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74LS00 Nand Gate: 4 Simple Circuit Analysis

  • Contents

I Description

This blog introduces and analyzes 4 simple and easy 74LS00 Nand Gate circuit diagrams. It’s including Square Wave Generator CircuitPulse Generator CircuitLED Light Circuit. And in the end, we will analyze the circuit that turns the timer into a countdown timer in detail.

This Video is An Introduction of 7400 Logic Devices

Catalog

I Description

II Square Wave Generator Circuit

III Pulse Generator Circuit

IV LED Light Circuit

V Turn Timer into Countdown Timer

5.1 Scheme Design

5.2 Implementation of Scheme Design

FAQ

Ordering & Quantity

II Square Wave Generator Circuit

Let’s take a look at the figure below. It’s a square wave generator circuit. This circuit contains a 74LS00 Nand Gate integrated circuit

NAND gates circuit

Figure 1. Square Wave Generator Circuit Diagram

 Among this circuit diagram:

  • NAND gates 1, 2, and external RC time constant components form an oscillator circuit
  • NAND gate 3 is a buffer output stage.

As long as the capacity of C is changed, square wave outputs of different frequencies can be obtained.

III Pulse Generator Circuit

Figure 2. Pulse Signal Generator Circuit Diagram

The circuit diagram is shown in Figure 2 and it’s a simple pulse signal generator circuit. The signal generator mainly uses two TTL integrated circuits (74LS00 and 74LS221 ). So why choose these two circuits? That is because these two circuits can be used to generate a pulse signal of τ=4μs. Besides, it uses fewer components and is convenient for debugging and maintenance.

IV LED Light Circuit

This circuit is made with  NE555, 74LS00, 74LS154, 74LS193, and LED lights, and the production process is very simple. When we turn on the power, here is how it works is:

  • When the output Q0 of the 74LS154 decoder is low, the 74LS193  is a positive counter. At this time, the LEDs are individually lit from D1...D16;
  • When the output Q15 of the 74LS154 decoder is low, the 74LS193  is a countdown counter. At this time, the LEDs are individually lit from D16...D1.

LED light circuit diagram

Figure 3. LED Light Circuit Diagram

From the above we can see that: the LED lights turn on from D1 to D16, and then back to D1 from D16, and so on.

V Turn Timer into Countdown Timer

Generally, there are two design ideas for turning a timer into a countdown timer: First, change the counting chip in the timer; Second, reset the function of the chip.

Besides, there is actually another way to achieve this goal: By applying the 74LS00 and 74LS20 chips to "reverse" the results on the display, so as to achieve the purpose of counting down.

5.1 Scheme Design

The result displayed by each digit of the timer is an incremental value, such as 0.1.2.3.4.5.6.7.8.9. Yet, the countdown timer displays a decreasing value, such as 9.8.7.6.5.4.3.2.1.0. As long as the display result conversion is completed with a suitable logic circuit, the timer can be turned into a countdown timer.

At first, we need to find the logical relationship between the timer display result and the countdown timer display result. Table 1 below lists the BCD codes corresponding to each display result of the timer and countdown timer. From this table, you can easily find the BCD codes of the timer and countdown timer:

  • The lowest bit Q1 and Y1 are opposite; While Q2 and Y2 are the same;
  • Relationship between Q3 and Y3: Y3 of the countdown timer is the exclusive OR of timer Q3 and Q2;
  • Relationship between Q4 and Y4: The Y4 bit of the countdown timer is the opposite value of the OR of Q4, Q3, and Q2 of the timer, which is also equal to the non-re-AND of Q4, Q3, and Q2.

corresponding BCD code displayed by (down)timer

Table 1. Corresponding BCD Code Displayed by (Down)Timer

 

 

The above logical expression is:

Therefore, as long as you choose a circuit that can complete the above logic conversion relationship, you can realize the design from a timer to a countdown timer. The figure? shows a two-digit timer circuit. After adding the above conversion circuit, it becomes figure 3 shows the countdown circuit.
 

timer circuit
Figure 4. Timer Circuit Displaying 2 Digits

 

countdown circuit diagram
Figure 5. Countdown Circuit Diagram

5.2 Implementation of Scheme Design

Two kinds of chips 74LS00 and 74LS20 are used here. The former are four two-input NAND gates, which are used to complete the conversion of Y1 and generate the negation of Q4, Q3, and Q2. The latter are two four-input NAND gates, which are used to obtain Y4 from the non-reAND of Q4, Q3, and Q2. In summary, we can follow the logical relationship as follows:  

The logic diagram is shown in Figure 6.

Logic Diagram

Figure 6. Logic Diagram

Knowing that the XOR gate operation can be completed, the Y3 conversion can be completed. The connection circuit diagram of the above-mentioned Y4, Y3, Y1 conversion specific physical objects is shown in Figure 7. 

Figure 7. Y4, Y3, Y1 Conversion Specific Physical Connection Diagram

Figure 8 shows the actual picture of the countdown timer.

countdown timer

Figure 8. Countdown Timer

So far, the timer has become a countdown timer. Through this design method, there is no need to change the original counter circuit, is it particularly trouble-free?

 

FAQ

  • 1.What is a 74LS00?

74LS00 is NAND gates-based IC. It has 14 pins which all connected with 4 NAND gates. Due to the NAND gate known as universal gate, 74LS00 can be converted into OR and NOT gate easily. The IC comes in three packages, SOIC, PDIP, and SOP.

 

  • 2.What is NAND logic gate?

In digital electronics, a NAND gate (NOT-AND) is a logic gate which produces an output which is false only if all its inputs are true; thus its output is complement to that of an AND gate.

 

  • 3.What is a 74LS08?

74LS08 is a Quadruple 8-bit Two Input AND IC. Gate AND gate is a digital circuit used to convert the logic state to a specific logic. In AND gate two logics state signals are used.

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