Yesterday I saw the “The Practical Meter” on Kickstarter – a small unit that is placed inline with the USB when charging your phone and shows the actual charging power (in watts) on a row of LEDs.
I wanted to have a go at it as well so I did the following last evening.
First I did a version with one of the newer 8-pin PIC and a low side current sense resistor, but to get a bit better resolution I also had to include a low voltage (about 0.8-1.1 volts) vref/vreg. Then I realized that the golden oldie ATTiny85 is much better for this application because it can do differential ADC readings and it also have an internal 20x gain amplifier for the ADC. So I changed into the tiny85 and a high side current sense resistor. And since the Reset pin on the Tiny can also double as one additional ADC input I could then also read the actual voltage for more accurate power calculation.
For more detail: PAW17 – USB Spypow
