Specifications

CONSTRUCTION
may be. Due to conduction of diode D7,
transistor T2 will be ‘on’ state and thus
binary counter (IC5) is in runing
condingion. At the same time, ‘auto re-
set’ transistor T4 will also be in ‘on’
state, with its collector pulled low. Thus,
IC5 will continue to operate normally.
When slide switch S1 is slided from
‘set’ position to ‘start’ position, the red
LED1 immediately glows. Transistor T4
goes to 'cut-off' and its collector tran-
sits from 'low' to 'high' state. The high-
going spike is coupled through capaci-
tor C3 to reset pin 12 of IC5. Thus, IC5
is reset and starts counting from begin-
ning. During this operation, pin 10 of
both IC3 ad IC4 are held low to select
countdown mode of operation. Transis-
tor T1 goes to 'cut-off' state. Thus carry
'out' pin 7 of IC3 and carry 'in' pin 5 of
IC4 are cascaded through resistor R16.
The one-pulse-per-minute is applied to
pin 15 of both ICs (IC3 and IC4) through
diodes D3 and D4. Now the digits dis-
played in DIS.1-DIS.2 combination start
decrementing once every minute. When
digits displayed in DIS.1-DIS.2 become
'00', carry 'out' pin 7 of both IC3 and
IC4goes 'low' and transistor T2 does not
conduct. As a result, collector of tran-
sistor T2 goes 'high' and the binary
counter stops counting. Simultaneously,
transistor T3 conducts and activates the
buzzer (functioning in interrupted
mode). Thus, interrupted beep sound is
heard from the buzzer, indicating that
preset time duration has ended. Pin 7
of both IC3 and IC4 goes 'low' during
display of digits '00' in DIS.1 and DIS.2
During display of any digits other than
'00', the carry 'out' pin 7 of either IC3
or IC4 will be high or both may be 'high'.
When the timer is in countdown mode,
do not press switch S2 or S3 to avoid
disturbance in timer setting.
This circuit, apart from using as a
conference timer, may be converted into
programmable 2-digit 'on' or 'off' timer
to switch 'on'/'off' any electrical or elec-
tronic appliance after 1 minute to 99
minutes duration by incorporating ad-
ditional add-on circuit shown in Fig.2.
During 'on'/off timer operation, the dig-
its displayed in DIS.1-DIS.2 help one to
know the exact leftover time to switch
‘on’ ‘off’ the appliance. When using this
circuit as ‘off’ timer, slide switch S4 to
‘off’ position and, for ‘on’ timer opera-
tion, slide switch S4 to 'on' position.
When displayed digits become '00', the
relay will be energised to turn 'on'/'off'
the load.
The actual-size, single-sided PCB for
the circuit in Fig.1 is shown in Fig. 3,
while its component layout is given in
Fig. 4.
G
96