Specifications

CONSTRUCTION
95
operation, this pulse (one-per-minute)
is simultaneously applied to pin 15 of
IC3 and IC4 through switch S1 (in start
position), and diodes D3 and D4. For
presetting this timer, bounceless pulses
are required at clock pin 15 of both IC3
and IC4. For this reason 1Hz
(bounceless) pulses available from pin 7
of IC5 are used to preset the timer.
IC1 and IC2 (CD4511B), 7-segment
latch and driver, accept BCD input code
from up/down counters IC3 and IC4
(CD4510B). They convert the
BCD code to 7-segment posi-
tive logic output code to dis-
play the equivalent decimal
digits. While displaying deci-
mal digits 9 and 6, their tails
are not displayed. The store
function available in these ICs
is not used in this circuit and
hence the store pin 5 of IC1
and IC2 is made permanently
low.
CD4510B is a divided-by-10 BCD up/
down counter. This counter increments
or decrements by one count for every
low-to-high transition of the clock pulse
applied to its clock pin15, depending on
the logic level at its pin 10. Thus, when
pin 10 of IC3 and IC4 are held high, the
counters increment by one count for ev-
ery clock pulse, and when they are held
low, the counters decrement by one
count for every clock pulse. The up
counting mode is used to preset the con-
ference timer while its down counting
mode is used for normal timer opera-
tion.
When presetting, the carry 'pout' pin
7 of IC3 and carry 'in' pin 5 of IC4 are
not cascaded, to permit presetting of
tens and units digits independently (us-
ing push-to-on tactile switches S2 and
S3, respectively). In countdown mode
carry 'out' pin 7 of IC3 and carry 'in'
pin 5 of IC4 are cascaded for 2-digit
countdown timer operation. For preset-
ting function, 1Hz (approx.) pulses
are used, while for normal cas-
caded countdown operation of the
timer, pulse rate of one-pulse-per-
minute is used. As stated earlier,
these two types of pulses are
available from pins 7 and 15, re-
spectively, of IC5.
Transistor T2 is used to stop
or activate the IC5 binary
counter. When transistor T2 is in
'cut-off'' state, its collector volt-
age goes 'high'. As a result, the
positive supply rail is extended
to pin 11 of IC5 via resistor R23
and diode D6 to stop IC5 from
counting further. When transis-
tor T2 conducts, its collector volt-
age goes low and counter IC5 be-
comes active. The stop and run
functions of IC5 binary counter
are used during countdown opera-
tion only. While presetting, the
IC4 binary counter will be in run-
ning condition.
When slide switch S1 is slided
to 'set' position, pin 10 of both IC3
and IC4 is taken 'high' to select
the countup mode for presetting
th timer. As the same time, tran-
sistor T12 gets forward biased and
conducts. As a result, its collec-
tor as well as pin 5 of IC4 go 'low'.
Pin 5 of IC3 is permanently low
and both these ICs are not cas-
caded. The one-pulse-per-minute
(from pin 15 of IC5) is no longer avail-
able to diode D3-D4 junction, while 1Hz
pulse (available from pin 7 of IC5 ) may
be applied to hte clock input in 15 of
IC3 or IC4 by pressing the respective
tactile switches S2 and S3. For preset-
ting the timer, depress tactile switch
S2 and S3 until desired count is dis-
played in unt and tens digit (DIS.1 and
DIS.2). When desired digit has been dis-
played in DIS.1 or DIS.2, immediately
release switch S2 and S3, as the case
Fig. 2: Appliance ‘on/off’ switching application for timer
Fig. 3: Actual-size, single-sided PCB layout
Fig. 4: Component layout for the PCB