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In the last several years the electronics industry has released hand-generator powered flashlights which are advertised as the end of battery powered flashlights. This has become possible because of recent advances in capacitor, magnet, and LED technology. Nevertheless the physics behind these flashlights is fairly simple.
New capacitor technology has increased voltage and capacitance while maintaining a small size. Neodymium magnets have both large magnetization and large coercive fields while remaining small in size. Improvements in LEDs make them comparable with the brightness of conventional incandescent bulbs while using significantly less power. The drawback of LEDs is they are polarity sensitive devices and therefore only operate efficiently with DC sources.
The shaker flashlights used in this experiment have only five components: a hand-generator, an LED, a capacitor, a switch, and a full-wave bridge rectifier. Combining these components to make a flashlight fosters a rich understanding of some interesting physics:
• The students learn how mechanical energy is converted into electrical energy.
• They learn how alternating current (which is ill-suited for storage) can be rectified to be well-suited for storage.
This experiment is from our conceptual physics lab course for non-science majors in which we strive to emphasize the everyday applications of each lab. It is an ideal conclusion to an electricity section because the technology involved is both cutting edge and common to student’s everyday life. Upon completion of this lab the students will not only have built a shaker flashlight, but will have a good understanding of the AC adapters (which contain full-wave bridge rectifiers) they use to charge their cell phones and mp3 players.
Full-Wave Bridge Rectifier
Shaker Flashlight Hand Generator
For improved lifespan of materials, we attached the remaining electronic components to acrylic “flashlight” boards. Alternatively, each electronic part could be used individually for easier production.
Shaker Flashlight Board
Diode Flashlight Schematic
References
For an excellent article on the engineering of the Forever Flashlight, see R. J. Nelson, “The Forever Flashlight II – Batteries Not Required,” (November 2006)
Author: Emily Alden, 2006
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109