, OLED module Vcc
Battery -ve -> ATtiny85 pin 4, INA219 module GND, OLED module GND, screw terminal block middle pin
ATtiny85 pin 5 -> INA219 module SDA, OLED module SDA
ATtiny85 pin 7 -> INA219 module SCL, OLED module SCL
INA219 module Vin- -> screw terminal block left pin
INA219 module Vin+ -> screw terminal block right pin

Step 6: Testing

Turn on the switch, you should see V and mA figure is 0.
Connect testing power source +ve and -ve to screw terminal right pin and middle pin respectively, you should see V figure become the power source Voltage value and mA still 0
Connect testing loading device +ve and -ve to screw terminal left pin and middle pin respectively, you should see mA figure become the loading device drawing current value