Data Sheet

-78-
134. Calculating RC Time Constant
Build the circuit shown on the left and turn on the switch (62). Place the magnet
(7) near the reed switch (83) and you will see the colorful LED (72) light the ber
tree (40). Move the magnet (7) away from the reed switch (83) and the colorful
LED (72) will stay bright for a little while and then turn dim. As discussed in the
previous project, the time constant of an RC circuit is R*C, which in this case is
1000*(470 x 10
-6
) = 0.47 second. If the 1kW resistor and 470mF capacitor (74)
represented the only resistance and capacitance in the circuit, then 0.47 second
would represent the time for the voltage on the 470mF capacitor (74) to reduce
from its maximum value to ~63% of its maximum value.
135. Relative RC Time Constant
Replace the 1kW resistor (42) in project #134 with the 5.1kW resistor (43) and
turn on the switch (62). Place the magnet (7) near the reed switch (83) and you
will see the colorful LED (72) light the ber tree (40). Move the magnet (7) away
from the reed switch (83) and the colorful LED (72) will stay bright for a while
and then turn dim. The time constant of this circuit is 5100*(470 x 10
-6
) = 2.4
seconds. The colorful LED (72) will likely take longer than 2.4 seconds to dim in this
circuit due to other resistance and capacitance not accounted for in the circuit, but
relatively speaking it should take about 5 times longer for the colorful LED (72) to
dim in this circuit compared to the previous circuit.
136. Intermittent Windshield Wipers
Replace the 1kW resistor (42) in project #134 with the 10kW resistor (44) and then
turn on the switch (62). Place the magnet (7) near the reed switch (83) and you will
see the colorful LED (72) light the ber tree (40). Move the magnet (7) away from the
reed switch (83) and the colorful LED (72) will stay bright for a several seconds and
then turn dim. This circuit could be used to create the delay for your windshield wipers.
137. Resistor Color Code Table
Replace the 1kW resistor (42) in project #134 with the 100kW resistor (45) and
then turn on the switch (62). Place the magnet (7) near the reed switch (83) and
you will see the colorful LED (72) light the ber tree (40). Move the magnet (7)
away from the reed switch (83) and the colorful LED (72) will stay bright for a long
time before turning dim. Project #128 introduced the 4-band resistor. The rst
three bands dene the resistance of the resistor using the table shown on the left.
So for instance, a 100W resistor would have the colors brown, black and brown as
the rst three bands (10 x 10
1
= 100).
Color
1st Band
(1st digit)
2nd Band
(2nd digit)
3rd Band
(Multiplier)
Black 0 0 10
0
Brown 1 1 10
1
Red 2 2 10
2
Orange 3 3 10
3
Yellow 4 4 10
4
Green 5 5 10
5
Blue 6 6 10
6
Violet 7 7 10
7
Gray 8 8 10
8
White 9 9 10
9
+
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