Specifications

193
CONSTRUCTION
tinue to
work as
usual and
hence the
accuracy
of the pe-
riods is
not af-
fected by
the ‘on’ and ‘off’ times of transistor T4.
To count and display the current pe-
riod, a two-digit counter is designed us-
ing two CMOS decade counter cum 7-
segment decoder/driver CD4026 ICs
(IC5 and IC6) and two 7-segment com-
mon-cathode displays (LT543). The pulse
obtained every 40 minutes from pin 3
of IC10 is also connected to the input of
this two-digit counter. This counter
counts these pulses and displays them
via the LT543 (showing the current pe-
riod number in progress). The two-digit
counter counts and displays the period
number up to 11.
The segment ‘d’ output for most sig-
nificant digit (MSD) and segment ‘c’ out-
put for least significant digit (LSD) from
IC5 and IC6 are connected to the bases
of transistors T1 and T2 respectively,
via 2.2-kilo-ohm resistors R1 and R2.
The collectors of the two transistors are
connected together, working as a NOR
gate. When ‘d’ and ‘c’ segment driving
outputs from IC5 and IC6 respectively,
go low simultaneously (just at the be-
ginning of 12
th
period), the output (com-
mon collector voltage of transistors T1
and T2) goes high. This output is also
connected to clock pin 3 of IC2 (IC 7474),
which is a dual ‘D’ flip-flop.
Only one of the two flip-flops is used
here in toggle mode by connecting its
Q pin 6 to data (D) pin 2. The
flip-flop toggles after every clock pulse.
The ‘Q’ output of this flip-
flop drives relay RL1 through
transistor T3, and thus
switches off the supply to
parts II and III of the sys-
tem, just at the beginning of
12
th
period (i.e. at the end of
11
th
period). Resumption of
the supply may take place
the next day after momen-
tarily pressing switch S3. For
power supply, a 12V car bat-
tery with charging facility is
recommended.
An actual-size, single-
sided PCB for the circuit
(Fig. 2) is shown in Fig. 5 and
the component layout for the
PCB in Fig. 6. The total cost
of the system, excluding cost
of audio amplifier, tape re-
corder, horn, etc, is about Rs
1,000.
Operation
After completing the circuit,
test the circuit according to
circuit description, as discussed above.
For operation of the circuit, switch S3 is
momentarily pressed for resumption of
the supply to parts II and III of the
circuit, as relay RL1 is energised. Pe-
riod-displaying 7-segment displays DIS.5
and DIS.6 will display any random num-
ber, which is reset to 00 by momentary
depression of
switch S4.
Further,
switch S5
(DPDT) is
pressed and
then released
exactly at the
time when
the first pe-
riod is to
start. This re-
sets IC7
through IC10.
The output
Q0 at pin 3 of
IC10 will go
high, to
energise the
relay and
thus switch
on the bell for
a few seconds
and advance
Fig. 3: Pin configuration of
MM5369
Fig. 5: PCB layout
Fig. 4: Pin configuration of IC MM5387/LM8361