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AC measurement circuit question

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Tom18991:
Please see the schematic below, which is one of many similar versions on the internet for measuring AC voltage on a microcontroller.  If the AC source is independent of the microcontroller's power input (for example, the AC is coming from a sensor that generates its own signal and the microcontroller is powered with a 9v battery), then how can the microcontroller measure the DC voltage on the output of this circuit?

Benta:
Don't do this. You have 230 V live voltage on your MCU and display, it's potentially lethal.

Tom18991:
Thanks and no worries.  I don't play with that kind of power.  :)   I'm borrowing the concept to measure the voltages off a variable reluctance sensor.  The volts might climb up to 20-30v, but without the current of a power line.  I see there's a ground at the bottom of the circuit, which I presume is tied to the arguido ground, which probably explains my confusion.  I'll try it on a (safe) function generator and see how I get on...

Kleinstein:
The circuit is a crude form of a peak detector. It's slow and temperature dependent.

With a decent ADC inside the µC and only mains frequency it is usually better to do direct AC sampling and that do RMS calculation in software. Even a 8 Bit µC with 10 Bit ADC like 8 bit AVR can do that surprisingly well.

Tom18991:
Mains frequency?  Meaning the frequency of the 220v?  My actual project involves a low-current ac sensor, and the waveform is not a sign wave.  I'll play with various circuits, and am looking for something cheap :) without the need to be all that accurate -- +/- 5% is plenty good enough for my purpose.  I was just confused how this could even work, but probably just missed the fact that the sensor circuit appears tied to the Arduino ground plane.

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