Fundamentally this project is an exercise in low power electronics wrapped up to look like jewellery. In efforts to reduce fiddly battery changing and the inevitable “I will get around to it” followed by the ring never being worn again version 2 and 3 are powered by solar/ambient light. This choice puts a hard limit on the amount of power that the ring is allowed to use. In the case of replaceable batteries a heavy user of the ring could just replace the batteries more frequently, though that user is unlikely to spend more time outside just to give the ring more power!

Therefore we should have an understanding of how much energy the ring will use in a typical day, and how much it would generate.

LED Power

By far the highest current draw in the system is lighting the LEDs. This ranges from 100uA in a dark room, to 12mA in direct sunlight. Typical indoor usage is estimated to be about 3mA to give a legible display.

If we estimate that the user will look at the display for 1 second every 30 minutes for 16 hours a day, a reasonable estimate of the LED power use is: 3mA*3.3V*(32s/(24*3600)s = 3.66uW, or 0.317J per day.

Standby Power

We can estimate the standby power by looking at the data sheets for the current draw for all relevant components. A rough table is provided below:

ComponentCurrent nANotes
Harvest Chip144
Boost Chip128.595% 30nA 5% 2uA
Super Capacitors500This has complicated dependencies.
MCU + RTC600
Reverse Leakage Diode28Worst case.
MCU (intermitant)30Book keeping every ~5min
Power (@3.3V) W4.72065E-06
Energy per day (J)0.40786416

Generated Power

The power generated from harvesting light will depend strongly on the user’s habits, going outside is hugely valuable! Lets make the following assumptions about their day:

ConditionTimePower Generated uWGenerated Energy J
Overcast Daylight0.53000.54
Inside (daytime)870.2016
Inside (evening/morning)820.0576

These values are hard to justify, as calculating values from data sheets is possible, though the fudge factors end up dominating. Examples of these fudge factors: shading of the solar panels by the hand, wavelengths available in the indoor lighting, brightness of the day etc. A flat array in direct sun can generate ~6mW, this could provide the 0.7J required in 2 minutes! Though account for the array being round, and indirect sunlight, you can see that perhaps a factor of 15 seems reasonable, or about 30minutes outside.

A flat array also has been measured to be self sustaining in a standard office lighting condition (200lux LED lighting), the roundness of the array would be less of a disadvantage in this ambient lighting condition. Consequently my estimates is that the ring will generate no power in the dark, will glide downwards in residential lighting, will break even in working lighting and be very profitable when outdoors.