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How to detect a change in Peak to Peak Voltage?

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mathew086

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Hello

I am having a circuit that produces square waves as output. Each time a parameter is varying, the voltage of the output wave changes ( peak to peak voltage changes) It decreases from 8 V to 1.5 V with a step of 80mV.
How is it possible to get an output without using a voltimeter??

Something like a High and Low value if the voltage is changing from 8 V

Any ideas ??
Thanks
 

Hi mathew086,

Please explain your problem in detail. I think you could use an ADC.
 

TRhe circuit is shown in the attchment.
A sqaure wave generator is made using a 555 timer. The output from that is connected to a T bridge capacitor. As the value of teh red circled test capacitor changes, the output signal changes its peak to peak voltage. The output for 10pF is shown below. ( in the diagram)
The value of test capacitance chnages from 10 pF to 500 pF
 

Hi mathew086,

I don't understand: 'Something like a High and Low value if the voltage is changing from 8 V'.

One method is you can low pass filter the square wave to get a DC voltage. But first you should clamp the square wave so that we get a square pulse train ie, no negative voltage. Then you can use an ADC or LM3914 display driver(only 10 levels).

Or use a peak detector circuit to find the peak voltage of the waveform. Then you may use an ADC or the LM3914 IC. But you will have to discharge the capacitor in the peak detector circuit before making a measurement so that it previous peak value is removed and the capacitor is allowed to charge to new peak value.
 

I am getting a waveform as shown in post #2. It is too crowded because the frequency is 4KHz. When the test capacitance changes, the peak to peak voltage changes. i.e sometimes the upper peak changes keeping the lower same or vice versa. SO it is not possible to just detect the upper or lower alone.
Is there a way to calculate that peak to peak voltage ??
OR
Is there a way to detect a change in peak to peak voltage??
The changes are like this :

10pF -------8.18 V
11pF--------8.10 V
12pF--------8.03 V

i.e around 80mA for 1pF change
 

Hi mathew086,

The 555 works as an astable multivibrator. You may modify the ckt to produce a square wave of 50% duty-cycle. So at output if you have 8V peak-to-peak then the peak value of positive half would be 4V ie, just the half. This may help.
 

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