Noise-signal discriminator
Electronic
74HC02

When listening to an AM radio, we often hear a scratching or grating noise which is very offensive to the ear.

In this project, we're going to make a circuit that can discriminate between noise and signals using a NOR gate. The discrimination can be made by the signal width, because noises are small in width but signals are large in width.

The schematic shows you that the LEDs are used for noise and signal indication. S1 is used to generate signals, and LED 1 lights up when a noise is generated. Noise and signal discrimination is made by NOR gate IC - it doesn't light up LED 1 unless the signal received has a certain width (see Fig. 1).

When you've wired the project, turn power ON and press S1 just for a moment (less than 1 second). You'll notice that LED 2 lights up but LED 1 doesn't. This is because a noise is generated.

Now press S1 for more than 1 second. This time LED 2 lights up immediately, and LED 1 stays ON for about 5 seconds after S1 is released.

Finally, press S1 for more than 1 second and release it, then press it again for 1 - 2 seconds. LED 2 lights up immediately, but LED 1 indicates only the first input signal (generated by pressing S1 for more than 1 second), but not the second input signal (generated by pressing S1 for 1 - 2 seconds).