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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchYes. An AC-input optocoupler such as the H11AA1 can provide an isolated signal near each AC zero crossing. It does not timestamp the exact instant the waveform reaches 0 V: its output changes when the optocoupler and output circuit cross their switching thresholds. That is often adequate for line synchronization and cycle timing, but precision phase control may need calibration or a faster isolated comparator.
Choose a detector, not a zero-crossing triac driver
“Zero-crossing optocoupler” can refer to two different devices. An AC-input optocoupler, such as the H11AA1, reports an approximate crossing through an isolated transistor output. A zero-crossing triac driver, such as the MOC306x, is designed to trigger a power triac near zero voltage; it generally does not provide a logic-level timestamp for a microcontroller. onsemi explains the difference between zero-crossing and random-phase triac drivers in its triac-driver application note.
For arbitrary phase-angle control, use a detector to inform the MCU and a random-phase optotriac to fire the power triac at the chosen delay. A zero-crossing triac driver intentionally waits until the AC voltage is near zero, so it is not suitable for arbitrary firing angles.
How an H11AA1 detects crossings
A zero crossing is where an AC waveform changes polarity through its reference level, normally 0 V. For a sine wave, v(t) = VPK sin(2πft), with VPK = VRMS√2. There are two crossings per line cycle: one in each direction. The interval between adjacent crossings is 10 ms at 50 Hz and about 8.333 ms at 60 Hz.
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The H11AA1 contains two inverse-parallel infrared LEDs and a phototransistor output, so one LED conducts on each AC half-cycle without an external bridge rectifier. Away from zero, LED current turns on the phototransistor and pulls its collector low. Near zero, the LED current falls far enough that the phototransistor stops conducting; a pull-up then raises the collector, creating a pulse around the crossing. See the H11AA1 datasheet for its device specifications.
AC line ── series resistor(s) ──┐
│
H11AA1
│
AC neutral ─────────────────────┘
Low-voltage, isolated side:
VCC ── Rpull-up ──┬── MCU input
│
collector
H11AA1
emitter
│
GNDIn this common arrangement, the collector is low for much of each half-cycle and high in a window around each crossing. An inverter, buffer, or different input arrangement can reverse that polarity, so select the MCU interrupt edge from the signal actually present in your circuit.
Why the pulse is near zero, not exactly at zero
The LED does not switch at the mathematical instant the input voltage is 0 V. The detector changes state when the input current and resulting optical output are no longer sufficient to keep the phototransistor and its load at the chosen logic state. The effective threshold depends on LED behavior, current-transfer ratio (CTR), pull-up resistance, MCU input thresholds, temperature, device variation, noise, and phototransistor saturation and storage time.
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If the complete circuit has an effective turn-off threshold of VT, an approximate timing offset from the ideal crossing for a sinusoid is:
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When VT is small compared with VPK, this is approximately VT / (2πfVPK). This is a design relationship, not a guaranteed H11AA1 delay: determine the effective threshold from the complete circuit under its worst-case operating conditions. Because the threshold is a voltage window, changing line amplitude can change pulse width and transition timing.
Size the mains-side resistor network
The input LEDs need current limiting. For a simple resistor-fed input, an initial estimate for peak current is IF,PK ≈ (VRMS√2 − VF) / RTOTAL, or RTOTAL ≈ (VRMS√2 − VF) / IF,PK. Use the device datasheet and the full operating range to check the design; the H11AA1’s listed forward voltage is not by itself the circuit’s switching threshold.
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For illustration only, assuming a 1 mA peak current and a 1.2 V LED forward voltage, 120 V RMS gives about 170 V peak and roughly 169 kΩ total resistance. At 240 V RMS, about 339 V peak gives roughly 338 kΩ. These examples are not universal recommended values. In a resistor-only approximation, dissipation is P ≈ VRMS² / RTOTAL: about 85 mW for the 120 V example and 170 mW for the 240 V example.
Before choosing parts, evaluate the maximum line voltage, current and resistor heating, working-voltage and surge ratings, fault protection, and the applicable safety requirements. Splitting resistance across series components can distribute both voltage and heat, but the number and ratings must be chosen for the actual circuit and standard. TI’s isolated zero-cross design demonstrates dividing a resistor string for its own comparator circuit; its values should not be copied directly into an H11AA1 design.
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Connect and condition the isolated output
A phototransistor output needs a pull-up to the low-voltage supply and a compatible MCU input. Keep the logic-side ground and supply on the isolated side of the optocoupler. The H11AA1 is not a Schmitt-trigger logic device, and the pull-up affects collector current, transition speed, and potentially saturation delay. Validate the chosen resistance against the datasheet switching characteristics and your MCU’s input thresholds.
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- Use a Schmitt-trigger buffer or comparator with hysteresis if the MCU input transitions are slow or noisy.
- Use filtering only after accounting for its timing shift. A large capacitor at the input can make edges look cleaner while moving them away from the crossing.
- Capture both edge polarities if you need an event on every crossing; the collector pulse has two edges per window.
- Use modest firmware rejection for implausibly close transitions, but do not rely on debounce to correct a poorly conditioned electrical signal.
A standard unidirectional optocoupler connected directly to AC may be reverse-biased on one half-cycle. For full-wave detection, use an inverse-parallel AC-input part or a suitable bridge rectifier so the optocoupler LED sees the correct polarity. Microchip shows an external optocoupler adaptation circuit in its PL460 zero-crossing documentation.
Timestamp crossings in firmware
Use a hardware timer where possible. A basic interrupt can reject spurious edges and record the interval between valid crossings:
void zero_cross_isr(void)
{
uint32_t now = timer_read();
uint32_t interval = now - last_crossing;
if (interval > MIN_VALID_INTERVAL) {
half_cycle_period = interval;
last_crossing = now;
schedule_triac_trigger(now + phase_delay);
}
}MIN_VALID_INTERVAL and phase_delay depend on the timer scale, mains frequency, detector pulse shape, and load. In a real implementation, update the timestamp only after the interval passes the validity test, and account for timer wraparound. Measure whether the chosen edge corresponds to the crossing direction you intend to use. Calibrate against the actual AC waveform if timing matters; nominal LED forward voltage is not enough to predict the edge.
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Is this detector accurate enough?
An H11AA1-style circuit is a practical, low-component-count choice when approximate crossing timing is acceptable—for example, cycle counting, relay timing, and some moderate-precision phase control on a reasonably sinusoidal 50/60 Hz supply. Its threshold window, CTR variation, output loading, and temperature response make it unsuitable as an automatically precise 0 V timestamp.
| Approach | Useful when | Main trade-off |
|---|---|---|
| H11AA1-class AC-input phototransistor | Isolation and simple, approximate line timing are priorities. | Threshold timing and output edge vary with the complete circuit. |
| Bridge plus unidirectional optocoupler | You have a standard optocoupler and need both half-cycles represented. | Requires bridge adaptation; phototransistor timing limitations remain. |
| Logic-output optocoupler, such as an H11L1-class device | A cleaner logic transition is useful. | Input-current and protection network still need design; verify current availability and suitability. |
| Isolated comparator | Defined threshold, hysteresis, and fast output timing matter. | Requires careful front-end, resistor, layout, and isolation design. TI reports no more than 220 ns measured transition delay for its demonstrated AMC23C10 circuit, not for optocouplers generally. |
| Transformer-based detector | A low-voltage AC waveform is already available and phase fidelity is valuable. | Adds transformer size, cost, and implementation constraints. |
| MCU-integrated zero-cross peripheral | The selected MCU supports the required input and system architecture. | Still requires the specified external adaptation/current-limiting network; the input reference need not equal an ideal 0 V threshold. |
For comparator circuits, hysteresis helps avoid repeated transitions when noise is present near the crossing; onsemi discusses separate switching thresholds in its zero-cross comparator application note. Microchip’s ZCD documentation describes an integrated-peripheral approach and its required input adaptation: Microchip ZCD peripheral.
Troubleshoot common failures
- No pulses: Check the mains-side resistor path, LED current, output pull-up, transistor pinout, and MCU input configuration. Verify the low-voltage side has a defined supply and ground.
- Pulses on only one half-cycle: Confirm the device is AC-input rated or that a bridge is correctly oriented. A single LED optocoupler used directly across AC can be reverse-biased on alternate half-cycles.
- Multiple interrupts near a crossing: Look for noise, slow edges, and inadequate logic hysteresis. Improve the signal conditioning and use a suitable validity interval in firmware.
- Excessive jitter or delayed edges: Check line-amplitude variation, waveform distortion, pull-up value, deep phototransistor saturation, and any RC filter. Compare the conditioned output with the AC waveform using safe measurement equipment.
- Works at nominal voltage but fails at low line: Recheck minimum LED current and worst-case CTR against the required collector current and logic threshold.
- Output never reaches logic high: Check the pull-up supply, transistor wiring, pull-up resistance, and whether the phototransistor is remaining on or saturated.
- Triac fires at the wrong phase: Confirm which edge is being timestamped, whether the optotriac is random-phase or zero-crossing, and how the measured detector offset relates to the intended firing delay.
Mains safety is part of the circuit design
Mains voltage can cause fatal shock, fire, or equipment damage. An optocoupler’s isolation rating alone does not make a finished board safe. The entire design must account for resistor voltage and surge ratings, input protection, fusing where required, PCB creepage and clearance, enclosure, connectors, and applicable standards. Do not treat a hobby circuit as suitable for certified mains equipment without the required engineering and compliance work.
Never attach the ground clip of a non-isolated oscilloscope directly to a mains-referenced node: it can create a dangerous short to earth. Probe the isolated low-voltage side, or use properly rated differential measurement equipment and appropriate laboratory safety procedures.
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