Specifications

CONSTRUCTION
mon-cathode 7-segment displays,
and a few passive components. For
more details of the digital clock, the
readers may consult ‘Car Clock
Module’ project in September 1986
issue or Electronics Projects (Vol.
7) published by EFY. Push-to-on
switches S1 and S2 (slow and fast
time set) may be used to set the
time of the digital clock.
(Note. For ready reference, pin
configurations of ICs MM5369 and
MM5387/LM8361 are reproduced
here in Figs 3 and 4, respectively.)
The standard 1Hz pulse is
taken from pin 39 of IC4 and con-
nected to clock input pin 14 of de-
cade counter IC7 (CD4017). The
carry pin 12 of IC7 outputs a pulse
every 10 seconds, which is con-
nected to clock pin 14 of the next
CD4017 decade counter (IC8). The
reset terminal (pin 15) of IC8 is
connected to pin 5 (output No. 6)
of the same IC. This IC thus di-
vides the signal by a factor of 6
and its pin 12 (carry pin) gives an
output of one pulse every minute.
This pulse is applied to IC9
(CD4017), where it is further di-
vided by a factor of 10 to produce
an output pulse at every 10-minute
interval. Finally, a pulse at every
40-minute interval is obtained from
IC10 (CD4017), which is configured
as divide-by-four counter, since its
reset pin 15 is shorted to Q4 out-
put pin 10 of IC10.
The output pulse at pin 3 of
IC10 remains high for ten minutes
and low for 30 minutes. This out-
put pulse (every 40 minutes) is con-
nected to the base of transistor T4
through a combination of capacitor
C4 and resistance R47, to energise
the relay and sound the bell. The
capacitor-resistor combination of
C4-R47 acts as a differentiator cir-
cuit, while diode D7 clips off the
negative going portion of the pulse.
The delay time may be adjusted by
choosing proper C4-R47 combina-
tion values.
After the preset time delay of a
few seconds, the transistor goes
into cut-off and the relay gets de-
energised, to switch off the bell.
However, the clock circuit of part I
around IC4 and divider circuit
formed by IC7 through IC10 con-
Fig. 2: Schematic diagram of electronic bell system for institutes
192