Hola soy nuevo en esto de arduino y he investigado mucho y probado tambien pero no he obtenido buenos resultados ya que los sketch que he probado se demora mucho tiempo en calcular la frecuencia la idea es que bajo los 100 hertz (onda cuadrada) mantenga un led apagado y sobre estos lo mantenga encendido obviamente en el menor tiempo posible espero que me puedan ayudar saludos y gracias de antemano . 
buscando por hay encontre este frecuenciometro pero veo que contiene algunos delay se podra modificar este codigo para lo que necesito ???
aqui dejo el codigo saludos
/*
Frequency Counter Arduino Sketch
by: Jim Lindblom
SparkFun Electronics
License: Beerware
This isn't the most accurate frequency counter, but it's simple to
program and understand, and it works in a pinch. I've tested it to
be accurate to at least 96% (usually around 99.5%). It should be
able to measure frequencies up to about 6.5MHz(8MHz optimally)
The circuit: Powered at 5V (5V LCD), Arduino running at 16MHz
D2 - RS (LCD)
D3 - R/W (LCD)
D4 - E (LCD)
D5 - Frequency input
D6 - DB4 (LCD)
D7 - DB5 (LCD)
D8 - DB6 (LCD)
D9 - DB7 (LCD)
D10 - Gate of NPN transistor (Collector tied to 5V, emitter tied to LCD backlight pin)
For the LiquidCrystal library, much thanks to:
David A. Mellis
Limor Fried (http://www.ladyada.net)
Tom Igoe
*/
#include <LiquidCrystal.h>
// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(2, 3, 4, 6, 7, 8, 9);
int brightness;
unsigned int tovf1 = 0;
unsigned long frequency = 0;
unsigned long temp = 0;
// Timer 1 is our counter
// 16-bit counter overflows after 65536 counts
// tovfl will keep track of how many times we overflow
ISR(TIMER1_OVF_vect)
{
tovf1++;
}
void setup() {
pinMode(5, INPUT); // This is the frequency input
pinMode(10, OUTPUT); // Backlight control pin
digitalWrite(10, HIGH); // Turn backlight on
// Timer 1 will be setup as a counter
// Maximum frequency is Fclk_io/2
// (recommended to be < Fclk_io/2.5)
// Fclk_io is 16MHz
TCCR1A = 0;
// External clock source on D5, trigger on rising edge:
TCCR1B = (1<<CS12) | (1<<CS11) | (1<<CS10);
// Enable overflow interrupt
// Will jump into ISR(TIMER1_OVF_vect) when overflowed:
TIMSK1 = (1<<TOIE1);
// set up the LCD's number of rows and columns:
lcd.begin(16, 2);
// Print a splash screen to the LCD.
lcd.print("FrequencyCounter");
lcd.setCursor(0, 1);
lcd.print(" v1.0 ");
delay(2000);
}
void loop() {
// Delay a second. While we're delaying Counter 1 is still
// reading the input on D5, and also keeping track of how
// many times it's overflowed
delay(1000);
lcd.clear();
frequency = (TCNT1H<<8)|TCNT1L;
frequency = (TCNT1H<<8)|TCNT1L;
// Correct weird counter bug
// A small range of frequencies (~30k-50k) are getting
// 42949 appended to the front of them
// Will look into this but this works for now
if (frequency > 40000000)
frequency -= 4294900000;
// 65536 (2^16) is maximum of counter
// We'll multiply that by how many times we overflowed
temp = 65536*(unsigned long)tovf1;
// Add the overflow value to frequency
frequency += temp;
// Print the proper amount of spaces to make it look pretty
lcd.setCursor(0,1);
if (frequency < 100)
lcd.print(" ");
else if (frequency < 10000)
lcd.print(" ");
else if (frequency < 1000000)
lcd.print(" ");
else
lcd.print(" ");
lcd.print(frequency);
lcd.print(" Hz");
// Reset all counter variables and start over
TCNT1H = 0;
TCNT1L = 0;
tovf1 = 0;
}