Elenco Basic Electronic Experiments - Manual (Page 46)

46
EXPERIMENT #30: THE ALARM
This circuit is unusual in that you turn it on by disconnecting a wire and turn it off by connecting the wire. Connect the
circuit, including a long wire as the “trip” wire. Notice that there is no sound. Now disconnect the trip wire and you hear a
sound, an alarm.
This type of circuit is used to detect burglars or other intruders. If you use a longer trip wire, you can place it across a
doorway or window and when someone goes through the doorway or window they will trip on the wire (disconnecting it)
and the alarm will sound. This is how professional burglar alarms work, although some use beams of light across the
doorway or window instead of wire for the “trip” mechanism. The trip wire could also alert your local police station instead
of turning on the alarm here.
This circuit is the same oscillator circuit you just used except that the trip wire was added. The trip wire creates a “short
circuit” across the transistor base, so no current flows into the base and the transistor stays off. Disconnecting the trip wire
eliminates the short and the oscillator works normally.
If you like, you can adjust the loudness of the alarm by replacing the 3.3k
Ω resistor with the variable resistor.
SPEAKER
LONG “TRIP” WIRE
+9V
S
P
1M Ω
502
3.3K Ω
.005 μF