Yesterday I saw the “The Practical Meter” on Kick starter – 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.
The schematics is easy enough. R5 is the current sense resistor – I plan to use a 0.1ohm resistor to get a reasonably high drop to measure accurately at low power but still a not too high drop at 2 amps. R6/R7 is a voltage divider for measuring the actual voltage being delivered to the output (after the drop caused by the sense resistor). And then there are 6 charlieplexed LEDs for displaying the power consumption plus a one power-on led.
Read More: PAW17 – USB Spypow