Elenco Basic Electronic Experiments - Manual (Page 60)

60
EXPERIMENT #43: ALARM WITH SHUT-OFF TIMER
Let’s demonstrate a use for the timer circuit you just built by combining it with Experiment 30, the Alarm. Connect the circuit
(noting that the transistor bases and transformer signals are not connected although their wires cross in the schematic).
Connect the alarm trip wire and then connect the battery wire to turn the circuit on. Press the switch once. Now disconnect
the trip wire to activate the alarm. The alarm stays on for a few seconds and then goes off. Re-connect the trip wire and
press the switch to reset the alarm and timer. If you only re-connect the trip wire without resetting the timer then the alarm
won’t work the next time. You could use a circuit like this where you get lots of false alarms and you want to shut off the
alarm before the battery gets weak. Automobile alarms, for example, get lots of false alarms.
+9V
100
μ F
100k
Ω
10k Ω
3.3k
Ω
470 Ω
33k Ω
S
P
473
“TRIP” WIRE
DIODE