Every month Australian electronics magazine Silicon Chip publishes a variety of projects, and in December 2012 they published the USB Power Monitor by Nicholas Vinen. Jaycar picked it up and now offers a kit, the subject of our review. This small device plugs inline between a USB port and another device, and can display the current drawn, power and voltage at the USB port with a large LCD module. This is useful when you’re experimenting with USB-powered devices such as Arduino projects or curious how external USB devices can affect your notebook computer’s battery drain.
The kit arrives in typical Jaycar fashion:
… everything necessary is included with the kit:
The instructions arrive as an updated reprint of the original magazine article, plus the usual notes from Jaycar about warranty and their component ID sheet which is useful for beginners. The PCB is quite small, and designed to be around the same size as the LCD module:
As you can see below, most of the work is already done due to the almost exclusive use of SMD components:
That’s a good thing if you’re in a hurry (or not the best with surface-mount work). Therefore the small amount of work requires is simply to solder in the USB sockets, the button and the LCD:
It took less than ten minutes to solder together. However – take careful, careful note of the LCD. There isn’t a pin 1 indicator on the module – so instead hold the LCD up to the light and determine which side of the screen has the decimal points – and line it up matching the silk-screening on the PCB. Once finished you can add the clear heatshrink to protect the meter, but remember to cut a small window at the back if you want access to the ICSP pins for the PIC microcontroller:
How it works
The USB current is passed through a 50 mΩ shunt resistor, with the voltage drop being measured by an INA282 current shunt monitor IC. The signal from there is amplified by an op amp and then fed to the ADC of a PIC18F45K80 microcontroller, which does the calculations and drives the LCD. For complete details purchase the kit or a copy of the December 2012 edition of Silicon Chip.
First you need to calibrate the unit – when first used the meter defaults to calibration mode. You simply insert it into a USB port. then measure the USB DC voltage brought out to two pads on the meter. By pressing the button you can match the measured voltage against the display as shown below – then you’re done.
Then you simply plug it in between your USB device and the socket. Press the button to change the measurement. The meter can measure the following ranges:
For an operational example. consider the next three images are from charging my phone – with the power, current and voltage being shown:
“P” for power…
current in mA
“b” for bus voltage
If you want to use the USB ports on the right-hand side of your computer, just press the button while inserting the meter – and it flips around:
Finally – here’s a quick video of the meter at work, whilst copying a file to an external USB hard drive:
I really like this – it’s simple and it works. Kudos to Nicholas for his project. You can purchase it from Jaycar and their resellers, or read more about it in the December 2012 edition of Silicon Chip. Full-sized images available on flickr. This kit was purchased without notifying the supplier.
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