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Inspired by this post on Electronics Stack Exchange, I made up a simple amplifier:

Inspired by this post on Electronics Stack Exchange, I made up a simple amplifier:

You can use the analog compator to detect an analog signal passing a threshold, like this:

You can use the analog comparator to detect an analog signal passing a threshold, like this:

Alternative method with analog comparator

Based on suggestions by Chris Stratton, there is another way of doing this.

You can use the analog compator to detect an analog signal passing a threshold, like this:

Analog comparator schematic

I have set up the voltage divider to trigger at 0.265V, so that a 500 mV signal should provide an acceptable triggering range.

As for the sketch, at these frequencies we can do something simpler than what I posted before:

volatile bool counting;
volatile unsigned long count;
ISR (ANALOG_COMP_vect)
 {
 if (counting)
 count++;
 }
void setup ()
 {
 Serial.begin (115200);
 Serial.println ("Started.");
 ADCSRB = 0; // (Disable) ACME: Analog Comparator Multiplexer Enable
 ACSR = bit (ACI) // (Clear) Analog Comparator Interrupt Flag
 | bit (ACIE) // Analog Comparator Interrupt Enable
 | bit (ACIS1); // ACIS1, ACIS0: Analog Comparator Interrupt Mode Select (trigger on falling edge)
 } // end of setup
unsigned long startTime;
void loop ()
 {
 if (!counting)
 {
 startTime = micros ();
 count = 0;
 counting = true;
 }
 else
 {
 // is a second up? (1000000 microseconds)
 if (micros () - startTime >= 1000000)
 {
 counting = false;
 Serial.print (count);
 Serial.println (F(" Hz."));
 } // end of a second being up
 
 } // end of if
 
 } // end of loop