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Last Updated on: Tuesday, June 01, 2021 03:06 PM

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Shake to Charge Flashlight --
designed by David A. Johnson, P.E.

Reed Relay to turn the
device on & off

Super Capacitors
used to store energy

The light from this Reed Relay is weak.  It took about a minute of vigorous shaking to get the first bit of light from it.  After a few more minutes of shaking, the capacitor was fully charged.  It might be okay for finding your keys in the dark but not much more.
The complete schematic for this device is shown below.  A few things should be pointed out.  First, they use some very cheap 1N4001 diodes in the bridge rectifier circuit instead more efficient Schottky diodes.  They also use a small 0.5 Farad cap with a 5.5 volt rating.  I noticed that that this kind of super capacitor was originally designed for maintaining data in memory chips and has a rather high internal equivalent series resistance.  This reduces the overall efficiency, since the device canít be charged or discharge very quickly.  Some of the power that should go to the LED will end up being dissipated inside the capacitor. Better super capacitors do exist.

Click on Circuit Below to view PDF of Schematic



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