Elenco Projects 306-511 - Manual (Page 45)

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Project #433
OBJECTIVE: To build a manual timer using a relay and whistle
chip.
This circuit is similar to project #431 except now the whistle chip (WC)
will also make sound.
Manual 7 Second
Timer (II)
Project #434
OBJECTIVE: To build a circuit that sounds the speaker for 15
seconds.
As in project #431, the transistor (Q2) acts as a switch, connecting the
relay (S3) and the alarm IC (U2) to the batteries. As long as there is
a voltage on the transistor's base, the alarm IC sounds.
Turn on the slide switch (S1) and then press the switch (S2). The
transistor turns on, the capacitor (C5) charges up, and the alarm
sounds. Release the press switch (S2). As the capacitor discharges,
it keeps the transistor on. The transistor will turn off when the
capacitor is almost discharged, about 15 seconds. The relay contacts
will switch and the alarm will turn off.
15 Second Alarm