555

Astable (oscilating) circuit

The oscilating circuit diagram is pretty simple, featuring the 555, and then a couple of resistors & capacitors that define the actual frequency of the oscillation. Pay attention to the pin numbers.

In reality, when you build it out, the circuit should look like this one:

TODO: image

You can calculate frequency using this formula:

T = 0,693 × (R1 + 2×R2) × C1

C1 is measured in Farads, R1 & R2 are in Ohms, so for values in schematic:

T = 0,693 × ( 100 000 + 2 × 100 000) × 0.000001 = 0,2 seconds (or 4,8 Hz)

If you want a different frequency, you would change the values of C1, R1 & R2. Changing the capicitor is the easiest to imagine -- if you go from a 10 µF capacitor to 1 µF, it will take 1/10 the time to charge, so your frequency will go up by a factor of 10.

Measuring the frequency

volatile long lasttime = 0; // volatile is necessary, since this value is changed in an interrupt

void setup()
{
  Serial.begin(9600);
  Serial.println("Measured time [ms]:");

  pinMode(3, INPUT);                    // make pin D3 (PD.3=INT1) input 
  attachInterrupt( 1, onTick, RISING);  // set interrupt 1 (from pin 3) to call 'onTick'
                                        // when the signal rises.
}


void loop()
{
  /* Nothing to do here, everything happens in interrupt service routine onTick */
}


void onTick()   // print out how many milliseconds occurred between the last
                // clock tick and this one.
{
  long thistime=millis();
  char tmp[7];
  sprintf(tmp, "%6d", thistime-lasttime);
  Serial.print(tmp);
  Serial.print("\r");
//    Serial.println(thistime-lasttime);
   lasttime = thistime;
}