数电课程设计——考试天数倒计时

课程设计名称：电子技术课程设计

题 目： 考试天数倒计时器

课程设计任务书 一、设计题目 考试天数倒计时器的设计 二、设计任务 显示两位数倒计时，如99到1 1.当到9时喇叭自动响0时结束 2.用LED数码管显示结果 3.可以实现预置数功能 三、设计计划 电子技术课程设计共1周。 第1天：针对选题查资料，确定设计方案； 第2天：方案分析比较，电路原理设计，进行元器件及参数选择； 选用芯片参考：减法器、74160A、74LS48、LED数码管。 第3～4天：利用Multisim电路仿真，画电路原理图； 第5天：编写整理设计报告。 四、设计要求 1. 画出整体电路图。（Protel或Altium Designer或Multisim,禁止用VISIO画电路图）。 2. 对所设计的电路全部或部分进行仿真，使之达到设计任务要求。有余力的同学尝试制作PCB单面板图。 3. 写出设计报告书。

目录

1 电路原理图设计 ``````````````````````````````````````````````````````````````````````````````````1

1.1 设计原理`````````````````````````````````````````````````````````````````````````````````````````````1 1.2 设计方案 ```````````````````````````````````````````````````````````````````````````````````````````1

2 各分电路功能分析`````````````````````````````````````````````````````````````````````````````````4

2.1 脉冲发生电路`````````````````````````````````````````````````````````````````````````````````````4

2.1.1 555定时器``````````````````````````````````````````````````````````````````````````````````````4 2.1.2 用555定时器构成多谐振荡器```````````````````````````````````````````````````````4

2.2 计数器电路````````````````````````````````````````````````````````````````````````````````````````5 2.3 译码显示电路``````````````````````````````````````````````````````````````````````````````````````7 2.4 控制电路`````````````````````````````````````````````````````````````````````````````````````````````9

2.4.1 报警电路`````````````````````````````````````````````````````````````````````````````````````````9 2.4.2 置数、暂停、清零、控制电路```````````````````````````````````````````````````````10

3 总体电路仿真````````````````````````````````````````````````````````````````````````````````````````12 4 设计小结````````````````````````````````````````````````````````````````````````````````````````````````13

参考文献````````````````````````````````````````````````````````````````````````````````````````````````````14

目录

课程设计成绩评定表 ........................................................................................... 错误！未定义书签。 摘要 ...................................................................................................................................................................... 4 1 电路原理图设计 .............................................................................................................................................. 1

1.1 设计原理 .............................................................................................................................................. 1 1.2 设计方案 .............................................................................................................................................. 1 2 各分电路功能分析 .......................................................................................................................................... 4

2.1 脉冲发生电路 ...................................................................................................................................... 4

2.1.1 555定时器 ............................................................................................................................. 4 2.1.2 用555定时器构成多谐振荡器 ............................................................................................ 4 2.2 计数器电路 ........................................................................................................................................ 6 2.3 译码显示电路 ...................................................................................................................................... 7 2.4 控制电路 ............................................................................................................................................ 10

2.4.1 报警电路 ................................................................................................................................ 10 2.4.2 置数、暂停、清零、控制电路 ............................................................................................ 10

3 总体电路仿真 ................................................................................................................................................ 12 4 设计小结 ........................................................................................................................................................ 13 参考文献 ............................................................................................................................................................ 14

摘要

倒计时器在日常生活中应用比较广泛，比如篮球比赛是用的30秒倒计时，还有交通灯使用的60秒倒计时等等，但是这些倒计时器仅用在特定的场合，通用性比较差。为此，本文通过简单的数字逻辑电路器件，搭建成了可以实现预置数功能的倒计时器。本倒计时器主要有六个模块构成：脉冲发生器、计数器、译码显示电路、置数电路、控制电路和报警电路；本倒计时器主要采用555 构成的振荡电路, 由74LS190、74LS48 和七段共阴LED 数码管构成计数译码显示电路,由74LS147，74LS04构成的置数控制电路。本计时器具有启动计数、暂停/连续计数，清零以及置入任意两位数作为计数起点，在最后十秒发出声光报警等功能。本倒计时器比较好的解决了目前倒计时器通用性差的问题。

关键词：任意置数倒计时；译码显示；控制；声光报警

1 电路原理图设计

1.1 设计原理

我们可以用555时基电路构成的多谐振荡器来产生频率为1Hz的脉冲，即输出周期为1秒的方波脉冲，将该方波脉冲信号送到计数器74LS192的CP减计数脉冲端,再通过译码器74LS48把输入的8421BCD码经过内部作和电路“翻译”成七段（a，b，c，d，e，f，g）输出，显示十进制数，在74LS192的四个输入端接74LS147的输出，在74LS147的输入用拨码开关输入，然后在适当的位置设置开关或控制电路即可实现计数器的直接清零，启动和暂停/连续、译码显示电路的显示，置入任意两位数作为计数起点，以及声光报警等功能。

1.2 设计方案

经过初步的设计，可以确定该系统应包括秒脉冲发生器、计数器、译码显示电路、置数电路，控制电路和报警电路等6个部分构成。其中，计数器和控制电路是系统的主要部分。计数器完成倒计时功能，而控制电路具有直接控制计数器的启动计数、暂停、连续计数、译码显示电路完成显示功能。为了满足系统的设计要求，在设计控制电路时，应正确处理各个信号之间的时序关系。在操作直接清零开关时，要求计数器清零，数码显示器显示零。当启动开关闭合时，控制电路应封锁时钟信号CP，同时计数器完成置数功能，当启动开关断开时，计数器开始计数；当暂停、连续开关拨在暂停位置上时，计数器停止计数，处于保持状态；当暂停、连续开关拨在连续时，计数器继续递减计数。当倒计时到9时，数码管显示09，报警电路发出声光报警，给予提示。系统设计框图如图1-1所示。

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