Solar-powered motor runs on 10 nA

Designs for solar-powered applications with low-duty-cycle requirements can often rely on capacitors for energy storage in place of less reliable batteries. Typical applications include solar positioning, telemetry transmitters, chemical pumps, data loggers, and solar-powered toys. The circuit in Figure 1 can run a small pager motor from the output of a small calculator-type solar cell in near darkness. The circuit works by repeatedly charging a 4700-µF capacitor, C₁, to 1.75V and then dumping the charge into the motor. Only the self-leakage current of the solar cell limits low-light operation. The circuit itself has such low leakage currents and trigger-current requirements that it can run the motor on 10 nA of current if you use a low-leakage energy-storage capacitor. Transistors Q₁ and Q₂ form a regenerative pair similar to a thyristor. The 1N4007 diodes take the place of pullup and pulldown resistors, and the diodes bypass the leakage current of the transistors and LED.

As the C₁'s charge approaches 1.75V, the green LED starts to conduct, causing Q₁ to turn on and feed current to the base of Q₂. The amplified base current appears as a disturbance at the collector of Q₂. The emitter-base drop of output transistor Q₄ isolates the collector of Q₂ from the output transistor, and the emitter-base-drop of Q₃ and the 10-nF capacitor, C₂, isolate Q₂ from the dc bias at the base of Q₂. However, the nanoamp-magnitude ac disturbance at the collector of Q₂ couples into the base of Q₁ via C₂, causing fierce regenerative action. You achieve nanoamp triggering and charging of C₁ through the use of leakage diodes in place of pullup resistors, through isolation of the load at the start of regeneration, and through the dc isolation of Q₁'s bias voltage from the collector of Q₂ at start of regeneration. As regenerative action continues, a dc latching path appears between the base of Q₁ and the collector of Q₂ through transistor Q₃. At this point, output transistor Q₄ also enters saturation, and the motor runs.

The high motor load quickly discharges C₁ toward 1.1V, at which point Q₁ can no longer sustain regenerative action because of the voltage loss in the emitter-collector junctions of Q₁ and Q₃. The 100Ω resistor and the reverse charge on C₂ drive Q₁ into cutoff and another energy-storage-capacitor charging cycle begins. Substitute a blue LED for the green one or add diodes in series with the LED to increase circuit-firing voltage beyond 1.75V. You can use 10-MΩ resistors in place of 1N4007 diodes to improve noise immunity if you don't need less-than-1-µA operation. Capacitors become leaky if you leave them in storage. You may need to condition such capacitors by applying a 9V battery to the capacitor for a few days. Use two solar panels in series to provide enough voltage for very-low-light operation.

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