# Can this circuit detect change in AC signal?

Discussion in 'General Electronics Chat' started by assassin___0, May 10, 2011.

1. ### assassin___0 Thread Starter New Member

May 10, 2011
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The idea is that there will be an AC signal and whenever there is a change in the signal (whether it increases or decreases, even for a split second), i want it to be detected.

The schematic isn't complete (I want to just verify what I have so far is correct), but the plan is that I take that one quick signal and run it through a flip flop that lights a bulb whenever the pulse (disturbance) triggers the flip flop.

What do you think?

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2. ### ErnieM AAC Fanatic!

Apr 24, 2011
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I don't like it. You have an indeterminate voltage on the cap steady state as you don't know how large the diode drops are so the cap is basically charging with the current below the knee voltage of two diodes. You are driving TWO diodes with 1 V of signal.

Additionally, there is no way shown to discharge the cap. So if it actually does detect a peak voltage it will tend to stay there, so the next trigger will need a larger input then the first trigger, and so on.

3. ### assassin___0 Thread Starter New Member

May 10, 2011
19
0
Let me see if I understood you correctly...

So if I account for the voltage drops across the diodes, then I can determine the max voltage on the Cap? And if I set that to the negative terminal of the op amp, then it should work?

As far as discharging goes, it is enough for me to detect just one disturbance in the AC input. So after that one disturbance it doesn't matter how many happen after that.

4. ### Jaguarjoe Active Member

Apr 7, 2010
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What Ernie is saying is that when you first turn your circuit on with a particular ac input, the cap will charge up to a certain voltage. If that AC input then decreases at all, the cap will not follow that decrease because it has no discharge path to get to that lower voltage.
This circuit will only respond to increases in input.

5. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Actually, I said it will change up to an uncertainvoltage.

"Indeterminate" was my exact word, and I meant it. If you place 1 volt across two diodes and see what voltage leaks out onto a cap... it ain't zero but dang no one could tell you what it will be.

And you need to know that voltage to set the comparison voltage

assassin: can you define the input pulse?