Self-Powered Solar Data Logger

Introduction:

My project is a self-powered solar data logger. Put out in the sunlight, it will measure the light level and log this to memory to be later downloaded to a computer. The system is powered by a small solar panel and battery.

Summary:
The solar logger I built uses a photodiode to measure the solar insolation level. It converts the analog signal from the photodiode to a digital value that is stored in flash memory. Every time the system logs a data point, it also logs the time and date so that the data can be analyzed in the future. The logged data is available for a user to download to a computer for analysis.
While the system is logging, real time data is displayed on a small LCD screen as well as information about the battery voltage, the length of time the system has been logging and the length of time it can continue to log before running out of memory.
The logger has a dedicated solar charging system to provide the needed power. A very simple charge controller regulates charging of a small, sealed gel-cell lead-acid battery by a small solar panel. This charge controller is a simple on-off switch that disconnects the PV panel when the battery voltage rises too high.
The user sets the time and date each time the system is reset as well as the frequency with which data is stored. The user can also clear the memory or continue appending data to the previously logged data and extract logged data to a computer for analysis.

High level design:

Rationale and sources of project idea:
There are many reasons for logging solar insolation data for solar electric systems. When planning to install a solar electric power system, one must be able to predict the power output in order to determine the financial costs and benefits of the installation.
When planning an off-grid solar electric power system, the output must be closely matched with the load so as to provide sufficient power without considerable waste. The power output is directly related to the insolation level. While seasonal and annual average insolation levels for most major cities in the United States are available (http://www.solar4power.com/solar-power-insolation-window.html), cloud cover and other weather effects can be very localized depending upon the topography, making extrapolations from the large cities with established data to neighboring areas unreliable. In addition, this data is not available for every part of the world,
For a large installed solar electric system, one would typically install an insolation monitor to determine if the system is performing as expected. If the system output drops below the level expected given the insolation, an alarm would be set off to indicate that the system needs servicing.