573 Relays and Solenoids

Learning Objectives

After completing this chapter, you will be able to:

Understand relay operation and specifications
Interface relays safely with microcontrollers
Control solenoids for linear actuation
Implement flyback protection for inductive loads
Design safe high-voltage switching circuits
Use solid-state relays (SSR) for silent, high-speed switching

573.1 Relay Fundamentals

Relays are electrically-operated switches that allow low-power circuits (microcontrollers) to control high-power loads (motors, heaters, lights).

Key Benefits:

Electrical isolation: Complete separation between control and load circuits
High current switching: 10A, 20A, or more
AC and DC loads: Can switch both types
Low control current: Typically 20-100mA coil current

573.1.1 Relay Specifications

Parameter	Typical Values	What It Means
Coil voltage	3.3V, 5V, 12V, 24V	Voltage needed to activate relay
Coil current	50-100mA	Current drawn by coil (needs driver!)
Contact rating	10A @ 250VAC	Maximum load current and voltage
Contact type	SPST, SPDT, DPDT	Number of poles and throws

573.1.2 Relay Control Circuit

Never Connect Relay Coil Directly to GPIO!

Most relay coils draw 50-100mA, far exceeding the 20-40mA GPIO limit. Always use a transistor driver!

// Relay control with transistor driver
#define RELAY_PIN 25

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, LOW);  // Relay OFF (normally open)
}

void loop() {
  // Turn relay ON (closes normally-open contact)
  digitalWrite(RELAY_PIN, HIGH);
  Serial.println("Relay ON - Load powered");
  delay(3000);

  // Turn relay OFF
  digitalWrite(RELAY_PIN, LOW);
  Serial.println("Relay OFF - Load unpowered");
  delay(3000);
}

573.1.3 Relay Module Wiring

Most relay modules include the transistor driver and flyback diode:

ESP32          Relay Module         High-Power Load
GPIO25 ------> IN (control)
GND ----------> GND
3.3V/5V -----> VCC
               COM ----------------> Load common
               NO (normally open) -> Load hot (when relay ON)
               NC (normally closed)-> Load hot (when relay OFF)

573.2 Solenoid Control

Solenoids provide linear push/pull motion for locks, valves, and latches.

Characteristics:

Fast response (5-50ms)
Binary operation (on/off only)
High inrush current
Requires flyback protection

#define SOLENOID_PIN 26

void setup() {
  pinMode(SOLENOID_PIN, OUTPUT);
}

void loop() {
  // Activate solenoid (pull/push)
  Serial.println("Activating solenoid lock...");
  digitalWrite(SOLENOID_PIN, HIGH);
  delay(1000);

  // Release solenoid
  Serial.println("Releasing solenoid lock...");
  digitalWrite(SOLENOID_PIN, LOW);
  delay(3000);
}

// Smart lock example
void unlockDoor() {
  digitalWrite(SOLENOID_PIN, HIGH);
  Serial.println("Door unlocked");

  // Auto-lock after 5 seconds
  delay(5000);

  digitalWrite(SOLENOID_PIN, LOW);
  Serial.println("Door locked");
}

573.3 Flyback Protection

573.4 Critical: Inductive Kickback Protection

Relay coils and solenoids are inductors. When power is cut, the collapsing magnetic field generates a high voltage spike (potentially 100V+ from a 12V supply) that can destroy transistors and microcontrollers!

Required Protection Circuit:

GPIO --> [Transistor/MOSFET] --+-- Relay Coil --+
                               |                 |
                               +-- [Diode] <----+
                               |
                              GND

Diode specs:
- Voltage rating > supply voltage
- Current rating >= coil current
- 1N4007 for relays, 1N5819 Schottky for motors

573.5 Solid-State Relays (SSR)

For silent, high-speed, and maintenance-free switching, use solid-state relays.

Advantages over mechanical relays:

No mechanical wear
Silent operation
Faster switching (microseconds vs milliseconds)
No contact bounce
Works with PWM for heater control

Disadvantages:

Voltage drop across output (1-2V)
Requires heatsink for high loads
More expensive
No electrical isolation (optocoupler-type has some isolation)

// SSR for heater PWM control
#define SSR_PIN 27

void setup() {
  // SSR can handle PWM for proportional heating
  ledcSetup(0, 1, 8);  // 1 Hz, 8-bit (slow PWM for heating)
  ledcAttachPin(SSR_PIN, 0);
}

void setHeaterPower(int percent) {
  // 0-100% maps to 0-255
  int duty = map(percent, 0, 100, 0, 255);
  ledcWrite(0, duty);
}

573.6 Valve Control

Solenoid valves control fluid flow in irrigation, HVAC, and industrial systems.

#define VALVE_PIN 25
#define FLOW_SENSOR_PIN 34

void setup() {
  pinMode(VALVE_PIN, OUTPUT);
  pinMode(FLOW_SENSOR_PIN, INPUT);
  digitalWrite(VALVE_PIN, LOW);  // Valve closed
}

// Water for specified duration
void water(int seconds) {
  Serial.print("Watering for ");
  Serial.print(seconds);
  Serial.println(" seconds");

  digitalWrite(VALVE_PIN, HIGH);  // Open valve
  delay(seconds * 1000);
  digitalWrite(VALVE_PIN, LOW);   // Close valve

  Serial.println("Watering complete");
}

// Water until flow sensor detects specified volume
void waterVolume(float liters) {
  float flowRate;  // Liters per minute
  float totalVolume = 0;

  digitalWrite(VALVE_PIN, HIGH);

  while (totalVolume < liters) {
    // Read flow sensor (pulse counting)
    // This is simplified - real implementation needs interrupt
    flowRate = readFlowSensor();
    totalVolume += flowRate / 60.0;  // Convert to liters/second
    delay(1000);

    Serial.print("Volume: ");
    Serial.print(totalVolume);
    Serial.print(" / ");
    Serial.print(liters);
    Serial.println(" L");
  }

  digitalWrite(VALVE_PIN, LOW);
}

573.7 Safety Considerations

573.8 High Voltage Safety

When working with mains voltage (120V/240V AC):

Use properly rated relay contacts (check current AND voltage rating)
Ensure electrical isolation between low-voltage and mains circuits
Use enclosed, insulated enclosures for all high-voltage connections
Install fuses or circuit breakers on the mains side
Follow local electrical codes (NEC, IEC, etc.)
Consider hiring a licensed electrician for permanent installations

573.9 What’s Next?

Now that you understand switching actuators, you’re ready to explore visual and audio output devices.

Continue to Visual and Audio Actuators →