Thames & Kosmos Electronics - Sensors & Detectors

EXPERIMENT 

Flashing alarm
system
YOU WILL NEED
 your flasher circuit from
Experiment 14
  short wire bridge
As soon as you mount the wire bridge on
your flasher circuit, the base (B) and
emitter (E) terminals of transistor T1 are
short-circuited, and the transistor can’t
work. The flashing immediately stops,
and the red LED shines uninterruptedly.
When you pull the wire bridge back out
again, the circuit starts to flash: Alarm!
That’s how easy it is to turn your flasher
circuit into an alarm system…
 WHAT’S HAPPENING?
HERE’S HOW
1. Insert a small wire bridge into the
contact clips at the bottom left.
What change do you notice?
2. Remove the wire bridge again.
+
–
9V
T2
T1
A
B
R 2
220 kΩ
+
10 µF
C 2
+
100 µF
C 1
LED 1
R 4
470 Ω
R 3
470 Ω
R 1
100 kΩ
LED 2
Tip
Instead of the small wire
bridge, of course, you could try
using a tripwire or a door
contact, as you did in the alarm
system in Experiment 5.
LED1
LED2
T1
+
–
T2
A
A
C
C
C1  µF
R3  Ω
C2  µF
R
 Ω
R2
 kΩ
+
+
A
B
R
 kΩ