Logic State Indicator

Sometimes when we test digital equipment at home we require to know the binary state at any particular point. For that you can use this simple logic state indicator circuit. This circuit uses four LED's to give you the indication of the logic state. Like if Green LED1 glows means you have provided the 10 as binary state and so on.

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Fig. 1: Prototype of Logic State Indicator Circuit on Breadboard

This simple logic state indicator circuit is based on inverter IC 7404 with four resistor and four LED’s.

CD7404 is the most commonly used Complementary Metal Oxide Semiconductor (CMOS) chip. It comes in a 14 pin Dual Inline Package (DIP). It has small notch on one side which identifies as pin 1.It consists of 6 independent NOT gate on a single chip. Each gate has 1 inputs and 1 output. Working voltage range of IC is from 5V to 16V. It can deliver approx.10mA at 12V but this can be reducing as power supply voltage reduces.

IC consist of 14 pin in this pin 7 and 14 is connected to battery or DC power supply. Negative is connected to pin 7 and pin 14 is connected to power supply. As we know it has six gates we call it NI1, NI2, NI3, NI4 and so on. In first gate NI1 pin 1 is for inputs and pin 2 is for output. As you can see in the circuit below. Its outputs go low when input is high. And its output becomes high when input is low that’s why it is known as inverter IC.

Pin Diagram for IC CD7404

Fig. 2: Pin Diagram for IC CD7404

For testing the logic state use the probe connected at point A and B. Consider a case you have apply 0 and 0 at logic probe A and B both then Red LED4 will glow because when input is low 7404 IC output is high that is pin 2 is high and LED 4 become forward bias and it will conduct. And rest LED’s will be in reverse bias state hence does not conduct.

Similarly when 1 and 0 is provided to probe A and B respectively, Green LED 1 will glow because output will become high it will reverse bias LED 4 and LED 2 as 0 is provided to probe B and 1 is provided to probe A and LED 3 is also in reverse bias condition because of logic level at probe A and B. So in short at 1 and 0 LED 1 will glow. Similarly when 0 and 1 is provided at probe A and B respectively reverse action will take place and RED LED2 will glow.

Now when 1 and 1 is provided at probe A and B both then LED 1 and LED 2 are reverse bias because of logic level at point A and B. And in this case Green LED 3 will glow. Following table will help you to identify the state of LED’s according to the probe connected.

Logic State	LED1	LED2	LED3	LED4
Probe A Probe B	GREEN	RED	GREEN	RED
0	0	×	×	×	Glows
1	0	Glows	×	×	×
1	1	×	×	Glows	×
0	1	×	Glows	×	×