Specifications

CONSTRUCTION
I
n this innovative project, a simple
electronic bell system using com-
monly available ICs is presented for
use in educational institutes. This
simple and easy-to-fabricate project has
the following features:
It sounds the bell automatically
after every period of 40 minutes.
It displays in digital form the cur-
rent time and period number of the class
going on.
The system automatically switches
off after the last period (11
th
period). The
digital clock showing the current time,
however, continues working as usual.
The principle
Fig. 1 shows the block diagram of the
system, which has three parts. Part I
has the usual digital clock comprising
quartz crystal oscillator cum frequency
divider IC MM5369, clock chip MM5387,
and 7-segment common-cathode displays.
The 1Hz pulse (i.e. one pulse per
sec.) is taken from the digital clock and
used in part II of the circuit. The accu-
racy of the system depends on this 1Hz
pulse, obtained from the standard digi-
tal clock circuit. In part II of the sys-
tem, the 1Hz pulse is used to obtain
one pulse after every 40 minutes, by
employing a four-stage counter circuit.
The pulses obtained at 40-minute in-
tervals drive transistor T4 (see Fig. 2)
into saturation for a few seconds
(the exact duration being decided by the
delay circuit comprising 56-kilo-ohm re-
sistor R47, 100µF capacitor C4, and di-
ode D7). When the transistor goes into
saturation, relay RL2 is energised and
the bell sounds for a few seconds.
Any electronic horn/siren using an
audio power
amplifier of de-
sired wattage
may be used for
the bell. In the
prototype, the
author used an
audio tape re-
corded with the
usual sound of
brass bell, with
tape recorder/
player of 150
watts rating,
driving four 20-
watt speaker
units. It was
found adequate
for the campus
of any educa-
tional institute.
The readers
may, however,
use any other
sound system
according to
their require-
ments.
Part III consists of the period counter
and display. It displays the current pe-
riod in progress. The number of pulses
received at 40-minute intervals are
counted by this counter circuit and the
display unit displays the period number.
One additional relay circuit is used
so that the power supply given to parts
II and III of the system is automati-
cally interrupted at the end of the elev-
enth period. Next day the system has
to be reset, and the cycle repeats.
The circuit
Fig. 2 shows the detailed circuit diagram.
The clock circuit of part I of the system is
designed using 3.58MHz quartz crystal,
MM5369 crystal oscillator and divider
(IC3), MM5387 clock chip (IC4), four com-
ELECTRONIC BELL
SYSTEM
Dr D.K. KAUSHIK
S.C. DWIVEDI
PARTS LIST
Semiconductors:
IC1 - 7805 +5V regulator
IC2 - 7474 dual ‘D’ flip-flop
IC3 - MM5369 oscillator/driver
IC4 - MM5387/LM8361 clock chip
or equivalent
IC5, IC6 - CD4026 decimal up-counter
with 7-segment driver
IC7-IC10 - CD4017 decade counter
T1, T2 - BC107 npn transistor
T3, T4 - 2N2222 npn switching
transistor
D1-D8 - 1N4001 rectifier diode
LED1, LED2 - Red LED
Resistors (all ¼-watt, ±5% carbon, unless
stated otherwise):
R1, R2 - 2.2-kilo-ohm
R3, R44, R50 - 1.5-kilo-ohm
R4 - 4.7-kilo-ohm
R5, R6, R45
R46, R48 - 10-kilo-ohm
R7-R43 - 330-ohms
R47 - 56-kilo-ohm
R49 - 20-mega-ohm
Capacitors:
C1, C4 - 100µF, 25V electrolytic
C2 - 30pF ceramic disk
C3 - 30pF trimmer
Miscellaneous:
S1-S4 - Tactile switch (SPST)
S5 - Tactile switch (DPDT)
X
TAL
- 3.57945MHz crystal
RL1-RL2 - 12V, 200-ohm relay (SPST)
DIS.1-DIS.6 - LT543 common-cathode
7-segment display
- Power amplifier with
loudspeaker
Fig. 1: Block diagram of the electronic bell system
191