TTL NAND enable circuit
Électronique / Electronic
7400

NAND gates can also be "electronic sentries". If you don't want a signal to pass a NAND gate can make sure it doesn't. We call this Project a "NAND enable circuit" because that's what the NAND gate does, it enables signals to pass through a channel.

The two LEDs will let you see whether or not the signal shown at LED 1 will be allowed to pass to LED 2. You probably recognized one circuit in the schematic right away, the multivibrator.

You can see the output of the multivibrator by watching LED 1. You'll also notice that the multivibrator provides one of the inputs to the NAND gate. What do you suppose happens when the Select Switch is set to A? To B?

Can you figure out what LEDs 1 and 2 will do with the Select Switch at A or B? Be sure to make some mental notes because we're about to find out.

As you build this circuit, set the Select Switch to B. When you finish the wiring, put the power ON and look at LEDs 1 and 2.

You'll see LED 1 "blink" to indicate the output of the multivibrator. But look at LED 2. You'll see that it stays lit all the time, indicating that something's preventing the signal at LED 1 from reaching LED 2.

Now set the Select Switch to A and observe LED 1. What is happening?

Is the same thing happening to both LED 1 and LED 2?

You can see that LED 1 and LED 2 "take turns" going on and off. This is because we set one input of the NAND to 1 when we set the Select Switch to A. The multivibrator sends 0 and 1 signals to the other input.

When the signal is 1, LED 1 lights but both input signals to the NAND are then 1. That means the NAND's output is 0, and LED 2 goes out. But if the multivibrator's output is 0, LED 1 goes out.

Since this causes one of the inputs to the NAND is 0, its output goes to 1 and LED 2 lights, (Now be honest, did you figure all that out before building the circuit? We hope so).