There were two primary options that were considered: E-ink, 7 Segment LEDs. Both have their allures!
E-ink
There are E-ink modules that are small enough and flexible that could potentially be put into the ring design. However, these were primarily designed for low cost products like smart cards with embedded counter displays. Hence the available ones do not sleek enough to put in a jewellery design. Power is also a consideration. On the upside these displays use no power at all when static, allowing the time to be displayed constantly without the user needing to trigger it. The downside however is that the display must then be updated every minute to allow the displayed time to remain correct, even if the user is not looking at the display in that time. Updating the E-ink displays is fairly energy intensive, requiring a large voltage which is challenging to generate in a compact circuit. Overall this means that the power draw of this display type is prohibitive.
A particular advantage of this type of display is that there is no upper limit to the ambiant light it can be viewed in, even direct sunlight it should remain clear. However, the opposite is not true, when it is dark the display will not be visible.
7 Segment LEDs
When using a set of normal LEDs in a 7 segment arrangement, no power is needed to update the display each minute when the user is not looking. Simply a blank display is presented. To view the time the user would have to activate the display with a button press (or similar). This means that the baseline power draw is zero for this display type.
Unfortunately the current needed to light the display when being viewed can be quite large. In typical indoor lighting it is usually around 1mA. Outdoors it is as much as 30-40mA. These current values are the aggregate across the display. The current is shared across all the LEDs via time domain multiplexing. Luckily, as the current design is solar powered, we can assume that we are also harvesting large amounts of energy when outdoors, hopefully recuperating this large current outlay.
While with adequate current the display should be visible in outdoors lighting, direct sunlight is likely to be an issue. However, in very low light the display will still be visible. This is the opposite trade-off to the E-ink dis
The Verdict
In the current design I have chosen to use the 7 Segment LED displays. This is because I anticipate that the viewing time of the display will be only a few seconds per hour, and that the energy use of the display correlates with the available energy harvesting from ambient light. (High currents are needed when bright light is available to offset it, display current is very low when in darkness, no updates are needed while the user is sleeping!). Further I believe that the aesthetics of the display are pleasing, and suite the jewellery design ambitions.