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Last year I went to a maker event where some guys used an ACS712 sensor with an Arduino to monitor the mains voltage and current to monitor the energy cost in the house.

One thing I saw there is that they were measuring the mains voltage with the ACS712 sensor (as its values had some 0.1V to 0.5V variance when I was talking to them).

I was playing at home with one of these sensors, but I couldn't find a way to measure the mains voltage with it. Is there some example or tutorial of how can I can do it?

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  • \$\begingroup\$ It measures currentl, and it's isolation voltage is relatively low. Don't do anything dangerous! \$\endgroup\$ – Gregory Kornblum Aug 16 '18 at 19:20
  • \$\begingroup\$ Are you sure he was talking about the mains voltage changing and not just the voltage at the VIOUT pin? \$\endgroup\$ – Darius Aug 16 '18 at 19:20
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One thing I saw there is that they were measuring the mains voltage with the ACS712 sensor

No, they weren't using that sensor to measure the mains voltage. It measures current only.

(as its values had some 0.1V to 0.5V variance when I was talking to them)

That variation in the reported mains voltage does not mean that it was being measured using an ACS712 sensor.

I couldn't find a way to measure the mains voltage with it

You couldn't find a way to do that, because there isn't a way to do that.

The "live side" measurement connections of the ACS712 (pins 1+2 and 3+4) are isolated from the Hall-effect sensor on the low-voltage side of the device (2.1kV RMS minimum isolation). Therefore the ACS712 cannot be used to measure the "live side" (e.g. mains) voltage.

ACS712 functional diagram

(Image from ACS712 datasheet)

In summary: The ACS712 + Arduino system which you saw, measured the mains voltage some other way. The ACS712 was used only for the current measurement.

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