Years ago I published a TRIAC dimmer that could be controlled by a simple microcontroller such as an arduino. Times have changed and right now it is of more importance to be able to control that lamp from a home automation system such as OpenHab, Homematic or NodeRed.
So, let’s have a look at the circuit first:
This is a rather classic circuit. It detects the zerocrossing on the grid and then a microcontroller ignites the TRIAC with a time delay that determines the brightness of the attached Lamp. The full cycle is 10mS (at 50Hz)
A circuit like this is easy to make for very low cost (1-2 Euro).
However, if you shy away from soldering such a circuit, there are ready made modules available as well:
You should not need to pay more than 3-4 USD for such a module. Anything above that is robbery
Software to control this dimmer would look something like this:
#include <Ethernet.h>
#include <PubSubClient.h>
#include <TimerOne.h>
#define CLIENT_ID "Dimmer"
#define PUBLISH_DELAY 3000
String ip = "";
bool startsend = HIGH;
uint8_t mac[6] = {0x00, 0x01, 0x02, 0x03, 0x04, 0x07};
volatile int i=0; // Variable to use as a counter volatile as it is in an interrupt
volatile boolean zero_cross=0; // Boolean to store a "switch" to tell us if we have crossed zero
int AC_pin1 = 4;// Output to Opto Triac
int dim1 = 0;// Dimming level (0-100) 0 = on, 100 = 0ff
int inc=1; // counting up or down, 1=up, -1=down
int freqStep = 100; // make this 83 for 60Hz gridfrequency
EthernetClient ethClient;
PubSubClient mqttClient;
long previousMillis;
void zero_cross_detect() {
zero_cross = true; // set the boolean to true to tell our dimming function that a zero cross has occured
i=0;
digitalWrite(AC_pin1, LOW); // turn off TRIAC (and AC)
}
// Turn on the TRIAC at the appropriate time
void dim_check() {
if(zero_cross == true) {
if(i>=dim1) {
digitalWrite(AC_pin1, HIGH); // turn on light
i=0; // reset time step counter
zero_cross = false; //reset zero cross detection
}
else {
i++; // increment time step counter
}
}
}
void setup() {
attachInterrupt(0, zero_cross_detect, RISING); // Attach an Interupt to Pin 2 (interupt 0) for Zero Cross Detection
Timer1.initialize(freqStep); // Initialize TimerOne library for the freq we need
Timer1.attachInterrupt(dim_check, freqStep);
pinMode(4, OUTPUT);
// setup serial communication
Serial.begin(9600);
while (!Serial) {};
Serial.println(F("dimmer"));
Serial.println();
// setup ethernet communication using DHCP
if (Ethernet.begin(mac) == 0) {
Serial.println(F("Unable to configure Ethernet using DHCP"));
for (;;);
}
Serial.println(F("Ethernet configured via DHCP"));
Serial.print("IP address: ");
Serial.println(Ethernet.localIP());
Serial.println();
ip = String (Ethernet.localIP()[0]);
ip = ip + ".";
ip = ip + String (Ethernet.localIP()[1]);
ip = ip + ".";
ip = ip + String (Ethernet.localIP()[2]);
ip = ip + ".";
ip = ip + String (Ethernet.localIP()[3]);
//Serial.println(ip);
// setup mqtt client
mqttClient.setClient(ethClient);
mqttClient.setServer( "192.168.1.103", 1883); // <= put here the address of YOUR MQTT server //Serial.println(F("MQTT client configured")); mqttClient.setCallback(callback); Serial.println(); Serial.println(F("Ready to send data")); previousMillis = millis(); mqttClient.publish("home/br/nb/ip", ip.c_str()); } void loop() { // it's time to send new data? if (millis() - previousMillis > PUBLISH_DELAY) {
sendData();
previousMillis = millis();
}
mqttClient.loop();
Serial.print("dim1 in loop = ");
Serial.println(dim1);
}
void sendData() {
char msgBuffer[20];
if (mqttClient.connect(CLIENT_ID)) {
mqttClient.subscribe("home/br/sb");
if (startsend) {
mqttClient.publish("home/br/nb/ip", ip.c_str());
startsend = LOW;
}
}
}
void callback(char* topic, byte* payload, unsigned int length) {
char msgBuffer[20];
payload[length] = '\0'; // terminate string with '0'
String strPayload = String((char*)payload); // convert to string
Serial.print("strPayload = ");
Serial.println(strPayload); //can use this if using longer southbound topics
Serial.print("Message arrived [");
Serial.print(topic);
Serial.print("] ");//MQTT_BROKER
for (int i = 0; i < length; i++) {
Serial.print((char)payload[i]);
}
Serial.println();
Serial.println(payload[0]);
dim1=strPayload.toInt();
}
The value freqStep is usually set at 75 or 78 with this type of circuits, which allows for 128 steps of dimming at 50Hz gridfrequency. I have set the value here purposely to 100 allowing for only 100 steps, which is convenient for use in OpenHAB as the slider goes from 0-100%.
The calculation is as follows:
EDIT 2019 There is a library for the Arduino, the ESP8266 and the ESP32.
50Hz
As we have two zerocrossings per sine wave, the frequency of zerocrossings is 100Hz. Therefore the period between two zerocrossings is 10mSec is 10000uS.
So if you want to have 100 levels of dimming the steps you need to take are 10000/100=100uS
60Hz
The frequency of zerocrossings is 120Hz, therefore the period between two zerocrossings is 8.3mS is 8300uS. So if you want to have 100 levels of dimming, the steps you need to take are 8300/100=83uS -> freqStep=83
Almost 3 years ago, I published a 
Arduino, ESP8266, ESP32 & Raspberry Pi stuff 