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Important: This project switches lethal mains voltage. It is an educational design concept for people competent to design, assemble, measure, and enclose mains-powered equipment—not a beginner wiring project or a certified household dimmer. Optical isolation protects the Arduino’s signal paths; it does not make the load circuit safe to touch. For permanent household use, choose a properly rated, certified enclosed dimmer.
An Arduino phase-angle dimmer uses an isolated zero-cross detector to time each AC half-cycle, then drives a random-phase optotriac such as the MOC3021 to trigger a power triac after a controlled delay. A suitable resistive load is the best first test. Component values, PCB spacing, thermal design, enclosure, and protection must all be engineered for the actual mains supply and load.
What this dimmer does
This is a phase-angle AC dimmer, not ordinary low-voltage PWM. A triac switches an AC load for part of each half-cycle. The later it is triggered after the waveform crosses zero, the smaller the portion of the half-cycle delivered to a resistive load.
- The AC voltage passes through zero.
- An isolated detector signals the Arduino.
- The Arduino waits for a selected delay.
- A pulse through the optotriac triggers the power triac.
- The power triac conducts for the remainder of that half-cycle, then turns off naturally when load current falls below its holding current near the next current zero.
A shorter delay generally means more conduction and more power for a resistive load. Neither electrical power nor perceived brightness changes linearly with a knob value: “50%” on a control is not necessarily 50% power or half perceived brightness. A calibrated curve or lookup table is more useful than mapping a control setting directly to half-cycle delay.
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Functional layout and isolation boundary
ISOLATION BARRIER
LOW-VOLTAGE SIDE MAINS SIDE
Arduino Nano <── detector ── zero-cross network ── AC line/neutral
│ │
└── optotriac LED ──||── random-phase output ── power triac ── load
│
snubber
The zero-cross detector and optotriac provide two separate optical signal paths across the barrier. The Arduino, its USB connection, and its control wiring belong on the low-voltage side. The detector input network, optotriac output, power triac, snubber, fuse, terminals, and load wiring are mains-side circuitry. The load remains electrically connected to mains; isolation does not make its terminals or the assembled device touch-safe.
The published project concept uses a TLP521-1 phototransistor optocoupler for zero-cross sensing, a MOC3021 random-phase optotriac, a BT138 power triac, and an Arduino Nano. The example assigns digital pin 2 to the detector signal and pin 10 to the optotriac LED drive. These are example choices, not universal requirements. See the project description and sketch; independently inspect any supplied PCB or Gerber files before considering them.
Why zero-cross detection matters
The detector gives the controller a repeatable timing reference. Mains frequency determines the available half-cycle:
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The reference design uses a mains-side resistor/rectifier network and TLP521-1. Its output is not necessarily a razor-thin pulse at the mathematical zero: detector threshold, optocoupler behavior, resistor tolerance, and signal conditioning influence when the Arduino sees a transition. The firmware must act on a single event, not repeatedly while the detector output stays HIGH.
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The original project reports visible flicker from timing jitter in some circumstances and uses polling instead of an interrupt. That is a report about that implementation, not a general rule: a carefully edge-detected interrupt or timer-driven state machine can provide deterministic timing. The important requirements are bounded jitter, one scheduling event per half-cycle, and a defined response when crossings stop. The EDN explanation also discusses timing and flicker concerns.
Choosing the switching parts
Zero-cross optocoupler
The TLP521-1 is the example detector optocoupler. Do not copy its mains-side resistor network blindly. Calculate resistor voltage stress, power, pulse capability, detector LED current, target mains voltage and frequency, and isolation requirements. Use the exact manufacturer datasheet for the chosen part and verify its pinout and ratings.
Random-phase optotriac
The example MOC3021 is a random-phase optotriac: it can trigger when commanded during a half-cycle, which is what phase-angle dimming needs. A zero-cross optotriac waits for the mains voltage to approach zero before switching. That is useful for on/off or burst switching with reduced switching noise, but it cannot provide arbitrary firing-angle control. These parts are not interchangeable for this purpose; see the project discussion of optotriac selection.
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The reference uses a BT138. Select a triac by more than its headline current rating. Check repetitive off-state voltage with margin for the local supply and transients, RMS and surge current, gate trigger current in the relevant quadrants, latching and holding current, on-state loss, thermal resistance, and immunity to rapid voltage and current changes. BT138 ratings, pinout, and package details depend on the exact maker and suffix; consult that part’s datasheet. Its mounting tab may be electrically connected to a triac terminal, so do not assume a heatsink is safe to touch or can be bonded to a metal enclosure.
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Resistors, capacitors, snubber, and protection
The design includes RC snubber components to help control false triggering from rapid voltage changes and improve operation with some difficult loads. There is no universally correct snubber value: the triac, load inductance, wiring, voltage, leakage-current tolerance, and emissions requirements all matter. Validate the network with the intended load and consult the triac maker’s application guidance.
For any capacitor connected across or from mains, use the appropriate approved safety class for its actual connection and purpose. A voltage marking alone, even a 400 V rating, does not establish that a capacitor is suitable as a mains safety component. Likewise, resistor wattage alone does not establish suitability: check working voltage, pulse rating, fault behavior, and spacing. Provide appropriately rated overcurrent protection and terminals as part of the complete design.
Timing and firmware approach
The project’s demonstration sketch uses pin 2 as the zero-cross input, pin 10 as the control output, and a 5,000-microsecond delay. At 50 Hz, 5 ms is approximately the midpoint of a 10 ms half-cycle; it is not a universal 50% power setting. It is also beyond the 4.17 ms half-cycle at 60 Hz, so it cannot be used unchanged there.
Its basic loop waits for a HIGH detector input, delays, then drives the optotriac output HIGH. That illustrates the idea, but it is not production firmware: it can react repeatedly during one HIGH interval, does not create a finite gate pulse, and has no explicit timeout or fault state. A more robust design follows this sequence:
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on a qualified zero-cross event:
cancel any pending gate event
if requested level is zero:
keep optotriac off
else:
calculate and clamp delay for this half-cycle
schedule a short gate pulse
when pulse width expires:
turn optotriac output off
if no valid crossing arrives before timeout:
cancel events and turn output off
Implement this with an edge-detected input and timer scheduling appropriate to the selected Arduino board. Avoid blocking delays in a design that must also service timing or fault logic. Set the pulse width and the earliest/latest firing angles from the optotriac and triac datasheets and validate them in the circuit. Clamp delay against the configured 50 or 60 Hz half-cycle. Make zero output an explicit off state; define startup, reset, and lost-crossing behavior so the gate is not left asserted. The published project is useful as a conceptual reference, not a certified design or drop-in firmware solution.
Load compatibility
| Load type | Practical guidance |
|---|---|
| Incandescent or other resistive lamp | Best initial test class, provided the lamp and switching components are properly rated. |
| Mains LED bulb | Use only if explicitly marked dimmable and compatible with leading-edge triac dimming. Capacitive input circuits may flicker, fail to start, or react to triac leakage. |
| Universal motor | May operate, but can produce EMI, acoustic noise, and commutation transients; validate the specific motor and control method. |
| Induction motor | Generally a poor match for a simple phase-angle dimmer; reduced voltage can cause stalling or overheating. |
| Transformer | Not a casual substitution: phase cutting can cause saturation and damaging current. Use a compatible control method. |
| Electronic power supply or low-wattage load | May require trailing-edge control, may not start reliably, or may draw too little current to keep the triac latched. |
Large inrush current can exceed the triac’s surge rating even when steady-state load wattage appears modest. Test one known-compatible load first; do not infer that a controller supports every AC load because it switches mains.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Thermal and mechanical design
A conducting triac dissipates heat. Estimate losses using expected RMS load current and the device’s on-state characteristics, then assess junction temperature against ambient temperature, enclosure airflow, thermal interface, and heatsink thermal resistance. Include a conservative margin and verify temperature under sustained operation with the actual load. The project notes its small heatsink was suitable only for short testing and that longer operation needs more heatsinking (project notes). If the package tab is live, use an appropriate insulating mounting arrangement and verify the complete assembly’s insulation.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsLay out the PCB with distinct mains and low-voltage zones. Creepage and clearance depend on working voltage, pollution environment, material group, and applicable local standards; there is no single spacing value that is safe in every context. Keep copper, vias, mounting hardware, and other conductive features from compromising the isolation barrier. Slots or cutouts may help increase creepage, but do not replace a standards-based layout review. Use mains-rated wire and terminal blocks, strain relief, a flame-retardant enclosure, secure insulated test points, and clear line, neutral, load, and low-voltage markings. Protect users from exposed solder joints and live heatsinks. Wider reinforced tracks and extra vias can address current capacity, but track width alone does not make a board safe; the layout discussion is not a substitute for electrical safety design.
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Commissioning and measurement
Do not connect a conventional grounded oscilloscope probe’s ground clip to a mains conductor. The scope ground is typically connected to protective earth; the resulting connection can create a short, destroy equipment, or cause fatal shock. The project explicitly warns against this practice (measurement warning). Use a properly rated differential probe or professionally designed isolated measurement setup, and understand the measurement equipment’s ratings and procedure. Never treat Arduino USB ground as a safe reference for mains measurements.
- With power disconnected, inspect polarity, soldering, clearances, component orientation, and mechanical security.
- Check for shorts and verify separation between mains and low-voltage sections with suitable equipment and procedures.
- Exercise Arduino-side timing logic without mains connected.
- Use an appropriately isolated, current-limited test arrangement only if competent to do so; isolation equipment does not make exposed conductors safe.
- Test first with a low-power, known resistive load, with the circuit enclosed before applying normal mains power.
- Monitor for abnormal heat, flicker, fuse operation, resets, noise, or smell; disconnect power immediately if anything is unexplained.
Troubleshooting by symptom
- Flicker: Check for repeated actions during the detector HIGH interval, threshold or timing jitter, a delay too near the end of the half-cycle, insufficient gate current, low load current, or an incompatible LED load. Also investigate electrical noise.
- No output: Verify zero-cross events and timing first on the low-voltage side. Then, with power disconnected, verify optotriac polarity, LED current, triac pinout, gate resistor design, trigger quadrant requirements, and whether the load reaches the latching current.
- Load stays on or triac misfires: Check whether the optotriac output is being held on too long, wiring is incorrect, dv/dt is excessive, or an inductive load is commutating poorly. Review snubber suitability and leakage.
- Arduino resets: Investigate conducted or radiated transients, poor high-current/logic layout separation, decoupling, low-voltage supply quality, and unsafe USB or enclosure current paths.
- Excessive heat: Recheck RMS current, inrush, thermal interface, heatsink, airflow, ambient temperature, and triac losses. Stop use until thermal design is corrected.
- Fuse opens or audible noise appears: Disconnect and inspect for shorts, unsuitable loads, excessive inrush, or EMI. Do not increase fuse rating without a fault and conductor protection analysis.
When another approach is better
A random-phase optotriac plus triac is appropriate for learning phase-angle control and some compatible lamps. A zero-cross optotriac or solid-state relay is generally better for isolated on/off or burst switching, not arbitrary dimming. Some electronic LED loads work better with a compatible trailing-edge MOSFET/IGBT dimmer. If the goal is an installed household control rather than an experiment, use a certified, enclosed dimmer specified for the exact load class and local electrical requirements.
This architecture follows a project published in June 2019 and mirrored across electronics sites. Its component examples and layout files are not evidence of certification or universal suitability. Treat them as a starting point for understanding the blocks, not a build-ready appliance design.
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Quick Recap
Before applying mains: final checklist
- Confirm the exact voltage, frequency, load type, and maximum steady and surge current.
- Verify every optocoupler, triac, resistor, capacitor, terminal, wire, fuse, and heatsink against its actual datasheet and use conditions.
- Review the isolation barrier, creepage, clearance, enclosure, strain relief, and touch protection for the applicable standards.
- Confirm defined firmware behavior at startup, zero command, reset, and loss of zero-cross detection.
- Use a known compatible resistive load first and a safe, correctly rated measurement method.
- Do not use the device as a permanent mains appliance unless the complete product has been appropriately designed, tested, and certified.
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