# Arduino controlled light dimmer software

I found this software:

``````/*

AC Voltage dimmer with Zero cross detection
Author: Charith Fernanado <a href="http://www.inmojo.com">  http://www.inmojo.com

</a>
Attach the Zero cross pin of the module to Arduino External Interrupt pin
Select the correct Interrupt # from the below table
(the Pin numbers are digital pins, NOT physical pins:
digital pin 2 [INT0]=physical pin 4 and digital pin 3 [INT1]= physical pin 5)
check: <a href="http://arduino.cc/en/Reference/attachInterrupt">  http://www.inmojo.com

</a>

Pin    |  Interrrupt # | Arduino Platform
---------------------------------------
2      |  0            |  All -But it is INT1 on the Leonardo
3      |  1            |  All -But it is INT0 on the Leonardo
18     |  5            |  Arduino Mega Only
19     |  4            |  Arduino Mega Only
20     |  3            |  Arduino Mega Only
21     |  2            |  Arduino Mega Only
0      |  0            |  Leonardo
1      |  3            |  Leonardo
7      |  4            |  Leonardo
The Arduino Due has no standard interrupt pins as an iterrupt can be attached to almosty any pin.

In the program pin 2 is chosen
*/
int AC_LOAD = 3;    // Output to Opto Triac pin
int dimming = 128;  // Dimming level (0-128)  0 = ON, 128 = OFF

void setup()
{
attachInterrupt(0, zero_crosss_int, RISING);  // Choose the zero cross interrupt # from the table above
}

//the interrupt function must take no parameters and return nothing
void zero_crosss_int()  //function to be fired at the zero crossing to dim the light
{
// Firing angle calculation : 1 full 50Hz wave =1/50=20ms
// Every zerocrossing thus: (50Hz)-> 10ms (1/2 Cycle)
// For 60Hz => 8.33ms (10.000/120)
// 10ms=10000us
// (10000us - 10us) / 128 = 75 (Approx) For 60Hz =>65

int dimtime = (75*dimming);    // For 60Hz =>65
delayMicroseconds(dimtime);    // Wait till firing the TRIAC
digitalWrite(AC_LOAD, HIGH);   // Fire the TRIAC
delayMicroseconds(10);         // triac On propogation delay
// (for 60Hz use 8.33) Some Triacs need a longer period
digitalWrite(AC_LOAD, LOW);    // No longer trigger the TRIAC (the next zero crossing will swith it off) TRIAC
}

void loop()  {
for (int i=5; i <= 128; i++){
dimming=i;
delay(10);
}
}
``````

But don't uderstand this part:

(10000us - 10us) / 128 = 75 (Approx) For 60Hz =>65

I know 10000us is 10ms, and we need to wait less then 10ms to dim light.

But don't understand why here we has -10us and /128.

Anyone know why is that?