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Controlling 8 relays with ESP32 and shift register

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In this series of tutorials, we will see how to drive a multitude of relays with a NodeMCU32S microcontroller and 74HC595 shift registers. We will then see how to address each relay individually. Finally, we will create a web interface to control each relay via the internet. This project is a good basis to develop a home automation system for your home.

This tutorial can be applied to any microcontroller. You will need to be careful to modify the connection diagram and code to suit your use.

Material

Principle

To control the 8 relay module, 8 digital outputs of the microcontroller must be reserved. In order to save the number of inputs/outputs of the microcontroller, different integrated circuits can be used. In particular, the shift register. The shift register 74HC595 has 8 programmable outputs, which can be set to 0 or 5V and only requires 3 digital inputs. This solution is therefore ideal to allow us to control the relays and keep the microcontroller outputs for other functions.

Schematic

To control the shift register, the NodeMCU is connected as follows:

The outputs of the integrated circuit 74HC595 (Q0-Q7) are directly connected to the inputs of the 8 relay module. In this tutorial we do not use the Enable (OE) and Reset (MR) pins.

Attention: The outputs of a microcontroller are current limited. To drive so many relays, it is strongly recommended to use an external voltage source.

Code

In the following code, we will first define the basic functions to manage the shift register. Then we will operate the relays one by one using a for loop.

//Constants
#define number_of_74hc595s 1
#define numOfRegisterPins number_of_74hc595s * 8
#define SER_Pin 25
#define RCLK_Pin 33
#define SRCLK_Pin 32

//Variables
boolean registers [numOfRegisterPins];

void setup(){
//Init Serial USB
Serial.begin(115200);
Serial.println(F("Initialize System"));
//Init register
pinMode(SER_Pin, OUTPUT);
pinMode(RCLK_Pin, OUTPUT);
pinMode(SRCLK_Pin, OUTPUT);
clearRegisters();
writeRegisters();
delay(500);
}

void loop(){
writeGrpRelay();
}

void clearRegisters(){/* function clearRegisters */ 
//// Clear registers variables 
for(int i = numOfRegisterPins-1; i >=  0; i--){
  registers[i] = HIGH;//LOW;
}}

void writeRegisters(){/* function writeRegisters */ 
//// Write register after being set 
digitalWrite(RCLK_Pin, LOW);
 for(int i = numOfRegisterPins-1; i >=  0; i--){
  digitalWrite(SRCLK_Pin, LOW); int val = registers[i];
  digitalWrite(SER_Pin, val);
  digitalWrite(SRCLK_Pin, HIGH);
}
  digitalWrite(RCLK_Pin, HIGH);
}

void setRegisterPin(int index,int value){/* function setRegisterPin */ 
////Set register variable to HIGH or LOW
registers[index] = value;
}

void writeGrpRelay(){/* function writeGrpRelay */ 
for(int i = numOfRegisterPins-1; i >=  0; i--){
   Serial.print(F("Relay "));Serial.print(i);Serial.println(F(" HIGH"));
   setRegisterPin(i, LOW);
   writeRegisters();
   delay(200); 
   Serial.print(F("Relay "));Serial.print(i);Serial.println(F(" LOW"));
   setRegisterPin(i, HIGH);
   writeRegisters();
  delay(500); 
      
}
}

Results

Once the components have been correctly connected and the code loaded into the microcontroller, the relays should activate one after the other.

Bonus: Control 16 relays with two shift registers.

Schematic

Code

The beauty of this code is that to make it work with a second shift register you only need to change the parameter:

#define number_of_74hc595s 2

Results

Next Steps

Sources

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