Equipment
for
"
All-Weather Testing of Natural and
Artificial UV Protection"
by
John Pickle, jpickle@mos.org
Museum of Science Boston, MA
March 17, 2003
Objective:
Make a pair of UV lights for controlled experiments in exploring
the protection of sunscreens, sunglasses, clothing, hair,
and other materials from the SunÍs ultraviolet radiation.
Materials per pair of lights:
- 2 UV
LEDs from All
Electronics Corp. (operates best 3.7 volts and 20
milliamps; $1.75 each at -- http://www.allelectronics.com/cgi-
bin/category.cgi?category=search&item=
ULED-1&type=store)
- 1
270-ohm resistors ($0.50
each at All Electronics
Corp. -- http://www.allelectronics.com/matrix/
One_Half_W_Resistors.html)
- 1
9-volt battery and holder (available at most local electronic
stores)
- 3
3-inch pieces of plastic covered wire with 0.25 to 0.5
inches striped from each end
- base
of plastic sandwich-size container (lid could be used
to protect LEDs during storage)
- screw-type
wire connectors (smallest size) available at most local
electronic and hardware stores for less than $0.10 each)
- 1
rubber band
If
you canÍt get the above UV LEDs, you will need to recalculate
the resistance of the resistor.
For
example, to obtain the required resistance for the above UV
LEDs:
(9
volt battery Æ 3.7 volts required for LED) / 0.020 amp = 265
ohms (270 ohms is close enough and is commonly sold in electronic
stores)
For
additional information on this calculation, see the Java
script calculator.
Connecting
the Wires
First,
Light Emitting Diodes (LEDs) require electricity to run in
the proper direction Æ they will not work if wired backwards!
So remember, the shorter wire of the LED must be connected
to the negative charge, and the longer wire of the LED must
be connected to the positive charge. Or, red to long,
and black to short.
You
will be making a parallel circuit Æ where the red wire coming
from the battery is connected to each long leg of the LEDs.
A 270-ohm resistor is connected to the short leg of each LED,
which in turn is connected to the black wire coming from the
battery.
Building
Steps
Working
with the plastic container, first use an awl, pin,
pen, or ice pick to poke 4 holes in the plastic container.
It might help to make a paper template so the spacing for
each set of 2 holes is placed equidistant from the edges of
the container. Each set of 2 holes are approximately 0.5 inch
apart.
Second,
place the LEDs on the inside of the box and push the wire
leads through the holes.
Third, connect the wires to the LEDs, resistor and battery.
- a)
Wrap the POSITVE (red) battery wire to the long lead of
one LED.
- b)
Wrap one end of a 3-inch piece of wire (green in this case)
with the red battery wire and the LED.
- c)
Twist a wire connector around the three wires.
- d)
Wrap the free end of the green wire to the long lead of
the second LED; twist a wire connector around the two wires.
- e)
Connect one end of each of the remaining two 3-inch pieces
of green wire to the short leads of each LED; twist a wire
connector around the two wires.
- f)
Connect the two free ends of the green wires around a lead
from the resistor (any end of the resistor will work). Twist
a wire connector around the three wires.
- g)
Connect the black wire from the battery to the free end
of the resistor and twist a wire connector around the two
wires.
Finally,
put the rubber band around the outside of the box and secure
the battery to the box with the rubber band. To operate, connect
the battery leads to the battery.
Make
sure the LEDs are pointing directly down when operating. To
secure the LEDs in place you can pinch together the LED leads
that poke out of the box Æ but do not let the wires touch!
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