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Digital circuit production 07-three-person voting device

Overview

Principle description

Let's analyze the circuit through the simulation diagram. The three input terminals of 74HC138 have a 10K resistor to pull up VCC, which can avoid misoperation. There is a low level display only when the button is pressed. 74HC138 is a three-line to eight-line decoder. Simply put, by controlling the level combination of A0, A1 and A2, a high level can be output in the corresponding Y0~Y7. Then the output end is connected to a 74HC20 four-input NAND gate for logical judgment. When any two or three buttons are pressed, the output Y3, Y5, Y6, and Y7 are all 1. At this time, the output of pin 6 is 0, 6 The output of the pin is used as the input of the second NAND gate. The corresponding output of pin 8 is 1. The green light indicating passing lights up and the buzzer sounds. When there is only one button or no button is pressed, the output of 138 has at least a low level, so the output of pin 6 is 1, and the corresponding output of pin 8 is 0, then the LED light passing the indication will not light up. rise. For 74HC138 functional testing and 74HC20 functional verification, please refer to the following projects:

74HC138

74HC20

Design Files

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Update:2025-06-27 18:54:32

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