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A programmable code lock can be used for numerous applications
in which access to an article/gadget is to be restricted to a limited number
of persons. Here is yet another circuit of a code lock employing mainly the
CMOS ICs and thumbwheel switches (TWS) besides a few other components. It is
rugged and capable of operation on voltages ranging between 6 and 15 volts.
The supply current drain of CMOS ICs being quite low, the circuit may be operated
even on battery.
The circuit uses two types of thumbwheel switches. switch
numbers TWS1 through TWS8 are decimal-to-BCD converter type while switch numbers
TWS9 through TWS16 are 10-input multiplexer type in which only one of the ten
inputs may be connected to the output (pole). One thumbwheel switch of each
of the two types is used in combination with IC CD4028B (BCD to decimal decoder)
to provide one digital output.Eight such identical combinations of thumbwheel
switches and IC CD4028 are used. The eight digital outputs obtained from these
combinations are connected to the input of 8-input NAND gate CD4068.For getting
a logic high output, say at pole-1, it is essential that decimal numbers selected
by switch pair TWS1 and TWS9 are identical, as only then the logic high output
available at the Specific output pin of IC1 will get transferred to pole-1.
Accordingly, when the thumbwheel pair of switches in each combination is individually
matched, the outputs at pole-1 to pole-8 will be logic high.This will cause
output of 8-input NAND gate IC CD4068b to change over from logic high to logic
low, thereby providing a high-to-low going clock pulse at clock input pin of
7-stage counter CD4024B, which is used here as a flip-flop (only Q0 output is
used here).The output (Q0) of the flip-flop is connected to a relay driver circuit
consisting of transistors T1 and T2. The relay will operate when Q0 output of
flip-flop goes low. As a result transistor T1 cuts off and T2 gets forward biased
to operate the relay.Switch S1 is provided to enable switching off (locking)
and switching on (unlocking) of the relay as desired, once the correct code
has been set.
With the code set correctly, the NAND gate output will stay
low and flip-flop can be toggled any number of times, making it possible to
switch on or switch off the relay, as desired. Suppose we are using the system
for switching-on of a deck for which the power supply is routed via the contacts
of the relay. The authorised person would select correct code which would cause
the supply to become available to the deck. After use he will operate switch
S1 and then shuffle the thumbwheel switches TWS1 through TWS8 such that none
of the switches produces a correct code. Once the code does not match, pressing
of switch S1 has no effect on the output of the flip-flop.Switches TWS9 through
TWS16 are concealed after setting the desired code. In place of thumbwheel switches
TWS1 through TWS8 DIP switches can also be used |
