Im having trouble trying to understqnd this circuit , as you can see from http://www.instructables.com/id/Arduino-Audio-Input/?ALLSTEPS its a circuit that will amplify a signal from 200mV to 2.5V and then apply an offset to it in order for it to be centered in 2.5v this way the entire signal can be read by the arduinos ADC

however, as you can see from my picture i dont understand the porpuse of the 10uF cap, and even worse i dont understand how the arduino its feeding itself with 5vDC when im actually aplying a voltage divider to an AC signal

enter image description here

This Voltage divider only makes sense when using 10v DC (which is actually the dc current used in the proyect) where

Vout = (10v * 100k)/(100k+100k) = 5v

However according to the schemmatic we are not using DC we are using a 2.5v AC signal


1 Answer 1


The capacitor is there to "AC-couple" the input. This removes any DC component from it, allowing only the AC in the waveform to pass through.

This circuit isn't feeding the 5V supply, it's drawing from it. The voltage divider biases the input at 2.5V, and the AC-coupled signal causes it to vary from that voltage in either direction.

  • \$\begingroup\$ The power supply for the Arduino may be 5 volts from USB, or 7 - 12 volts from Vin. As Ignacio says, the "5V" terminal on the Arduino is (or should be) an output, not the power supply input. \$\endgroup\$ Nov 21, 2014 at 1:46

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