## Control 7401 Quad 2 - Input NAND Gates using Switches

by realfinetime  |  in NAND at  12:06

We had already seen Controlling 7400, quad two input NAND gate using switches. 7401 is also quad 2-input NAND gate Integrated Circuit. But the difference is 7401 is Open collector and an external pull up resistor must be added in the circuit for the proper working of gates. Pull up resistor must be added across the Vcc and Output pin of logic gate.

Pinout diagram of 7401

Pinout diagram of 7401 is given below. Each 7401 has four NAND gates and hence the name "QUAD". All the NAND gates are two input gates as shown in the following diagram.

NAND gate gives a LOW output, if all the inputs are HIGH and gives a HIGH output, if any of the input is LOW.

Circuit to Control 7401 using Switches.

Controlling 7401 using switches is a simple task. A simple circuit for controlling 7401 using switches is given below.

Vcc is 7V and is supplied from a constant voltage source. Input to Gate (A1 - Pin 2 and B1 - Pin3 ) is supplied from this voltage source through a current limiting resistor. Two seperate switches ( S1 and S2 ) are connected in these lines. Gate input pins ( A1 and B1 ) is grounded through two pull down resistors. Output is connected to an LED through a current limiting resistor. Most important thing in 7401 circuit is the pull up resistor. A pull up resistor must be connected across Vcc and gate output. Truth table of NAND gate is given below.

Why pull up resistor?.

A pull up resistor must be connected across the Vcc and output pin of logic gate. If pull up resistor is not connected, current will not flow through LED. Because current required to turn on LED is supplied from Vcc and the only path through which current can reach LED is through the pull up resistor. But in 7400, there is an internal connection inside IC through which current will reach the LED. So external pull up resistor is not required.

Output pin of logic gate does not supply the voltage to turn on led. If both switches are ON, impedance of the gate will be very LOW. So current from the Vcc will flow through the pull up resistor to ground through GATE since current always choose low impedance path. This current will not flow through LED, because of the high impedance of  LED and current limiting resistor compared to GATE.

Similarly, if any of the switch is OFF, impedance of the GATE will be very HIGH compared to the impedance of  LED and current limiting resistor and the current from Vcc will flow through the pull up resistor to LED. This will turn on the LED.

Why Pull down resistors?.

Pull down resistors must be connected across the input pin of GATE and ground. This will pull the GATE input to ground which is logical LOW, when the switch is OFF. Similarly, When the switch turns ON, voltage from Vcc will drop in pull down resistors and this voltage will appear across the GATE input, which is the logical HIGH.

If the pull down resistors are not connected, GATE input will always remain at logical HIGH, even if the switch is ON or OFF, because if there is no connection to GATE inputs, 7401 will assume itself as logical HIGH. That is, in the absence of pull down resistors, when switch turns ON, GATE input will get connected to Vcc and hence will be at logical HIGH. When the switch turns OFF, GATE will assume itself as logical HIGH. So we cannot provide logical LOW to the GATE.