## How to Control 74HC00 / 74HCT00 NAND Gate IC using Switches?

by realfinetime  |  in TTL at  04:51

I had already published my blog about the basics of working of 74HC00 / 74HCT00 Quad 2-Input NAND gate ICs. Input side of each NAND gate is normally controlled by switches or by some programmable ICs. 7400 controlled using switches is given below. First set of NAND gate is used in this circuit.

Input pins of the NAND gate is connected to the positive terminal of the battery. Pull down resistors Rp (10 K) must be connected to the input pins as shown in the following circuit. Pull down resistor helps to pull the input pins to ground ( LOW ) when switch is off. Output pin is connected to an LED through a current limiting series resistance of 1 K. Vcc pin must be connected to the positive terminal of battery and GND pin must be connected to the negative terminal of the battery.

When S1 and S2 are ON ( HIGH ), LED will turn off ( LOW ). If any of the switch is OFF ( LOW ), LED will turn on ( LOW ). Truth table is given below.
Working of Circuit

Switch controlling of 7400 IC is simple. But the function of pull down resistors is a bit confusing. When the switch is ON, corresponding NAND gate input pin will get 5V and hence considered as logic HIGH. When the switch is off , we will think that, corresponding 7400 NAND gate input pin will get a LOW voltage. Problem arise there. Because, if the input pins of TTL logic circuits are left unconnected it will assume itself as logic HIGH. So to supply LOW level to TTL IC input pins, a proper technique should be adopted. Here comes the use of pull down resistor.

When the switch is OFF, corresponding NAND gate input pin get grounded through the 10K pull down resistor.  So the voltage will be LOW at that input. When the switch is ON, input voltage will appear across the NAND gate Input pins. Value of pull down resistance must be high to give maximum resistance to current when the switch is ON. If the value of pull down resistance is low, when the switch turns on, voltage get grounded through the pull down resistor due to low resistance and hence necessary voltage will not appear across the IC input pins to generate Logic HIGH.