WORKING PRINCIPLE

This circuit illustrates the use of XOR (Exclusive-OR) gates as bit comparators. Four of these
XOR gates compare the respective bits of two 4-bit binary numbers, each number “entered”
into the circuit via a set of switches. If the two numbers match, bit for bit, the green “Go”
LED will light up when the “Enter” pushbutton switch is pressed. If the two numbers do not
exactly match, the red “No go” LED will light up when the “Enter” pushbutton is pressed.
It is the nature of an XOR gate to output a “high” (1) signal if the input signals are not the
same logic state. The four XOR gates’ output terminals are connected through a diode
network which functions as a four-input OR gate: if any of the four XOR gates outputs a
“high” signal—indicating that the entered code and the key code are not identical—then a
“high” signal will be passed on to the NOR gate logic. If the two 4-bit codes are identical,
then none of the XOR gate outputs will be “high,” and the pull-down resistor connected to
the common sides of the diodes will provide a “low” signal state to the NOR logic.
The NOR gate logic performs a simple task: prevent either of the LEDs from turning on if the
“Enter” pushbutton is not pressed. Only when this pushbutton is pressed can either of the
LEDs energize. If the Enter switch is pressed and the XOR outputs are all “low,” the “Go”
LED will light up, indicating that the correct code has been entered. If the Enter switch is
pressed and any of the XOR outputs are “high,” the “No go” LED will light up, indicating
that an incorrect code has been entered.
A buzzer and a mod-5 down counter with preset value 4 have been added to the circuit. When
the count value reaches 5, the buzzer rings indicating that incorrect passcode has been entered
multiple times. After buzzer rings the counter is reset. Counter also gets reset when the
correct passcode is entered. When the passcode entered is correct, a motor connected to it
through a 5v relay opens the door.