Electronic circuits > Home/Garden > Programmable Digital Code Lock
Programmable Digital Code LockA 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
Published on: 2004-08-21 (3409 reads)
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