I have wanted one of those magnetic induction rechargeable flashlights for some
time, so when I saw one for $6 at a hamfest, I picked it up. Then at the Father's
day hamfest in Frederick MD, I saw them again for $3 each, or two for $5. I picked
up eight of them, thinking they would make wonderful Christmas presents.
On my way out I heard someone say that there was no magnet inside the flashlight.
Instead of going right back to the vendor, though, I went home and ran an
experiment. I left one of the flashlights on while I went out for a couple hours.
When I returned, the flashlight was dark. I picked it up and shook it, but no amount
of shaking would make it shine again.
(See photo #1
) A closer look inside showed what looked like two
button-cell batteries. I took the unit apart and inside there were two 2032 button
cells, an on-off switch that was attached to the batteries, and two wires from the
coil going to a circuit board with nothing on it--no diodes, no chips, nothing but
unused solder pads.
I then took the "magnet" out and found that it was not a magnet--just a lump of
non-magnetic metal. A closer look at the coil showed that it was one layer of wire only,
hardly enough to generate any useful electron flow even if the metal slug was
I'm not saying that the flashlights were not worth the $2.50 I paid, but they were
not as advertised. The flashlight box, in poor but understandable English, clearly
said that the flashlight did NOT use batteries and all you had to do to make it
light was to shake it back and forth.
So, simply, buyer beware. The cheap LED shaking-flashlight you picked up may, in
fact, be an LED 2032 battery-operated flashlight made to look like a shaking
The figure below shows the box and the wording, as well as a flashlight taken apart
to show the two button batteries used to run the LED.
(See photo #2
At the Berryville hamfest, I found a number of vendors selling the above flashlights
and informed them that they were not what they proposed to be. All of the vendors
took them off the tables and apologized, saying they had not known.
There was one vendor who clearly said to beware of the other bogus flashlights but
that his were real! In examining them, I saw a much thicker coil, four diodes, and
what looked like a big capacitor. I bought two of these so I could rip one apart to
see for sure. The coil is only about 10-20% as thick as the coil in the real
magnetic induction flashlight I do now have.
The "capacitor" turned out to be labeled Ni-MH 3.6V 40mAh, 14h 4mA - clearly NOT a
capacitor. The circuit on the other hand was a full-wave bridge, properly wired. The
magnet was real and very strong as well. I tried to measure the voltage generated by
shaking, using a 20,000 ohm/volt needle meter, but the best I could measure was a
bit more than a volt, not the best way to measure this instantaneous voltage, but it
gave me hope.
I removed the battery and replaced it with a 1000uf 25V capacitor and then charged
it, using a DC power supply, to 4.2 volts. It lasted about 0.1 second but was still
able to make the LED glow at around 2.5 volts, long enough to measure. The unit was
put back together and then shaken to see if it would charge the 1000uf cap. It did.
The cap got up to 10.55 volts very shortly and, of course, that proves that this
flashlight is not bogus, though I can't imagine why they used a Ni-MH battery
instead of a real capacitor.
I also tried a 5.5 V 1.5 Farad cap but it took over 10 minutes of shaking to get it
from ~0.5 volts, which was essentually completely empty, to 4.2 volts, where the LED
produced a nice bright beam. My guess would be around 0.05F at 5V for 15 - 30
seconds of shaking would produce a bright light. (If we divide the above rated
capacitor by 10 to get one tenth of 1.5F, we get 0.15F which we can assume would
take one tenth the amount of time to charge from 0 to bright LED for 1 minute of
shaking. Now, to get this in the realm of 15-20 seconds, we divide this value by 3
to get one third of 0.15F and that gives a value of 0.05F. So if we can find a 0.05F
5.5 V capacitor, we can expect that for about 20 seconds of shaking the LED will
light brightly, that is IF the basic assumtion used here -- that the rates are
linear -- is valid.)
The real magnetically-induced flashlight takes about 15 sec to go from being able to
see that the LED is on to useful lighting. The magnet also doesn't bang against the
ends as much, the bumpers must be of a softer material, and the ends are just far
enough away that the entire battery clears the coil by about 20% of its length.
Also, the magnet is longer by another 50% of the length of the cheap unit. In the
cheap unit, the magnet hits bumpers so that about 33% of the length of the magnet
clears the coil.
The cheaper unit was manufactured like a magnetically-induced flashlight, but it
seems that with the right value/size cap it could actually work well as a
magnetically-induced flashlight. Some other caps -- 0.33F, 0.1F, and .047F -- were
ordered to see if this would be the case. I first tried the 0.047F cap and found
that even it required about 60 shakes to get it from empty to enough voltage to
light the LED, plus another 30 shakes to get it to a nice bright light. A larger cap
would require more work, so I took apart the second flashlight and replaced its
battery with another 0.047F cap and it responded the same way. (See Photo #3)
I now have two more useful magnetically-induced rechargeable flashlights. They cost
about $3 each at the hamfest, plus another $1.76 for the capacitors and another $1
each for shipping. For a total of about $6 with some work, these flashlights now do
73 and good Hamfesting!
Phil Karras, KE3FL
PS. CR2032 cells can be bought at this site for about $0.60 each, for 50: