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AUTOMATIC SCHOOL BELL C I R C U I T I D E A S
ELECTRONICS FOR YOU
OCTOBER 2004
C
onsider that a school has a total of
eight periods with a lunch break
after the fourth period. Each period
is 45 minutes long, while the duration of
the lunch break is 30 minutes.
To ring this automatic school bell to
start the first period, the peon needs to
momentarily press switch S1. Thereafter,
the bell sounds every 45 minutes to indi-
cate the end of consecutive periods, ex-
cept immediately after the fourth period,
when it sounds after 30 minutes to indi-
cate that the lunch break is over. When
the last period is over, LED2 glows to in-
dicate that the bell circuit should now be
switched off manually.
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. Each pushing of switch S3 ad-
vances the time by 4.5 minutes. If the
school is closed early, the peon can turn
the bell circuit off by momentarily press-
ing switch S2.
The bell circuit contains timer IC
NE555 (IC1), two CD4017 decade counters
AUTOMATIC SCHOOL BELL
RAJ KUMAR MONDAL
S.C. DWIVEDI
(IC2 and IC3) and AND gate CD4081 (IC4).
Timer IC1 is wired as an astable
multivibrator, whose clock output pulses
are fed to IC2. IC2 increases the time
periods of IC1 (4.5 and 3 minutes) by ten
times to provide a clock pulse to IC3 ev-
ery 45 minutes or after 30 minutes, re-
spectively. When the class periods are go-
ing on, the outputs of IC3 switch on tran-
sistors T1 and T2 via diodes D4 through
D12.
Resistors R4 and R5 connected in se-
ries to the emitter of npn transistor T2
decide the 4.5-minute time period of IC1.
The output of IC1 is further connected to
pin 14 of IC2 to provide a period with a
duration of 45 minutes. Similarly, resis-
tors R2 and R3 connected in series to the
emitter of npn transistor T1 decide the 3-
minute time period of IC1, which is fur-
ther given to IC2 to provide the lunch-
break duration of 30 minutes.
Initially, the circuit does not ground
to perform its operation when 12V power
supply is given to the circuit.
When switch S1 is pressed momen-
tarily, a high enough voltage to fire sili-
con-controlled resistor SCR1 appears at its
gate. When SCR1 is fired, it provides
ground path to operate the circuit after
resetting both decade counters IC2 and
IC3. At the same time, LED1 glows to in-
dicate that school bell is now active.
When switch S2 is pressed momen-
tarily, the anode of SCR1 is again
grounded and the circuit stops operating.
In this condition, both LED1 and LED2
dont glow.
When the eighth period is over, Q9
output of IC3 goes high. At this time, tran-
sistors T1 and T2 dont get any voltage
through the outputs of IC2. As a result, the
astable multivibrator (IC1) stops working.
The school bell sounds for around 8
seconds at the end of each period. One
can increase/decrease the ringing time of
the bell by adding/removing diodes con-
nected in series across pins 6 and 7 of
IC1.
The terminals of the 230V AC
electric bell are connected to the nor-
mally-open (N/O) contact of relay RL1.
The circuit works off a 12V regulated
power supply. However, a battery source
for back-up in case the power fails is also
recommended.