毕业设计：数字式电容测量仪的设计

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1、摘要本设计是基于555定时器，连接构成多谐振荡器以及单稳态触发器而测量电容的。单稳态触发器中所涉及的电容，即是被测量的电容。其脉冲输入信号是555定时器构成的多谐振荡器所产生。信号的频率可以根据所选的电阻，电容的参数而调节。这样便可以定量的确定被测电容的容值范围。因为单稳态触发器的输出脉宽是根据电容值的不同而不同的，所以脉宽即是对应的电容值，其精度可以达到0.1%。然后在电路中加入一个由LM741以及一个电容和一个电阻构成的阻容平滑滤波器，将单稳态触发器输出的信号滤波，使最终输出电压与被测量的电容值呈线性关系。最后是输出

2、电压的数字化，将输入到7448译码器中翻译成BCD码，输入到LED数码管中显示出来关键词：电容，555定时器，滤波器，线性，译码器，LED数码管15目录一、测量系统的方案设计··············································31.1、测量部分的系统方案设计·····································31.1.1、恒亚充电法测量···················································31.1.2、恒流充电法

3、测量···················································31.1.3、脉冲计数法测量···················································31.2、测量信号数字化系统方案选择································31.2.1、利用单片机进行编程翻译··········································41.2.2、利用译码器进行翻译···················

4、···························4二、单元电路的设计及原理·············································4152.1、电容值测量电路及原理·······································42.1.1、多谐振荡器电路图及工作原理·······································62.1.2、单稳态触发器电路图及工作原理·····································72.1.

5、3、滤波器工作电路图及原理··········································82.2、模拟信号的处理以及数字化显示·····························9三、系统参数设定······················································10四、结论及谢词························································114.1、结论························

6、·····································114.2、谢词····························································1115参考文献··································································12附表：元器件明细表······················································13一系统方案设计1.1测量部分的系统

7、方案设计1.1.1:恒压充电法测量。用一个电阻和电容串联，用恒压源对电容进行充电，然后根据电容充电的曲线超过某个固定电压所需要的时间，利用曲线拟合的方法测量。测量所使用的原始公式是：。可见电容的值和电压以及时间呈微分关系。用这种方法测量，时间和容值是非线性的。因此测量难度高，精度低，并且难以实现数字化。1.1.2：恒流充电法测量。用恒流源对电容充电，此时电容的容量和充电时间是成正比的，所以可以利用AD或者比较功能同某个固定电压比较，来实现电容测量。测量所用的原始公式是：..所以。恒流源的电流大小是已知的，时间和电压也可以

8、测量出来。由上面的公式即可求得电容的大小。使用这种方法来测量，精度较上一种方法有所提高，且便于操作和实现。但要使用恒流源，恒流源的的设计要求很高，且达不到测量所需要的精度要求，因此这种方法也不适用。1.1.3：用脉冲计数法测量电容。由555定时器两个电阻以及一个电容，构成的多谐振荡电路，产生较为稳定的振荡频率计算的公