Meet the Relay — Switching High Power with Low Power

By the end of this mission, you will be able to:

• Explain what a relay module is and what problem it solves

• Describe the difference between active-HIGH and active-LOW relay modules

• Wire a relay module to the ESP32 safely

• Turn a relay ON and OFF using code

The ESP32's pins can only handle a tiny bit of power — nowhere near enough to run a lamp, fan, or appliance directly. So how do smart plugs and home automation systems control full-powered devices? The answer is the relay.

Robo says: “A relay is like a tiny robotic hand that flips a real power switch for you, based on a signal from your code. Let's meet it.”

A relay is an electrically-controlled switch. A small, safe signal from the ESP32 (just a few volts) controls an internal switch that can turn a completely separate, much higher-power circuit on or off — without the two circuits ever touching each other directly. This is what lets a tiny microcontroller safely control something like a lamp, fan, or appliance.

Important: many relay modules are “active LOW,” meaning they turn ON when the signal pin receives LOW, and OFF when it receives HIGH — the opposite of what you might expect from earlier lessons. Always check your specific module before wiring it up.

This lesson's circuit connects the relay like this:

• VCC → 5V

• GND → GND

• IN → GPIO 5

• Relay NO (normally open) → LED circuit for demo

Turn the LED ON and OFF via the relay.

int relayPin = 5;
void setup() {
pinMode(relayPin, OUTPUT);
}
void loop() {
digitalWrite(relayPin, HIGH); // Relay ON
delay(2000);
digitalWrite(relayPin, LOW); // Relay OFF
delay(2000);
}

🔧 Try It Yourself

Run the code and observe the relay, then write down what you noticed:

Example: The relay clicked audibly every 2 seconds as it switched.

• Did you hear the relay click as it switched?

• Was your module active HIGH or active LOW?

🤔 Discuss With a Partner or Write Your Answer

Why do you think it's safer to control a high-power appliance through a relay, instead of connecting the appliance's wiring directly to the ESP32?

1. A relay is an electrically-controlled ________.

2. A relay lets a low-power signal control a completely separate, ________-power circuit.

3. A relay module that turns ON when it receives LOW instead of HIGH is called active ________.

4. In this unit, you will only ever demo relays using a safe ________ circuit, never a real appliance.

Draw a line (or write the matching letter in the blank) to connect each word to its meaning.

• I can explain what a relay module is and what problem it solves

• I can describe the difference between active-HIGH and active-LOW relay modules

• I can wire a relay module to the ESP32 safely

• I can turn a relay ON and OFF using code

Question 1: What problem does a relay solve?

🔍 Reveal answer (drag-select the blank space): B

Question 2: What does “active LOW” mean for a relay module?

🔍 Reveal answer (drag-select the blank space): B

Question 3: In this unit's lessons, what do you use to safely demonstrate a relay instead of a real appliance?

🔍 Reveal answer (drag-select the blank space): B

🏆 Level Up

Can you make the relay switch on and off in a specific pattern (like two short clicks, then a long pause) instead of a steady 2-second interval?

Explain in your own words why a relay is needed to safely control something like a lamp or fan with a microcontroller.

🤖 Robo Says

You just safely switched real power with code! Next up: Lesson 2 — you'll add a button so you can flip that switch yourself.