- Open Source Hardware Definition v1.0
- Creative Commons - Attribution - ShareAlike 3.0
Although a simple concept, there are various factors that need to be characterized to gain the highest efficiency, tempered by cost considerations.
Operating conditions may range from freezing to surface temperatures of +50'C, so the types of rechargeable cell chemistry need careful consideration versus operating temperature. As does material selection for the housing.
The level of daylight also plays an important part in whether sufficient energy is collected during the day. Crystalline cells have the highest efficiency, but do not function as well as amorphous cells in lower light conditions. The sizes of cells are likely to be well standardized, which will provide a driver for the overall device dimensions.
The light source (LED) needs to provide an output to meet minimum observable distances. And the LEDs themselves need to have sufficient lifespan for the worst case operating duration. To address this the range of possible LEDs will need to be tested to establish
- How much heat they generate in various modes, as heat damage is the main cause of reduced lifespan in LEDs.
- Variation of phosphor duration; if there are LEDs with longer duration phosphors then the pulse frequency might be reduced to save energy
- Output variation of pulse current versus continuous
- Visibility distances for output power, duration, light colour/temperature, spreading angle
The housing/enclosure will need to meet sufficient IP rating (likely IP68) to provide service as a buoy marker, this requirement may be found to be at odds with wanting to allow excess power to be extracted via a USB port. Similarly, the operating temperature range and UV exposure from the sun will influence the choice of materials.
Figure: Rough concept layout for solar cell, control board, storage cells and x4 light units
With the option to offer USB charging output to allow the most use to be made of the solar cell once the on-board cells are charged.
Control of the light sequencing to allow flexibility in local regulations and to accommodate setting of signalling of a morse code letter to allow identification of different units, is needed. With the controller being required to minimise power in use and maximise time in sleep/standby state.