Zero-Cross Not Detected: AC Dimmer Doesn't Respond to Commands

TL;DR: An AC dimmer can't regulate the load if the microcontroller isn't receiving the zero-cross signal (the sine wave crossing zero). Without this signal it's impossible to calculate the TRIAC firing moment. Causes: wrong pin (not interrupt-capable), incorrect ZC wiring, noise on the signal line, or a software error (wrong interrupt mode). Work through the checklist below step by step.

Problem Description

You connected an AC dimmer, uploaded sample code — but the load doesn't respond or is always at full brightness. The wiring looks correct, no compile errors.

The most common cause in these cases is that zero-cross is not being detected by the microcontroller. Phase-cut dimming works on this principle: detect the sine wave zero-crossing → wait the calculated delay → fire the TRIAC. Without zero-cross there is no timing, and the dimmer either doesn't turn on the load at all or keeps it at 100% with no regulation.

Typical symptoms:

• Load is always at full brightness (TRIAC always open)
• Load doesn't turn on at all
• setPower(50) has no effect — load is either fully on or off
• Zero-cross counter doesn't increment in Serial monitor (if you add debug)
• Code compiles without errors but the dimmer "doesn't work"

Common forum messages:

• "Dimmer doesn't dim — lamp always full brightness"
• "setPower has no effect"
• "Zero cross interrupt not triggering"
• "Dimmer works sometimes, randomly"

Root Cause

The zero-cross signal is generated by a zero-cross detector circuit built into the AC dimmer or connected externally. Typically it's an optocoupler + resistor divider that produces a short pulse at each sine-wave zero crossing.

Pulse frequency: 100 Hz (50 Hz mains) / 120 Hz (60 Hz mains). One pulse per half-cycle.

To receive these pulses the microcontroller uses an external interrupt (attachInterrupt()). If the interrupt is not configured correctly or the signal doesn't reach the right pin — the ISR handler is never called, timing can't be computed, and the dimmer doesn't work.

Root causes of failure:

1. Wrong pin (not interrupt-capable) — most common error on Arduino
2. Incorrect ZC wiring — signal doesn't reach the pin
3. Wrong interrupt mode (FALLING instead of RISING or vice versa — depends on the circuit)
4. Noise — multiple false triggers on one half-cycle
5. ZC circuit power issue — no VCC on the zero-cross detector

Solutions

🟢 For beginners: DimmerLink — zero-cross built in

Don't want to deal with interrupt pins, RISING/FALLING, and electrical noise? DimmerLink has its own zero-cross detector and controls the TRIAC autonomously.

DimmerLink detects zero-cross in hardware and handles TRIAC firing timing internally. Your microcontroller only sets the brightness level — no ISR, no interrupts, no pin headaches.

When to choose this solution:

DimmerLink → Arduino/ESP32 wiring:

• VCC → 3.3V (ESP32) or 5V (Arduino)
• GND → GND
• SDA → SDA (GPIO 21 on ESP32, A4 on Arduino Uno)
• SCL → SCL (GPIO 22 on ESP32, A5 on Arduino Uno)

Code:

// DimmerLink — zero-cross detector and TRIAC control inside the module
// Microcontroller only sets the brightness level
// Docs: https://www.rbdimmer.com/docs/dimmerlink-I2CCommunication
#include <Wire.h>
#define DIMMER_ADDR 0x50
#define REG_LEVEL   0x10
void setLevel(uint8_t level) {  // level: 0–100%
    Wire.beginTransmission(DIMMER_ADDR);
    Wire.write(REG_LEVEL);
    Wire.write(level);
    Wire.endTransmission();
}
void setup() {
    Wire.begin();        // default SDA/SCL for your board
    setLevel(50);        // 50% brightness
}
void loop() {
    // No ISR, no interrupts, no zero-cross on the MCU
}

Result: Dimmer works without depending on interrupt pins or zero-cross signal on the microcontroller.

🔵 For advanced users: diagnose and fix the ZC

Want to keep the direct ISR connection? Follow the diagnostic steps.

Diagnosis: steps 1–5

Step 1: Verify the pin is interrupt-capable

This is cause #1 on Arduino.

// Arduino Uno / Nano / Mini:
// ✅ Interrupt-capable: ONLY pins 2 and 3
// ❌ All other pins (4, 5, 6...) — interrupts not supported
// Correct:
attachInterrupt(digitalPinToInterrupt(2), zeroCrossISR, RISING);  // pin 2 ✅
attachInterrupt(digitalPinToInterrupt(3), zeroCrossISR, RISING);  // pin 3 ✅
// Wrong:
attachInterrupt(4, zeroCrossISR, RISING);  // ❌ pin 4 — not interrupt-capable
// On Uno, attachInterrupt(4) refers to INT4, which doesn't exist

On ESP32 — any GPIO supports interrupts:

// ESP32: any GPIO — all work
attachInterrupt(digitalPinToInterrupt(18), zeroCrossISR, RISING);  // ✅
attachInterrupt(digitalPinToInterrupt(34), zeroCrossISR, RISING);  // ✅

On ESP8266 — all GPIOs except GPIO 16 support interrupts; recommend 4, 5, 12, 13, 14 (no boot-mode dependencies).

Step 2: Add a counter to verify

The simplest test — count pulses over 1 second:

// Zero-cross diagnostic: expect ~100 pulses/sec (50 Hz mains)
// or ~120 pulses/sec (60 Hz mains)
#define ZC_PIN 2  // ← make sure this is an interrupt-capable pin
volatile uint32_t zcCount = 0;
#ifdef ESP32
void IRAM_ATTR zeroCrossISR() {  // IRAM_ATTR required on ESP32
#else
void zeroCrossISR() {             // Arduino/ESP8266: IRAM_ATTR not needed
#endif
    zcCount++;
}
void setup() {
    Serial.begin(115200);
    pinMode(ZC_PIN, INPUT);
    attachInterrupt(digitalPinToInterrupt(ZC_PIN), zeroCrossISR, RISING);
}
void loop() {
    delay(1000);
    Serial.print("ZC impulses per second: ");
    Serial.println(zcCount);
    zcCount = 0;
    // Expected results:
    // ~100 — 50 Hz mains, all OK
    // ~120 — 60 Hz mains, all OK
    //   0  — ZC not detected (wiring or pin problem)
    //  >200 — noise, false triggers (RC filter problem)
}

Step 3: Check the ZC wiring

Typical RBDimmer → Arduino/ESP32 wiring:

RBDimmer  →  Arduino/ESP32
-------      -------------
VCC       →  5V (Arduino) / 3.3V (ESP32)
GND       →  GND
ZC        →  Interrupt-capable pin (2 or 3 on Arduino Uno)
DIM       →  Any digital output

Checklist:

Step 4: Check the interrupt mode (RISING/FALLING/CHANGE)

The mode depends on the ZC detector circuit of your specific module:

// Most RBDimmer modules: RISING (pulse on rising edge)
attachInterrupt(digitalPinToInterrupt(ZC_PIN), zeroCrossISR, RISING);
// Some circuits with inverting optocoupler: FALLING
attachInterrupt(digitalPinToInterrupt(ZC_PIN), zeroCrossISR, FALLING);
// If unsure — try CHANGE (catches both edges):
// Note: with CHANGE the counter will read ~200 Hz instead of 100 Hz
attachInterrupt(digitalPinToInterrupt(ZC_PIN), zeroCrossISR, CHANGE);

Use an oscilloscope or logic analyzer to check the ZC signal shape — identify whether you need to catch the rising or falling edge.

Step 5: Filter noise

If the counter shows >200 (on a 50 Hz grid) — false triggers due to noise. This is especially common with long wires or proximity to the load:

// Software debounce for ZC: ignore pulses too close to the previous one
// Half-cycle period at 50 Hz = 10 000 µs → filter anything shorter than 8 000 µs
volatile uint32_t lastZC = 0;
void IRAM_ATTR zeroCrossISR() {
    uint32_t now = micros();
    if (now - lastZC > 8000) {  // 8 ms minimum interval between ZC pulses
        lastZC = now;
        // your TRIAC timing logic here
    }
}

Hardware filter: an RC filter on the ZC line (1 kΩ resistor + 100 nF capacitor between ZC and GND) removes high-frequency noise.

Fix: choose your option

Option A: rbdimmerESP32 on ESP32 ✅

When: dual-core ESP32 with direct dimmer connection. The library handles ZC and timing automatically:

// Platform: dual-core ESP32
// Library: rbdimmerESP32
#include "rbdimmerESP32.h"
#define ZC_PIN  18  // any ESP32 GPIO
#define DIM_PIN 19  // any ESP32 GPIO
rbdimmer dimmer;
void setup() {
    Serial.begin(115200);
    dimmer.begin(ZC_PIN, DIM_PIN, 50);  // 50 Hz mains
    dimmer.setPower(50);
    Serial.println("Dimmer initialized");
}
void loop() {}

Common mistakes:

• ZC_PIN and DIM_PIN swapped in dimmer.begin() — check which pin goes to the load and which to zero-cross.
• Wrong mains frequency (third parameter) — 50 or 60 Hz.

Option B: Arduino Uno/Mega with RBDdimmer

// Platform: Arduino Uno / Mega / Nano (AVR)
// Library: RBDdimmer — https://github.com/robotdyn/dimmer
// WARNING: for ESP32 use rbdimmerESP32, not this library!
#include <RBDdimmer.h>
// ⚠️ Arduino Uno: ZC ONLY on pins 2 or 3
#define ZC_PIN   2   // interrupt-capable ✅
#define DIM_PIN  11  // any digital output
dimmerLamp dimmer(DIM_PIN, ZC_PIN);
void setup() {
    Serial.begin(9600);
    dimmer.begin(NORMAL_MODE, ON);
    dimmer.setPower(50);  // 50%
    Serial.println("Dimmer ready");
}
void loop() {}

⚠️ Common pitfalls

• "Using pin 4 on Arduino Uno — not working": Pins 4, 5, 6, ... on the Uno don't support external interrupts. Only pins 2 and 3 do. This is the most common beginner mistake.

• "attachInterrupt(4, ...) compiles — should work": It compiles, but it won't work. attachInterrupt(4, ...) on the Uno refers to INT4 (hardware interrupt number), not GPIO 4. Always use digitalPinToInterrupt(pin).

• "Added counter — always 0": Three possible causes: 1. Wrong pin (not interrupt-capable) — check above 2. No power on the ZC module 3. ZC pin not physically connected to the board

• "Counter shows 300–400 instead of 100": False triggers due to noise. Add software debounce (see above) or a hardware RC filter.

• "Pin 34 on ESP32 doesn't work for ZC": GPIO 34–39 on ESP32 are input-only — they support interrupts. But they have no internal pull-up/down. Add an external 10 kΩ resistor to 3.3V.

• "Works without load, breaks with load connected": Electrical noise from the load (especially motors, transformers) can couple into the ZC line. Physically separate power and signal wires.

Quick Check

Before posting on the forum, verify:

Interrupt Pin Compatibility Table

Related Issues

• ESP32 + AC dimmer: Guru Meditation Error → troubleshooting/esp32-iram-attr.md
• LED flicker with dimmer → load-types/led-flicker-triac-dimmer.md
• Dimmer doesn't regulate LED → load-types/led-lamp-compatibility-triac.md

Still have questions?

Ask on forum.rbdimmer.com or open a GitHub Issue.