Version 3 is the current version of the project. It has the following features:
- Very large surface area dedicated to solar panels (Photodiodes). They are arranged in 4 separate series strings allowing partial shading without loosing output from the other strings. This also allows a relatively high open circuit voltage, which can make harvesting more efficient.
- Super capacitor energy storage. 20x 11mF 3.3V. This is about an 1/3rd the energy as the ceracharge powered version 2. Though the lifespan of the capacitors should be much more. Also the max current of this system is > 100mA. This is no longer the limiting factor in making direct sunlight visible LEDs. (That is the current output of the GPIOs at 12mA.)
- Dual SMPS, one buck converter to take variable input from the solar panels and charge the super capacitors to 3.3V. A second one takes the variable voltage on the super capacitors and provides the MCU with 3.3V when the LEDs need to be lit. This boost SMPS can be turned off to allow the MCU to coast on a single 11mF super capacitor. This allows the average voltage supplied to the MCU to be lower, lowering the leakage current. It also removes the active quiescent current of the boost converter (~1uA). This is turned back on when the display needs to be lit, or if the voltage on the coast capacitor falls below a certain value. This is checked every 5 minutes.
- A sense photodiode is added to the display side of the ring. This allows the display brightness to be matched to the ambient lighting conditions.
- The programming pins are moved to the face of the ring. This allows the metal core to be a U-shaped channel all the way around, adding strength.
- The user now activates the display with a press of a button that is facing towards the finger the ring is worn on. The whole ring is pressed against the finger using the thumb to press the button. This frees up additional space on the top surface for more solar panels. Also this uses a trivial amount of current.
- The R1800K energy harvesting chip has a break even energy of 720nW, down from around 15uW for the previous design. This lets the harvester contribute even when in dimly lit rooms, helping the coast down in these conditions. It is still efficient enough in bright light to make the whole energy system self sustaining with ~20 minutes outside every 3 days.
- The programming pins can be used to charge the ring in its storage box, to allow long term storage without losing the time. Or for topping up if the user is aware they have been in the dark for too long.
- The programming pins can act as an additional input that can be activated by a press button in the charging box. This additional button is used to aid in the programming of the time for the end user.