ESP32 Relay Guide - Choosing the Right Module for Home Automation
Complete guide to ESP32 relay modules. Compare 1-channel, 4-channel, and solid state relays. Learn safe wiring for AC loads, common mistakes, and which relay to buy.
TL;DR - Quick Picks
Why You Need a Relay #
ESP32 GPIO pins output 3.3V at a maximum of around 12mA. That's enough to blink an LED, not enough to switch a lamp, fan, or anything connected to mains power.
A relay bridges the gap. Your ESP32 sends a small signal to the relay, and the relay switches a much larger load - up to 10A, 20A, or more depending on the relay.
ESP32 GPIO
3.3V, ~12mA max. Can control LEDs, transistors, relay coils (with driver).
Relay Output
Switches 120V/240V AC or high-current DC. 10A typical, some rated 30A+.
Safety First #
AC Voltage Warning
Mains voltage (120V/240V) can kill. If you're not comfortable working with AC wiring, use pre-built solutions like Shelly or Sonoff. Seriously - no project is worth it.
- Always disconnect power before wiring
- Use proper enclosures rated for mains voltage
- Check local electrical codes
- When in doubt, hire an electrician
Relay Types Compared #
| Type | How It Works | Best For | Downsides |
|---|---|---|---|
| Mechanical Relay | Electromagnet moves physical contacts | General switching, most projects | Clicks, wears out (100K cycles), slower |
| Solid State Relay (SSR) | Semiconductor switching, no moving parts | Heaters, motors, frequent switching | Costs more, needs heatsink at high loads |
| Optocoupler Module | Relay + optical isolation on one board | ESP32 projects (isolation included) | Most modules are this type already |
Quick Decision Guide #
| Controlling This? | Get This | Why |
|---|---|---|
| Single lamp or fan | 1-channel relay | Simple, cheap |
| LED strip (12V/24V DC) | MOSFET module | No relay needed, PWM dimming |
| Multiple outlets | 4-channel relay | One board, less wiring |
| Heater or motor | SSR (25A+) | Handles inductive loads, no arcing |
| Whole room / many devices | 8-channel board | Centralized control |
| Don't want to wire AC | Shelly or Sonoff | Pre-built, safe, works with ESP |
Relay Module Options #
1-Channel Relay Module #
The simplest option. One relay, one device. Most modules include an optocoupler for isolation, so the relay coil doesn't share ground with your ESP32.
✓ Pros
- Cheap. Under $3 shipped.
- Simple wiring. VCC, GND, IN - done.
- Optocoupler included. Most modules have isolation.
✗ Cons
- 5V trigger common. Some won't trigger reliably at 3.3V.
- Audible click. Mechanical = noise.
- One device only. Need more? Buy more modules.
3.3V vs 5V Trigger
Many relay modules expect 5V trigger signal. ESP32 outputs 3.3V. Look for modules specifically marked "3.3V compatible" or use a level shifter. Some modules work anyway - but test before installing.
1-Channel 5V Relay Module (Optocoupler)
4-Channel Relay Module #
Four relays on one board. Control lights, fans, pumps - whatever you need. The go-to for home automation projects.
Pro tip: These modules often have a jumper to separate relay VCC from signal VCC. Use separate 5V supply for the relay coils if your ESP32's regulator can't handle the current draw.
4-Channel 5V Relay Module
8-Channel Relay Module #
For serious automation. Eight relays, one board. Watch your power supply - all coils on draws significant current.
8-Channel 5V Relay Module
Solid State Relay (SSR) #
No moving parts. Semiconductor does the switching. Silent, fast, no contact wear.
✓ Pros
- Silent operation. No clicking.
- No contact wear. Millions of cycles.
- Fast switching. Good for PWM-style control.
- Handles inductive loads. Motors, solenoids.
✗ Cons
- Needs heatsink. At high currents, gets hot.
- Voltage drop. ~1.5V across SSR = heat.
- More expensive. $8-15 vs $2-3 for mechanical.
- Small leakage current. May not fully "off" LEDs.
When to use SSR: Heaters, heating elements, motors, any load that switches frequently, or anywhere you need silent operation.
Fotek SSR-25DA Solid State Relay
SSR-40DA 40A Solid State Relay
Pre-Built Smart Relays #
Don't want to wire mains voltage? These devices have relays built-in, proper enclosures, and certifications. Flash with Tasmota or ESPHome for ESP32-style control.
Shelly 1 #
Tiny WiFi relay that fits behind a light switch. Has an ESP chip inside. Works standalone or flash with Tasmota/ESPHome.
Shelly 1 WiFi Relay
Sonoff Basic #
Inline smart switch. Plug it between your device and the outlet. Easy to flash with Tasmota.
Sonoff Basic R3 WiFi Smart Switch
Shelly 2.5 #
Two relays in one device. Control two lights, or use for roller shutters/blinds (up/down).
Shelly 2.5 Dual WiFi Relay
Wiring Basics #
Understanding relay wiring is simpler than it looks. There are two completely separate circuits:
Most projects use COM + NO: power flows to your device only when the ESP32 activates the relay.
Active-Low vs Active-High
Many relay modules are "active-low" – the relay turns ON when you write LOW to the pin. This seems backwards, but it's how most modules work. Just test yours.
Quick Reference #
| Connection | From | To |
|---|---|---|
| Power | ESP32 5V (or VIN) | Relay VCC |
| Ground | ESP32 GND | Relay GND |
| Signal | ESP32 GPIO 5 | Relay IN |
| AC Live | Wall outlet | Relay COM |
| To Load | Relay NO | Your device |
Basic Code Example #
#define RELAY_PIN 5
void setup() {
pinMode(RELAY_PIN, OUTPUT);
digitalWrite(RELAY_PIN, HIGH); // Start with relay OFF (active-low module)
}
void loop() {
digitalWrite(RELAY_PIN, LOW); // Relay ON
delay(5000);
digitalWrite(RELAY_PIN, HIGH); // Relay OFF
delay(5000);
}For web control, timers, or Home Assistant integration, check out ESPHome - it handles all of that with simple YAML config.
Common Mistakes #
Using 5V relay module with 3.3V ESP32 without checking. Some work, some don't trigger reliably. Test first or buy 3.3V-compatible modules.
Powering relay coils from ESP32's 3.3V regulator. The regulator can't supply enough current. Use VIN (5V USB) or separate supply for relay VCC.
No flyback diode. When a relay coil switches off, it creates a voltage spike. Most modules have this built-in, but bare relays need one.
Undersizing the relay for the load. A "10A" relay shouldn't run at 10A continuously. Derate by 50% for reliability. Use 20A relay for 10A load.
Forgetting about inrush current. Motors and some lights draw 5-10x more current at startup. A lamp that draws 1A steady might draw 10A for a split second.
Mixing low-voltage and mains wiring. Keep them separate. Use proper enclosures. Don't run 3.3V signal wires next to 240V mains.
Relay Module Comparison #
| Type | Channels | Current | Trigger | Price | Best For |
|---|---|---|---|---|---|
| Mechanical | 1 | 10A | 5V (some 3.3V) | $2-4 | Single device |
| Mechanical | 4 | 4x 10A | 5V | $5-8 | Multiple devices |
| Mechanical | 8 | 8x 10A | 5V | $10-15 | Full automation |
| SSR | 1 | 25-40A | 3-32V DC | $8-15 | Heaters, motors |
| Shelly 1 | 1 | 16A | WiFi | $15-20 | Safe, no wiring |
| Sonoff Basic | 1 | 10A | WiFi | $8-12 | Budget smart switch |