Multichannel Access Control System

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This project deals with the implementation of a multichannel access control system which is also known as digital code lock. This system is used to prevent use of equipment by unauthorized persons. 4-digit code is needed for accessing any device which is connected to these control system. This code can be entered by using a keypad. The circuit will check the code entered via the keypad against a pre-programmed locking code. Only if these codes match, we will be able to gain access to the devices connected through the circuit. If the entered code is incorrect, access to the devices is denied. Maximum eight separated devices can be controlled by using only one code.

System Overview

A 10-digit decimal keypad comprising keys 0 through 9 is used for entering the decimal digits, which are decoded to their binary equivalents by decoder IC1. The binary outputs of IC1 are connected to the locking section via buffer B1 (half of IC2), the locking section via buffer B2 (second half of IC2), and a quad analogue switch (IC8).

To enter the 4-digit locking code, first press program switch S11. After entering the four digits sequentially, switch S11 is required to be pressed again. This depression of switch S11 enables the locking code section (comprising IC3 and IC4).Another depression toggles/restores the initial condition in which the locking section remains inhibited and the unlocking section is enabled.

The 4-digit locking and unlocking codes (converted to binary bits) are available at the output of the respective sections. These two 16-bit numbers are compared in a code comparator built around IC9 and IC10. If the locking and unlocking numbers match, the quad analog switch IC (IC8) is enabled while the clock gating circuit is initialized to freeze the locking and unlocking sections (and their outputs) until one of these is reset switch S12 or S13, respectively.

Thus once a match is found between locking and unlocking numbers, any further key depression results in the binary equivalent number at the output of IC1 to act as an address input for decoder IC11. The output corresponding to the address input goes high to toggle a specific flip-flop, which, in turn toggles the state of relay driven by its output. Addresses corresponding to keypad switches go high to toggle for digits 0 and 9 have not using the corresponding output pins of IC11. Thus switches corresponding to digits 1 through 8, when depressed, toggle the relay state (from energized to de-energized and vice versa) of the corresponding devices.

Sequential depression of the same keypad switch will not alter the state of the corresponding relay. Thus if the relay state of a specific value have been erroneously changed, depress a dummy number (0 or 9) before keying the same number again. Switch S14 is used to reset all devices simultaneously.

Circuit diagram 1

Circuit diagram 2

Circuit diagram 3

Circuit Overview

Keypad interface

The 10-digits keypad is wired to row and column inputs of IC 74C922 (IC1), which is a 16-key encoder IC with on-chip clock and debounce circuitry. The binary equivalent of the last key depressed of the last key depressed available at its output. The data available pin (Dav) of IC 74C922 (IC1) goes to high level when a valid key is made and returns to low level when the depressed key is released. The Dav output is used as clock for clocking and unlocking sections via 3-input AND gates N1 and N2, respectively. The Dav output is also used as to sound piezobuzzer PZ1via transistor T1 on entry of each digit via the keypad.

Locking and unlocking sections

The outputs of IC 74C922 (IC1) are buffered by two sections of an octal tristate buffer IC 74LS244 (IC2) before connecting the same to the locking and unlocking sections. IC 74LS244 (IC2) comprises two separately controllable 4-bit buffers designated as B1and B2.For enabling the buffers, counter IC CD4017 (IC 7) configured as a bistable flip-flop is used. Normally, the circuit is initialized for accepting the locking code since Q0 output of IC CD4017 (IC7) is initially (which disables buffer B1) and Q1 output is low (which enables buffer B2).

When depress program button S11 to enter the 4-digit locking code, Q1 output of IC CD4017 (IC7) goes high and Q0 output goes low. As a result, gate N1 is enabled and gate N2 is disabled. Simultaneously, buffer B1 is enabled and buffer B2 is disabled. The keyboard data is now routed to the locking section through buffer B1. The function of gates N1 and N2 is to allow the clock pulses (Davoutput of IC1) to reach only the selected section (locking or unlocking).

For storing and shifting the entered digits dual 4-bit static shift registers (four CD4015 ICs) have been used. The entered digits are shifted from right to left as in calculators. Shifting and storing for the locking section is done using IC3 & IC4, while the similar function for the locking section is achieved using IC5 & IC6.

Assume that the locking section using program switch S11 have been enabled. Now on pressing reset switch S12, the outputs of IC3 & IC4 are made low. When we press any digit on the keypad, 4-bit binary data is loaded into shift registers and the same appears at the first output of all the four shift registers. When the next key is depressed, the previously loaded data is shifted into the next output of each of the four registers and the new data appears at the first output. On the next key depression,

the second output data is shifted into the third output and the first output data is shifted into the second output, while newly entered data appears at the output of the registers. Again on the next key depression, the previously loaded third output data shifts into fourth output, the second output data shifts into the third output, the first output data shifts to the second output, and the newly entered data appears at the first output. For example, if the entered locking code is 4567, QA0 to QD0 is the nibble for units place, QA1 to QD1 is the nibble for tens place, QA2 to QD2 is the nibble for hundreds place, and QA3 to QD3 is the nibble for thousands place.

outputs

QD3

QC3

QB3

QA3

QD2

QC2

QB2

QA2

QD1

QC1

QB1

QA1

QD0

QC0

QB0

QA0

Reset

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

4

0

0

0

0

0

0

0

0

0

0

0

0

0

1

0

0

5

0

0

0

0

0

0

0

0

0

1

0

0

0

1

0

1

6

0

0

0

0

0

1

0

0

0

1

0

1

0

1

1

0

7

0

1

0

0

0

1

0

1

0

1

1

0

0

1

1

1

Table – 1: Sequential outputs of Shift register for Keyed Digits 4567

After entering the 4-digit locking code, depress program switch s11 again to inhibit the locking section and enable the unlocking section. The unit is now ready to accept the unlocking code, and if a valid code is entered via keypad, the switching section is activated.

2.3.3 The code comparator section

For comparing the locking code with the unlocking code a code comparator is needed. Two 8-bit magnitude comparator 74LS688 ICs (IC9 & IC10) are cascaded to form a 16-bit comparator here. The output of the comparator74LS688 is active-low (normally high). The two comparators have been used in conjunction with NOR gate N3 to get a high output when correct locking code is entered. This high output of NOR gate N3 enables quad bilateral electronic switches inside CD4066 (IC8) and the

compliment of NOR gate N3 is obtained from NOR gate N4 inhibits the AND gates N1 and N2, so no clock pulse is available for the shift registers of locking as well as unlocking sections and the existing outputs of all shift registers are frozen. In this condition the keypad has no effect on the locking and unlocking sections. Now the keyboard data is routed through bilateral switches of IC8.

2.3.4 The device/ relay switching section

The binary data corresponding to each keypad entry is routed via switches of IC8 to the input of 1-of-10 decoder CD4028 (IC11) to act as its address input. Q0 and Q9 outputs of IC11 have not been used. The outputs corresponding to keyed digits 1 through 8 cause outputs Q1 through Q8 of IC CD4028 to go high. Four dual-D CD 4013 flip-flops are used as bistable switches at outputs Q0 through Q8 of IC11 to hold the relays in ‘on’ or ‘off’ states. The outputs of IC CD4028 are used as the clock pulses for the corresponding flip-flops. Only the keys for digits 1 through 8are used for controlling the relays and other keys have no effect on the relay switching section. Reset switch S14 connected to reset pins of all the ‘D’ flip-flops can be used to reset all the devices/relays together.

Parts List

Semiconductors:

ICs

IC1 – 74C922 16-key encoder

IC2 – 74LS244 octal buffer

1C3-IC6 – CD4015 dual 4-bit static shift register

IC7 – CD4017 decade counter

IC8 – CD4066 quad analog switch

IC9& C10 – 74LS688 8-bit comparators

IC11 – CD4028 1-of-10 decoder

IC12, IC13 – CD4013 dual ‘D’ flip-flop

IC16 (N1, N2) – CD4073 3-input AND gate

IC17 (N3, N4) – CD4001 Quad 2-input NOR gate

Transistors

BC547 npn transistor

BC548 npn transistor

Diodes

1N4001 rectifier diode

Green & Red LEDs

Resistors:

4.7k, 2.7k, 1k, 220 ohms

Capacitors:

0.1uF ceramic disk

4.7uF, 10V electrolytic

Miscellaneous:

12V, 200W, 1C/O Relays

Push-to-on tactile switches

Piezobuzzer

4.5V battery

Procedure

The procedure for entering the locking code and unlocking codes, and device switching is summarised below:

For entering the locking code

¨ Reset the locking section using reset switch S12.

¨ Press program switch S11. LED 1 should glow. Now enter the enter 4-digit locking section via the keypad one by one.

¨ Press program switch S11 again.

For unlocking the device

¨ Enter the same four digits of locking code via the keypad. To indicate the matching, LED2 glows and the device is unlocked. Locking and unlocking sections are inhibited and Quad analog switch is enabled.

For device/relay switching

¨ Press the key on the keypad corresponding to the desired device number.

¨ The corresponding relay should energise (or de-energise if previously energized). Press S14 and observe that all relays are de-activated

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