AttatchInterrupt Constantly interrupts
As you've determined that kind of receiver will have a PWM output to control a servo, the Wikipedia Servo Control article has some good information about it. If you wanted to use it anyway the normal neutral position will be 1.5mS so you could use something like the following code that uses the pulseIn function to treat a pulse of 1.75mS or longer as being on:

#define MY_PIN 0

void setup() {
    Serial.begin(9600);
    pinMode(MY_PIN, INPUT);
}

void loop() {
   if (pulseIn(MY_PIN, HIGH) &gt;= 1750)
      Serial.println("TX on");        
}


Note that the typical period between pulses is 20mS so your code will be stalled in pulseIn for around that period. If that's a problem you could go back to using the interrupt and set it for CHANGE and keep track of the time between when the line goes high and low again. It would be something like the following untested code:

#define MY_PIN 0 // Make sure this matches the interrupt line
unsigned long start_micros = 0;
unsigned long last_duration = 0;

void setup() {
    Serial.begin(9600);
    pinMode(MY_PIN, INPUT);
    attachInterrupt(0 , PWM_handler, CHANGE);
}

void loop() {
   if (last_duration &gt;= 1750)
      Serial.println("TX on");        
}

void PWM_hander()
{
    if (digitalRead(MY_PIN) == HIGH)
        start_micros = micros();
    else
        last_duration = micros() - start_micros;
}