Data Sheet

-61-
92. Revisiting Kirchhoff’s Current Law
Build the circuit shown on the left, turn on the switch (62) or press the
press switch (61) and you will light up the heart LED (69). If you want to
light up the star LED (70), then you also need to move the magnet (7)
near the reed switch (83).
The LEDs in this project are in parallel just like they were in project #91
so we would get the same ammeter measurements as discussed in
project #91. If you hooked up an ammeter to the current path coming
out of the battery (91), you would measure about 84 mA. This shows
that the current being drawn from the battery (91) is equal to the
sum of the currents through the star LED (70) and heart LED (69)
(Kirchhoff’s Current Law in action!).
93. NOT Gate
Build the circuit shown, turn on the switch (62) and press the press
switch (61) and you will see both the LEDs are on. If you touch the reed
switch (83) with the magnet (7), you will see that the star LED (70) goes
off, but the heart LED (69) is brighter. This is because
by activating the reed switch (83) you are bypassing
the star LED (70) so the heart LED (69) sees the full
4.5V from the battery (91). In order to represent this
circuit’s logic, we need a NOT gate. The symbol for
a NOT gate and logic table are shown on the right.
The function of a NOT gate is to invert the input (if the
input is ON then the output is OFF and if the input is
OFF then the output is ON).
With the help of the NOT Gate, we can now represent the logic in this
circuit as shown below.
Input Output
A B
ON OFF
OFF ON
NOT Gate
+
–
+
–
1st level
2nd level
+
–
+
–