Data Sheet

-64-
98. Fire Drill Alarm
Build the circuit shown on the left making sure the alarm (78) is in the
correct direction. Press the switch (62) and you will hear the alarm (78)
and see the lamp (76) light.
This type of circuit could be used for re drill tests where a switch is
turned ON to set off the re alarm for the re drill and then turned OFF
when the re drill is over.
99. Resistors in Parallel
Build the circuit, turn on the switch (62) and you will see the heart LED
(69) is on. Take out either resistor and you will see the heart LED (69)
get dimmer. You may need to be in a dark room to see this. As shown in
project #53, two resistors in parallel have an equivalent resistance of:
R
equivalent
= (R
1
*R
2
)/(R
1
+R
2
)
In this case R
equivalent
= (5.1k * 10k)/(5.1k + 10k) = 3.4kW. So as you
can see, the equivalent resistance of the parallel combination of the two
resistors is less than the value of either resistor, so when you pull one
of the resistors out of the circuit it actually increases the resistance in
the circuit, which makes the heart LED (69) dimmer.
+
–
+
–