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POWER SUPPLY

The logger has a dedicated solar-charging system to enable for autonomous operation. It was built around a 3.2-W solar panel a Volkswagen dealer gave me to accessorize the originally diesel-fueled Volkswagen Golf TDI that I bought and converted to run on vegetable oil. But that car is another story.

Even without much optimization for efficiency, the power needs of the logger are very small. The maximum power drawn by each of the main components is 5 mW for the LCD, 75 mW for the ATmega32, and 75 mW for the photodiode for a total of 155 mW. If all components run at maximum power at all times, this would amount to 3.72 Wh/day. This does not account for losses in the voltage regulator or other minor components, but there is plenty of energy available for the system so this is not a problem.

The battery I chose was a 12-V, 5-Ah sealed gel cell lead acid deep cycle battery. This type of battery is the most cost effective when size and weight are not a concern, but safety, ease of handling, and the ability to deep-cycle the battery are. Although a 12-V, 5-Ah battery could provide 60 Wh, draining any battery too low (even a deep cycle battery) can cause damage. Five days of energy storage in Ithaca is considered conservative, but due to the reliability needs of an autonomous system such as this, it is worth having a good factor of safety. Note that 37 Wh of useful storage enables the system to ride through 10 days with no sun, giving plenty of leeway with the battery chosen.

Even with enough storage, it is necessary to be sure that the energy balance of the system is kept positive or the battery will eventually drain. Ithaca averages 2.3 sun hours per day in the winter.^[2] What this means is that a solar cell rated at 1 W would produce 1 Wh of electricity per sun hour. My 3.2-W solar panel would therefore be able to produce 7.4 Wh (on average) of electricity per day during the darkest time of year in Ithaca if kept at its maximum power point. Because there is no maximum power point tracking in this system, it can be expected that the panel will produce about 5.2 Wh per day in the winter, which still far exceeds the maximum load expected.

The PV panel, which is rated at 18.8 V at its maximum power point, was designed for trickle charging a 12-V car battery and so could be directly connected to the data logger. However, to prevent battery damage from overcharging, I needed a way to disconnect the panel once the battery was fully charged. Using a BUZ71 field effect transistor and a polling routine, the panel was effectively disconnected when the battery voltage rose above 12 V.

Depending upon the state of charge, the battery voltage will float around 12 V but will not remain steady. The logger components required a constant 5-V power supply. The ON Semiconductor LM2574, a 0.5-A, 5-V step down switching regulator (buck converter) regulates the voltage from the battery to the level needed for the system components (see Photo 2). This regulator has a typical efficiency of 72%, which is much higher than resistance-based voltage regulation.

An unresolved and bizarre result of running the logger off of the solar power supply as compared to the standard AC/DC power supply was that the on-off switch on the STK500 ceased to function. The only way to turn the logger on or off when connected to the solar power supply was to actually disconnect a battery lead.