I'm building a solar powered LED watch and I'm trying to calculate how much current the display will draw so that I can find out what kind solar panel I need. The display is a 4 digit, 7 segment display.
This is what I have so far:
LED current should be ~500uA or more preferably.
At any given time, when looking at the watch there will be ~16/28 segments lit.
The watch will be checked 50 max. in a day (random figure I found online, can't be bothered to actually count.
The display will be on for 3 seconds when checking the time.
Only one digit will be displayed at a time. Each digit will be on for 2ms before switching to the next digit, with no dead time in between digits and/or wrap-arounds (last digit back to first digit)
Since ~16 segments will be displayed, this means that on average 4 LEDs are on at any time. 16 segmets/4 digits = 4 segments.
4 LEDs for 3 seconds is the same as 12 LED's for 1 second.
12 LEDs * 500uA = 6000uA/s
6000uA/s * 50 times a day = 300000uA/s = ~83 uA/h
Assuming the watch can charge for 12 hours a day.
83uA/h = ~7 uA/12h
so the solar panel will need to provide at least 7uA continuously for 12 hours. And if I want each LED to get 1mA then the solar panel will need to provide at least 14uA for 12 hours.
Does any of this sound correct? And yes, I know I'm not taking into account active and sleeping MCU current draw, charging circuitry losses, etc.