Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 46)

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This circuit is unusual in that you turn it on by
disconnecting a wire and turn it off by connecting the
wire. Connect the wires according to the Wiring Checklist
and schematic, 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.
EXPERIMENT #28: The Alarm
Wiring Checklist:
o 27-to-24
o 19-to-25
o 18-to-43
o 5-to-21
o 6-to-22
o 54-to-23-to-30
o 31-to-42-to-53
o 20-to-26
o 26-to-53 (use a long wire,
this will be referred to as
the “trip” wire)
Schematic
Long “Trip” Wire