MadSci Network: Engineering

Re: I need to make a flashlight that works

Date: Tue Mar 7 18:34:17 2000
Posted By: Karl Kolbus, Staff, Data processing, Mequon Consulting Corp.
Area of science: Engineering
ID: 951953920.Eg

Hi, Carolyn:

Not knowing what grade level you are, we’ll start with the basics and work 
up from there.  All ordinary flashlights, such as the ones you may have 
around your house, need just a few things to make them work: 
1) An electrical power source - a battery. 
2) A way to change the electricity from the battery into light - a light 
3) A way to get the electricity from the battery to the light bulb - 
4) A way of turning the light on and off - an electrical switch.
5) A way to aim the light - a reflector.

	Get your parts together.
For your flashlight, you will need a battery (a flashlight “D” cell is 
fine), a 1 ½ volt flashlight bulb, a small board (about 4” by 6”) to mount 
the parts on, some duct tape, some insulated copper wire (it doesn’t have 
to be heavy-gauge; normal “hook-up” wire will do – about 2 feet long, some 
aluminum foil, some thin cardboard, some fine sandpaper, and 2 thumb 
tacks.  Drug, variety, and super-markets will usually have the battery and 
replacement flashlight bulbs.  You must get a bulb that is rated at 1-½ 
volts, because that is the voltage produced by the battery.  Sometimes 
they will label the bulbs by the number of batteries you are using such 
as: “… For flashlights with 2 “C” cells”.  In that case, you would get one 
that says: “… For flashlights or penlights with 1 cell.”  Don’t worry if 
it says size “AA” or “A” or “C” or “D” because the battery we are using, 
a “D” cell, has enough power for any of them.  The “D” cell is the big, 
fat one you find in flashlights.  The “AA” or “AAA” batteries are the 
small, skinny ones you would use in toys or portable radios.  All 3 will 
provide 1 ½ volts, but the larger “d” cell will last much, much longer. 
It’s like comparing a 12-ounce can of your favorite soft drink to a 2- 
liter plastic bottle of the same soft drink.  The 2-liter bottle will 
obviously last longer, providing you drink each at the same speed.

	Let’s start assembling.
Cut the wire into 3 pieces.  Make 1 piece 1 foot long, and the other 2 
pieces 6 inches long.  Strip one-half inch of insulation from both ends of 
all three wires. Use a wire stripper or knife (BE CAREFUL!).  Tape one end 
of the 12” wire to the bottom of the battery, making sure you have good 
contact between the copper of the wire and the metal base of the battery.  
Tape one end of a 6” wire to the top (the ‘button’ end) of the battery.  
Again, make sure you have good contact.  Make a small loop on the other 
end of the 6”wire you just attached to the battery, and on one end of the 
other 6” wire, which isn’t attached to anything yet.  Tape the battery to 
the board, being careful not to pull the wires loose.

If the thumbtacks are painted, use sandpaper to remove the paint from the 
top of them.  You must use metal thumbtacks.  The plastic “push-pins” 
won’t work because they will not conduct electricity.  Take a 2-inch 
square piece of aluminum foil and fold it in half, then fold it in half 
again until you have a strip two inches long and one-half inch wide.  This 
is your switch.  Push one of the thumbtacks through one end of the strip, 
about one-half inch from the end, and slip the loop of the wire from the 
battery around the tack.  Push the assembly into the board.  Take the 
second thumbtack, put it through the loop in the end of the wire that 
isn’t attached to anything yet, and stick it into the board right under 
the free end of the aluminum foil strip.  Make sure you place it so the 
foil will touch the top of the thumbtack when you press down on it.  That 
completes the switch.  Now attach the free ends of the wires to the bulb.  
Strip another 1” of insulation from the long wire coming from the battery, 
and wrap it tightly around the metal base of the bulb.  Secure it with 
duct tape.  Make sure you cover the whole base of the bulb with tape 
EXCEPT for the little blob of solder on the very end.  Place the end of 
the wire that comes from the foil switch, lay it directly over the solder 
blob and secure it with duct tape.  

You now have a working light source.  To test it, press the foil strip 
down so it touches the head of the other thumbtack.  That’s called closing 
the switch.  The bulb should light.  If it doesn’t, there are only three 
things that could be wrong:  1) One or more of your connections is bad, or 
2) the battery is low or dead, or 3) the bulb is burned out.  Check those 
items in the order given.  Now that you’ve gotten the bulb to light, lift 
the foil so it doesn’t touch the other thumbtack.  That’s called opening 
the switch, and the bulb should go out.       

But wait!  You still don’t have a flashlight.  When the bulb was lit, the 
light went in all different directions – It doesn’t help much to be able 
to light up a whole room when all you wanted to do was light a path for 
you to walk at night!  You need to gather all those beams of light and 
make them point in the same direction.  That’s where the reflector comes 
in.  To make it, coat a 4” square piece of thin cardboard with rubber 
cement or white glue, lay aluminum foil over it - shiny side up, smooth 
out any wrinkles, and let it dry.  Turn it over so the foil is face down, 
and draw a trapezoid (look it up in a dictionary or math book), as shown 
below.  Cut out the trapezoid and gently curve it, foil side on the 
inside, until the two same-size sides meet. You want to end up with 
something that looks like a funnel - a small hole on one end for the bulb, 
and a large hole on the other end for the light beam to come through.  Put 
tape around the outside to hold its shape.  The small hole will be large 
enough so you can move the bulb in or out of the cone.  Try several 
different positions and see how it changes the pattern of the light.  Good 
luck, and have fun!

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