In Depth Look at AC Power Control with Arduino

by Lewis Loflin

In the above video and the code below we take an in depth look at the hardware for using Arduino interrupts to control AC power through a triac. Using a zero-crossing detector Arduino will detect the pulse then calculate a delay to control the power output to a load. The complete circuit schematic.

For more on basic AC voltage see my video Basic Electronic Power Supplies

Fig. 2

Fig. 2 shows the 5-volt power supply for Arduino but includes blocking diode D2. On the cathode side we have filtered DC which is regulated to 5-volts through U2. On the Anode side we have unfiltered raw 120 Hz DC going to the LED in the 4N25 opto-coupler. The output from the photo-transistor collector goes to digital pin 2 of Arduino to interrupt 0. Potentiometer R3 goes analog pin 0 and is used to calculate the time delay for the half-cycle triac firing pulses.

The figure above shows the relationship of the zero-crossing pulse with the AC sine wave. By detecting the pulse and programming a delay one can control the power output level to a AC load.

Fig. 3

Fig 3 shows the triac firing circuit. The MOC3011 opto-coupler uses a photo triac as opposed to a transistor. Pulses synchronized to the AC sinewave half-cycle are output from Arduino digital pin 5 to the LED in the MOC3011, which also serves to isolate the high voltage AC from the low-voltage components.

Pressing the power switch will enable trigger pulses to the MOC3011 while the LED on digital pin 12 is a power on indicator. C1 and R6 form a snubber circuit for inductive loads. Without a snubber switching noise from inductive loads will cause miss-firing of the triac.

Note that on 60Hz power lines a half cycle is 8.35mSec. while a 50Hz system is 10mSec. Adjust the half-cycle time-delay for 50Hz.


/*
Purpose: to detect zero crossing pulse at 
 INT0 digital pin 2, which after delay 
 switches on  a triac. 
 Power output to triac activated by external switch.
 */

#define triacPulse 5
#define SW 4
#define aconLed 12 

int val;

void setup()  {
  pinMode(2, INPUT);
  digitalWrite(2, HIGH); // pull up
  pinMode(triacPulse, OUTPUT);
  pinMode(SW, INPUT);
  digitalWrite(SW, HIGH);
  pinMode(aconLed, OUTPUT);
  digitalWrite(aconLed, LOW);
}

void loop() {
  // check for SW closed
  if (!digitalRead(SW))   {
    // enable power
    attachInterrupt(0, acon, FALLING); 
    // HV indicator on
    digitalWrite(aconLed, HIGH);
  }  // end if
  else if (digitalRead(SW)) { 
    detachInterrupt(0); // disable power
    // HV indicator off
    digitalWrite(aconLed, LOW);
  }  // else
} // end loop



// begin AC interrupt routine
// delay() will not work!
void acon()  
{
  delayMicroseconds((analogRead(0) * 6) + 1000); // read AD1 
  digitalWrite(triacPulse, HIGH);
  delayMicroseconds(200);  
  // delay 200 uSec on output pulse to turn on triac
  digitalWrite(triacPulse, LOW);
}