a *really bright* flasher

A *very* bright flasher!

For quite some time now I have been experimenting with
different kinds of flashers: some time ago I wrote a short
message on this list about my very bright flashers, and since
then I have been receiving many questions about the details. 
So here the full story for the ones who requested it, and to
the list in general:

My experience is that LEDs do not give enough light. I have
tried to add more LEDs in parallel, but with little succes. So
I tried to use bulbs, the plain ones that can be found in
simple torches. The more voltage you apply to a bulb, the
brighter it lights up. For instance, when you apply 6 Volts to
a bulb which is rated for 2.5 Volts, it emits light as bright
as a photographic flasher. And that is what we want. The only
problem is that the bulb is dead before you could even lift
off your kite 8^(. The reason for burning the bulb is that
with such an overrated voltage, the heat which is developed
inside the bulb melts the glowing wire inside.

So what you do is not to allow the bulb to develop so much
heat, and in this case I solved this to minimize the time that
the bulb glows up, and then allow much time to let the bulb
cool off.

Let's go practical: I used the LM3909 LED Flasher/Oscillator
(National Semiconductor) to time the flashing. (The same timer
which is used in the LED flashers described before on this
list.) The bulbs light up about once a second, and for a
duration of estimated 0.1 seconds. This short time is long
enough to make the bulb light up fully, but just too short to
burn it. After one night of use you will note a dark coating
on the inside of the bulb, which is caused by vaporizing of
the glowing wire inside. It is advisable to have some extra
bulbs handy, because some do not last a full night. 

As you can see on the schematics below, there are only 3
parts. The bulb is rated at 2.5 Volts, and can be found on
torches that run on a single battery. (They can also be found
in shops 8^). Do not use these halogen type bulbs. The
batteries are 4 cells of 1.5 Volts. The current used is
little, so you can use the smallest batteries (AA) you can
find, if you need to save weight. Solder the batteries
together, and fix them well to your kite. They behave like
bombs if they jump out of a container up in the sky! Also fix
the bulb with a piece of tape, so that it does not come loose
up in the sky. On the diagram the LM3909 is drawn top view, a
little dot on the housing denotes pin number 1. The
electrolytic capacitor is 400 - 500 microfarad, 10 Volts; note
the polarity. The cost of one unit is about 10 guilders ($5).
Note that when a bulb of a different voltage is used, the
timing changes considerably. Also, to increase the flashing
speed by a factor of about 2, connect pin 8 with pin 1. This
ofcourse will decrease the life-time of the bulb.

The diagram:

   |    --------              |            |
   |    |      |              |            |
   |    |    ++0++++0++++0++++0++          |
   |    |    + 8    7    6    5 +          |+6 Volt
   |    |    +                  +    -------------
   X L  |     )    LM 3909      +        =====
   |    |    +                  +          |
   |    |    +.1    2    3    4 +          |
   |    |    ++0++++0++++0++++0++          |
   |    |           |         |            |
   |    |           |         --------------
   |    -----||-----| 
   |         C +    |
   |                |

Legend:          */\*
                 *\/*                       |+
   |                \                 -------------
   X L   Lightbulb, 2.5 volts             =====      Battery
   |                  \                     |
   --||--  Electrolytic Capacitor, 470 microF
     C +   note the polarity!
This little device competes very well with the expensive and
heavy photographic (Xenon-) flashers. But you can improve the
intensity by adding a reflector. The one I use is taken out of
my son's torch (he had no use of a torch without a bulb anyway
:-). Remember to direct the reflected light beam down.
Good Luck,                          \
Daniel Kropveld, Amsterdam           \ o
KROPVELD@AMC.UVA.NL                    ^

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