User manual

16
7 An alarm system
For this little alarm system, we will need a second cable. This
is used as the connection between the base, which shuts off
the collector current, and the emitter of the transistor. If you
pull out the cable, an alarm is triggered, and the red LED
goes on.
You could build an alarm system using a thin wire that
breaks, if someone opens a window or door. If formed as an
alarm loop, the wire could also secure several windows and
doors. Should a burglar discover and simply cut the wire to
turn off the alarm, then he will have a day of reckoning. In
such a case, the alarm will go off, too.
When in the alarm state, a small amount of current flows
through the green LED, the 330 kΩ resistor and through the
base terminal of the transistor. The transistor thus turns on
the collector current, and the red LED lights up. The alarm
loop forms a short circuit between base and emitter. This
turns off the base current. Without base current there is no
collector current flow and the red LED stays turned off. But
if you disconnect the alarm loop, the transistor will switch on.
A small current always flows, even without an alarm. The
green LED is dimly lit and shows that the alarm is enabled.
The battery will last for several years due to the very small
amount current used in standby mode. But if an alarm is trig-
gered, then more current will flow.