September 12, 1997

Solar cells implement low-cost illuminator

Dennis Eichenberg, Parma Heights, OH

The advent of inexpensive solar cells and low-cost "ultracapacitors" makes it possible to develop an entirely solar-powered inspection illuminator (Figure 1). The illuminator is useful in exterior locations having no convenient source of power but needing an occasional illumination source. A mailbox illuminator is a typical application for this circuit. A mailbox is located away from power sources; you usually open it once a day to inspect for deliveries. It's best to avoid the use of batteries in this application, because temperature extremes severely shorten battery life. The circuit in Figure 1 is inexpensive, compact, reliable, and rugged.

Six 0.5V solar cells, PV₁ through PV₆, form a series combination to provide 3V to the circuit under normal illumination. A 1F ultracapacitor connects directly in parallel with the solar cells. The 5-mA solar-cell current slowly charges the capacitor to 3V. Switch S₁ closes automatically whenever you open the mailbox door. The closure provides 3V to the remainder of the circuit. Phototransistor Q₁ keeps transistor Q₂ off if exterior light exists. This feature prevents C₁ from discharging unless it's dark out. When it is dark out, Q₁ turns off, and Q₂ turns on to provide approximately 10 sec of illumination from lamp L₁. The parts are readily available from most distributors, and total parts cost is approximately $8. (DI #2080)

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