Switching an AC load with an Arduino is rather simpel: either a mechanical Relay or a solid state relay with an optically isolated Triac.
It becomes a bit more tricky if one wants to dim a mains AC lamp with an arduino: just limiting the current through a Triac is not really possible due to the large power the triac then will need to dissipate, resulting in much heat and it is also not efficient from an energy use point of view.

The proper way to do it is through phase control: the Triac then is fully opened, but only during a part of the sinoid AC wave.
One could an Arduino just let open the Triac for a number of microseconds, but that has the problem that it is unpredictable during what part of the sinuswave the triac opens and therefore the dimming level is unpredictable. One needs a reference point in the sinus wave.
For that a zero crossing detector is necessary. This is a circuit that tells the Arduino (or another microcontroller) when the sinus wave goes through zero and therefore gives a defined point on that sinus wave.
Opening the Triac for a number of microseconds after the zero crossing therefore gives a predictable level of dimming.

This circuit, it is an example.

Such a circuit is easy to build: The zero crossing is directly derived from the rectified mains AC lines – via an optocoupler of course- and gives a signal every time the wave goes through zero. Because the sine wave first goes through double phased rectification (see figure below), the zero-crossing signal is given regardless whether the sinus wave goes up through zero or down through zero. This signal then can be used to steer an interrupt in the Arduino. sine Opening the Triac is done via a tried and tested opto coupled Triac circuit. For the Triac a TIC206 can be used, but a BT136 (a bit cheaper)  would probably work as well. With regard to the MOC3021: that has a forward voltage of about 1.2-1.4 Volts and a trigger current of 8-15 mA. An LED has a forward voltage of about 2Volts. Presuming a 5V digital steering signal that gives a resistor value between (5-3.2)/0.015= 120 Ohm and (5-3.2)/0.008=225 Ohm, making 220 a decent choice. In reality an LED may have a slightly lower forward voltage and a 330 Ohm resistor was found to work as well. It is important NOT to chose an opto-Triac here with an inbuilt zero-crossing filter. The MOC3041 or the MOC3061 for that matter therefore is not usable in this circuit. The MOC 3021 is. NOTE: some people have experienced flicker as a result of a weak input signal. As the voltage drop over an LED may vary depending on the color. it is safer to replace the LED by a wire bridge and increase the 220 Ohm resistor to 330 or 470 Ohm With regard to the 30k resistors: The 1/2 Watt should do it as the resistors dissipate some 400mW. If you think that is too much, replace the 4N25 with a 4N33 and the two 30k resistors with two 100 k resistors. Be careful though in using sensitive optocouplers as the 4N32 as they will easily be fully in saturation.

This is the complete circuit of the dimmer

All one needs then is the software. Fortunately someone already has done the basic work. I just adapted the code slightly:

AC Voltage dimmer with Zero cross detection
Author: Charith Fernanado http://www.inmojo.com
License: Creative Commons Attribution Share-Alike 3.0 License. 

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)

 Pin    |  Interrrupt # | Arduino Platform
 2      |  0            |  All
 3      |  1            |  All
 18     |  5            |  Arduino Mega Only
 19     |  4            |  Arduino Mega Only
 20     |  3            |  Arduino Mega Only
 21     |  2            |  Arduino Mega Only

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
/* Due to timing problems, the use of ‘0’ can sometimes make the circuit 
flicker. It is safer to use a value slightly higher than ‘0’
void setup()
  pinMode(AC_LOAD, OUTPUT);// Set AC Load pin as output
  attachInterrupt(0, zero_crosss_int, RISING);  
// Chooses '0' as interrupt for the zero-crossing
// 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

  // 10ms=10000us
  // (10000us - 10us) / 128 = 75 (Approx) For 60Hz =>65
  int dimtime = (75*dimming);    // For 60Hz =>65     
  delayMicroseconds(dimtime);    // Off cycle
  digitalWrite(AC_LOAD, HIGH);   // triac firing
  delayMicroseconds(10);         // triac On propogation delay
                                 //(for 60Hz use 8.33)
  digitalWrite(AC_LOAD, LOW);    // triac Off
void loop()  {
 for (int i=5; i <= 128; i++)

Code below uses the timer function instead of delays


Print design. Mirrorred.

On thing to consider is that if you are building a system in which you are not using a ready made PSU but rather supply your own PSU, built from a transformer and a rectifier, you could consider picking up the 0-crossing from the secundary side of the transformer.

A possible circuit could look like this:


E. ten Winkel