Electromechanical Devices

Relays are electromechanical devices that use electromagnets to connect or disconnect the plates of a switch. Relays are used to control high-power circuits with low-power circuits. Both circuits are electrically isolated; thus, the control logic is protected from high voltage, sometimes from the power mains. Relays are used in household appliance automation, lighting and climate control. Although the electromagnet's coil of the relay requires relatively low power compared to the power capability of the output circuit, it cannot be connected directly to the microcontroller's pin. Creating the transistor driver or using a relay module with the driver built-in is possible. The sample relay module is present in figure 1 and its connection to the Arduino development board in figure 2.

The following example code should work properly for the relay module. It turns the relay on while there is a “1” state at the microcontroller's output and turns the relay off while there is a “0” state at the output.

#define relayPin  4 //Define the relay pin
 
void setup()
{    
  Serial.begin(9600);
  pinMode(relayPin, OUTPUT); //Set relayPin to output
 
}
 
void loop()
{
   digitalWrite(relayPin,1);  //Turns relay on
   Serial.println("Relay ON"); //Output text 
   delay(2000); // Wait 2 seconds
 
   digitalWrite(relayPin,0);  //Turns relay off
   Serial.println("Relay OFF");
   delay(2000);
}

Solenoids use electromagnets to pull or push iron or steel cores. They are used as linear actuators for locking mechanisms indoors, pneumatic and hydraulic valves and in-car starter systems.
Solenoids and relays use electromagnets, and connecting them to Arduino is very similar. Coils need much power and are usually attached to the circuit's power source using a transistor driver. Turning the coil's power off makes the electromagnetic field collapse and creates a very high voltage. A shunt diode channels the overvoltage for the semiconductor devices' protection. For extra safety, an optoisolator can be used. Sample solenoid is present in figure 3 and connection to the MCU in figure 4.

The example code to control solenoid is present below:

#define solenoidPin  4 //Define the solenoid pin
 
void setup()
{    
  Serial.begin(9600);
  pinMode(solenoidPin, OUTPUT); //Set solenoidPin to output
 
}
 
void loop()
{
   digitalWrite(solenoidPin,1);  //Turns solenoid on
   Serial.println("Solenoid  ON"); //Output text 
   delay(2000); //Wait 2 seconds
 
   digitalWrite(solenoidPin,0);  //Turns solenoid off
   Serial.println("Solenoid OFF");
   delay(2000);
}