Specifications

CIRCUIT IDEAS
rent local call meter reading will blink
once every second.
In prototype circuit, the author used
TAKSUN calculator that costs around Rs
80. The display height was 1 cm. In this
calculator, he substituted the two button-
type batteries with two externally con-
nected 1.5V R6 type batteries to run the
calculator for more than an year.
The power ‘off’ button terminals were
made dummy by affixing cellotape on con-
tacts to avoid erasing of memory, should
someone accidentally press the power ‘off’
button. This calculator has auto ‘off’ fa-
cility. Therefore, some button needs to be
pressed frequently to keep the calculator
‘on’. So, in the idle condition, the ‘=’ but-
ton is activated electronically once every
second by transistor T1, to keep the cal-
culator continuously ‘on’.
Useful hints. Solder the ‘=’ button
terminals by drilling small holes in its
vicinity on PCB pattern using thin cop-
per wire and solder it neatly, such that
the ‘=’ button could get activated electroni-
cally as well as manually. Take the cop-
per wire through a hole to the backside
of the PCB, from where it is taken out of
the calculator as terminals G and H.
At calculator’s battery terminals, sol-
der two wires to ‘+’ and ‘–’ terminals.
These wires are also taken out from cal-
culator as terminals E and F. Affix COB on a gen-
eral-purpose PCB and solder the remaining compo-
nents neatly. For giving the unit an elegant look,
purchase a jewellery plastic box with flip-type cover
(size 15cm x 15cm). Now fix the board, calculator,
and batteries, along with holder inside the jewellery
box. Then mount the box on the wall and paste the
look-up table inside the box cover in such a way that
on opening the box, it is visible on left side of the box.
Caution. The negative terminals of battery A
and battery B are to be kept isolated from each other
for proper operation of this circuit.
T
he circuit diagram of a simple elec-
tronic code lock is shown in fig-
ure. A 9-digit code number is used
to operate the code lock.
When power supply to the circuit is
turned on, a positive pulse is applied to
the RESET pin (pin 15) through capaci-
tor C1. Thus, the first output terminal
Q1 (pin 3) of the decade counter IC (CD
4017) will be high and all other outputs
(Q2 to Q10) will be low. To shift the high
state from Q1 to Q2, a positive pulse must
be applied at the clock input terminal (pin
14) of IC1. This is possible only by press-
ing the push-to-on switch S1 momentarily.
On pressing switch S1, the high state
shifts from Q1 to Q2.
Now, to change the high state from Q2
to Q3, apply another positive pulse at pin
14, which is possible only by pressing switch
S2. Similarly, the high state can be shifted
up to the tenth output (Q10) by pressing
the switches S1 through S9 sequentially
in that order. When Q10 (pin 11) is high,
transistor T1 conducts and energises relay
RL1. The relay can be used to switch ‘on’
power to any electrical appliance.
Diodes D1 through D9 are provided
to prevent damage/malfunctioning of the
IC when two switches corresponding to
‘high’ and ‘low’ output terminals are
number or letter can be used to mark them. Switch
S10 is also placed together with other switches so
that any stranger trying to operate the lock fre-
quently presses the switch S10, thereby resetting the
circuit many times. Thus, he is never able to turn the
relay ‘on’. If necessary, two or three switches can be
connected in parallel with S10 and placed on the key-
board panel for more safety.
A 12V power supply is used for the
circuit. The circuit is very simple and can
be easily assembled on a general-purpose PCB. The
code number can be easily changed by changing the
connections to switches (S1 to S9).
pressed simultaneously. Capacitor C2 and
resistor R3 are provided to prevent noise
during switching action.
Switch S10 is used to reset the circuit
manually. Switches S1 to S10 can be
mounted on a keyboard panel, and any
SIMPLE ELECTRONIC
CODE LOCK
REJO G. PAREKKATTU
S.C. DWIVEDI
68