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CIRCUIT IDEAS

AUTOMATIC SCHOOL BELL
RAJ KUMAR MONDAL

DI
WIVE

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D
S

onsider that a school has a total of
eight periods with a lunch break
after the fourth period
...

To ring this automatic school bell to
start the first period, the peon needs to
momentarily press switch S1
...

Timer IC1 is wired as an astable
multivibrator, whose clock output pulses
are fed to IC2
...
5 and 3 minutes) by ten
times to provide a clock pulse to IC3 every 45 minutes or after 30 minutes, respectively
...

Resistors R4 and R5 connected in series to the emitter of npn transistor T2

gate
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At the same time, LED1 glows to indicate that school bell is now active
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In this condition, both LED1 and LED2
don’t glow
...
At this time, transistors T1 and T2 don’t get any voltage

when it sounds after 30 minutes to indicate that the lunch break is over
...

In case the peon has been late to start
the school bell, the delay in minutes can
be adjusted by advancing the time using
switch S3
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5 minutes
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The bell circuit contains timer IC
NE555 (IC1), two CD4017 decade counters

decide the 4
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The output of IC1 is further connected to
pin 14 of IC2 to provide a period with a
duration of 45 minutes
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Initially, the circuit does not ground
to perform its operation when 12V power
supply is given to the circuit
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As a result, the
astable multivibrator (IC1) stops working
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One
can increase/decrease the ringing time of
the bell by adding/removing diodes connected in series across pins 6 and 7 of
IC1
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The circuit works off a 12V regulated
power supply
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C

ELECTRONICS FOR YOU

OCTOBER 2004



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