The unknown is whatever it is your measuring. Thus whatever error your 5V supply has is a source of error in your measurement. One way to reduce that overall measurement uncertainty is to improve the accuracy of the known or reference voltage. With this input, it will supply a 2.
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Grounded terminals should be externally connected together! To do that we need a step down circuit which downs any input voltage to another one lower than 5V. For an AC voltage we can use a step down transformer, but measurements will not be accurate! For the autoranging purpose I used 4 optoisolators MOC or equivalent with triac in one side, this allows current to flow in either directions when the optoisolator LED is triggered. As shown in the circuit diagram, pin number 4 of each optoisolator optocoupler is connected to Arduino analog channel 3 A3 , pin number 6 of each opto-isolator is connected to 1 voltage divider output.
So, voltage divider ration equals to: The LM The 2nd role is to provide a precise DC bias of 2. As a result, connecting 2-phase voltage of V RMS to the above circuit will not do any damage to the ATmegaP microcontroller, even when the voltage divider with lowest ration is selected.
Arduino digital pin 6 is also connected to the common pin of the optoisolators through 10k resistor. The analog comparator is used to detect whether the voltage signal is AC or DC and also for counting AC voltage frequency. A anode and K cathode are the back light LED pins. Voltage type AC or DC is automatically detected, and range is automatically selected 4 ranges.
Arduino Based Autoranging AC-DC Voltmeter with TRMS