schematic arduino ac control
Basic schematic.

Using Hardware Interrupts to Control A.C. Power

by Lewis Loflin

The purpose of this project is to demonstrate the use of a hardware interrupt to control AC power levels to a load such as a lamp or small AC motor. Also see Hardware Interrupts Demo and Tutorial for ATMEGA168/Arduino Note the "power on" switch must be pushed for the circuit to operate.

SeeIn Depth Look at AC Power Control with Arduino

Full wave pulsating DC
Full wave pulsating DC.

In the main circuit diagram above transformer T1, D1, and D3 produce a positive going pulsating DC with a peak voltage of about 18 volts and a frequency of 120 Hertz. Diode D2 blocks the filtering effect of capacitor C2, which with U2 supplies positive five volts for the microcontroller. See Basic AC Rectification and Filtering

Zero crossing pulse
Zero crossing pulse from 4N25 in relation to AC sine wave.

The 4N25 opto-coupler provides a narrow 120 Hertz pulse at zero and 180 degrees of the sine wave. This pulse is fed to digital pin 2 (Dp2) of the controller to trigger an interrupt when the sine wave passes zero and 180 degrees. (There's 360 degrees in a sine wave.) See 4N25 Opto-Coupler (PDF file)

full wave DC
This illustrates to process with full-wave unfiltered D.C. but the process is identical with A.C.

By programming a delay of between .1 and 8.2 milliseconds based on the voltage value on pin Ad0 we control the firing point of a triac, transistor, or silicon controlled rectifier to control power output. Also see Basic Triacs and SCRs.

For this experiment one can use 24 volts A.C. and a 24 volt lamp. If the circuit is wired properly when the
control is adjusted the circuit should act as a lamp dimmer or speed control for a motor.
If the lamp dims as the control is adjusted clock-wise reverse the two outer connections on R3.


A circuit such as this can do far more than a $3 lamp dimmer. By connecting a thermistor to measure temperature (or an appropriate thermocouple circuit) to one of the analog inputs we have a proportional heat controller saving energy costs. We could control a heating element or the speed of a blower fan proportional to temperature.

By connecting a photocell we could control light intensity of lamps in proportion to natural light, saving energy costs with say sky lights. I use a version of this on my water heater to control a narrow temperature range.

In a proportional controller for heat as an example as the desired temperature is reached, power input is reduced automatically (delaying the firing point on the triac) and enough energy is added to maintain temperature. In a cheap mechanical thermostat power is all on or all off producing overshoot and undershoot. This is unacceptable in many industrial applications.

For an application of this circuit see Hatching Chicken Eggs with ATMEGA168/Arduino

photo resistor
Sample circuit for photocell or thermistor
connection to ADC (analog to digital converter) sensor pin.

See Controlling Low-Voltage Driveway Lights with the ATMEGA168/Arduino In that circuit we use a CdS photocell instead of a thermistor, but the circuit is identical.

// LED must be connected between digital pin and ground

#define triac_control 5
#define powerIndicator 12 // indicator
#define sensorPin 0 // potentiometer 
#define irq_Pin 2 
#define powerOn 4

// when using values in the main routine 
// and IRQ routine must be volatile value

volatile byte flag_bit1 = LOW; // declare IRQ flag
int analogValue = 0;

// HIGH = 1, LOW = 0

void setup()  {
  pinMode(triac_control, OUTPUT);  
  pinMode(powerIndicator, OUTPUT);
  digitalWrite(triac_control, 0); // LED off
  digitalWrite(powerIndicator, 0); // LED off
  pinMode(irq_Pin, INPUT);
  pinMode(powerOn, INPUT);
  digitalWrite(irq_Pin, 1); // pull up on
  digitalWrite(powerOn, 1); // pull up on
  attachInterrupt(0, flag1, FALLING);  
  // interrupt 0 digital pin 2 connected ZC circuit

void loop() {

  if (!digitalRead(powerOn)) digitalWrite(powerIndicator, 1); 
  else digitalWrite(powerIndicator, 0);  
  if ((flag_bit1 == 1) &
  (digitalRead(powerOn)== 0)) {
    analogValue = analogRead(sensorPin) + 1;
    delayMicroseconds(analogValue * 7);
    digitalWrite(triac_control, 1); //triac on
    digitalWrite(triac_control, 0);  //triac off
    flag_bit1 = 0; // clear flag
} // end loop

void flag1() // set bit
  flag_bit1 = 1; 

Added June 7, 2013:

You Tube Arduino Microcontroller Video Series March 2012: