Specifications

149
CONSTRUCTION
incremented automatically whenever a
pressed key is released (as during regis-
ter operation). If the data entered via
keyboard is found equal on comparison
with stored data, the output (O
A=B
) pin
19 of magnitude comparator IC3 will go
to logic low for the period the keypad
key is kept pressed. On release of the
key, pin 19 will come back to its previ-
ous state (logic 1), thus creating a pulse.
During logic 0 state at pin 19 of IC3,
LED2 will glow to indicate correctness of
the code entered. When this LED goes
‘off’, you may enter the next digit of the
secret code. If LED2 does not flash, it
means that the digit you entered was
not the right one. Now press reset switch
S3, and start entering the secret code
from the first digit onward again. This
kind of error and reset operation will,
however, not effect the lock status.
Pin 19 of IC3 is connected to clock
input pin 10 of IC6 (74HC4040). In verifi-
cation mode, whenever a correct digit of
secret code is entered, LED2 will flash
and IC3 will generate a pulse at its pin
19. This pulse will advance the counter
IC6, until the auto-reset function (depen-
dent on position of jumper JPN1) is in-
voked by current count value of IC8. If
all the digits of secret code match the
entered digits correctly, IC6 will provide
a pulse at one of its output pins (i.e. 7, 6,
5, 3, or 2), depending upon the selected
secret code length. Pin 7 will give this
pulse for 2-digit length, pin 6 for 4-digit
length, pin 5 for 8-digit length, pin 3 for
16-digit length, and pin 2 for 32-digit
length of secret code. One of these out-
puts has to be jumpered to pins 3 and 11
of IC10 using jumper JPN2. In fact, the
identical output pins of IC6 and IC8 have
to be connected to pins 3/11 (shorted) of
IC10 and pin 2 of IC9, respectively,
through jumpers JPN2 and JPN1.
IC10 (74LS74) is a dual J-K flip-flop
chip, in which both the flip-flops are con-
figured to work in toggle mode. Both the
flip-flops get same trigger input through
pins 3 and 11 respectively. One of the
flip-flops drives LED3 connected to Q1
(pin 5), while output Q2 at pin 9 con-
nected to the base of transistor T2,
through resistor R11, drives relay RL1,
whose contacts may be used for switch-
ing ‘on’/‘off’ supply to a lock or appliance.
One could even use it for locking/unlock-
ing of mains supply to any appliance.
When the lock is in ‘open’ state (i.e. RL1
energised), LED3 will be ‘on’. LED3, when
‘off’, will mean ‘closed’ state of the lock.
The whole circuit (excluding keypad)
Fig. 4: Component layout for the PCB
can be assembled on a 12x10 cm single-sided, gen-
eral-purpose PCB, using a few wire jumpers. How-
ever, an actual-size, single-sided PCB for the com-
plete circuit shown in Fig. 1, and that of power sup-
ply in Fig. 2, is shown in Fig. 3. The component lay-
out for the PCB is shown in Fig. 4.
The total cost of construction of this circuit will
not exceed Rs 800. The use of IC bases for ICs will be
a good practice. MOSFET BS170 used in the circuit
is very sensitive to static electricity, so it needs to be
handled with care. A 100nF bypass capacitor should
be used with each chip.
Operation
1. After assembling the circuit, recheck all the connec-
tions and apply power without putting the ICs into their
bases. Verify the supply and ground pin voltages at all
the IC bases. Then plug-in all the chips into their bases,
after turning ‘off’ the power.
2. Put switch S1 in ‘register mode’
position. Decide a secret code. Suppose it
is a 4-digit long code; select a page using
4-way DIP switch S2.
3. Use jumper JPN1 to extend pin 6
of IC8 to pin 2 of IC9 and jumper JPN2
to extend pin 6 of IC6 to pins 3/pin 11 of
IC10.
4. Turn ‘on’ power and press reset
switch S3 momentarily. Now press the
keypad key corresponding to the first digit
of your secret code. LED1 will light up
and buzzer will sound briefly. Then enter
the rest of the three digits of your secret
code one-by-one in a similar way.
5. Flip switch S1 to ‘operate/verify’
position, since secret code registration is
over. You have to remember the switch
S2 combination (i.e. page number) and
the secret code. To open or close the lock,
make sure that switch S2 is in the same
position as used during secret code regis-
tration. Now press reset switch S3 mo-
mentarily and enter secret code digits
one-by-one. On entry of each secret code
digit correctly, you will get a confirma-
tion signal through flashing of LED2. Af-
ter entering all digits, the lock will re-
spond (relay will energise). If the code
entered was correct, then LED3 will light
up. If secret code has not been entered
correctly, re-enter the same after press-
ing reset switch S3. Switch S1, along with
the whole circuit, must be kept hidden
while in use, except for the keyboard and
switches S2 and S3.
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