Why do smoke detectors go off when lightning strikes?

Just experienced this, I saw lightning outside my window (not hitting our building or anywhere close to it), and immediately after the smoke detector went off for a short while. Can anyone explain what caused this?

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optical or ionization? –  Federico Russo Jun 18 '11 at 13:18
I've had this same problem 3x in the past year. A nearby lightning strike triggers the interconnect smoke detectors in my house to sound. If it's truely an ionization issue, wouldn't my neighbors (who's houses are just feet from mine) experience the same issue? They have not. After the second time this happened, I even switched out all of my alarms for an equivalent product, but a different brand. My neighbors have the same detectors that we do. –  user13702 Oct 2 '12 at 13:42

5 Answers

Lightning is a nasty thing. Powerful. Very high current at very short rise time. This causes an strong EMP (ElectroMagnetic Pulse) which will be picked up by anything conducting. A 1m free-hanging wire may create a voltage peak between its ends. Even short connections may see spikes. Decoupling doesn't always work as the EMP can enter an IC directly; it doesn't have to come by the (power) wires.
So no wonder some products experience a temporary malfunction during a lightning bolt, and high impedance mean more sensitive. If the disturbance remains within the device's voltage range it may behave wrongly without suffering damage. Higher voltage spikes may destroy (parts of) the device.

I heard the story of a Dutch family where lightning had struck in the backyard. Every electronic product in the house was fried, from TV and PC to cameras and mobile phones. Se non è vero...

And David with his smoke alarm network/antenna, well... :-)

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Present! Lightning struck on a tree in the backyard of a house near mine (100 meters away). Many victims: two hiperlan radio routers, a Cisco 2950, my beloved Powerbook aluminium (that was POWERED OFF AND DISCONNECTED FROM ANY CABLE) and two TV tuners... :-( –  Axeman Oct 2 '12 at 14:29
At my first job we went out to diagnose a small business's hardware after a lightning strike about 20 feet away from the building. Just about everything in the building (fax machines, computers, copiers, printers) was fried. Every network card in the PCs had exploded chips on them. –  darron Mar 7 at 21:49

Just to add what the other folks have said... The fire alarms in my house are all inter-connected. When one goes off, they all go off. There are wires inside the walls/ceilings that connect them all. Those same wires, because they are long and unshielded, are excellent antennas and would easily pick up the EMI from a lightning strike. Also, the signal on these wires is very simple and the electrical noise generated could easily fool the alarm into thinking that some other alarm went off and so it should too.

What the other guys said could also be true (except the part about CO2 setting off an ionizing detector, it is actually a smoke particle that does it), but if the alarms are interconnected then that would be the weakest link. It's much easier for EMI to get into a 50 foot wire than something that is about an inch long.

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If you look inside of a smoke detector, you'll realize that those things aren't designed for EMI resiliency. They feature the lamest single-layer board design possible and are pretty much naked and open to anything coming down the mains wires or the interconnect signalling wire. I have one where static electricity discharges on a carpet on the opposite side of a subfloor will trigger a beep if the wire runs too close to the subfloor. Someone must be hauling home a fortune in profits on those things - they are as cheaply made as possible. –  Kuba Ober Mar 7 at 22:16

I think ionization and EMP glitches are missing the point. I find ionization in particular very hard to believe. How is lightning 100s of meters away going to ionize the air in my smoke detectors pretty much at the same instance as the lightning? It's not. I don't believe the EMP theory either. The pickups are sensitive and high impedance, but also shielded from external capacitive pickup. If not, ordinary power line hum and nearby static discharges would set them off, but they don't.

What is really going on is that the power got glitched. A lightning strike makes a mess of the power line for a few 10s of milliseconds. Most smoke detectors, including all the ones in my house, sound off for a short time whenever the power goes out. They are fairly sensitive to this, more so than most ordinary appliances. You may notice a small flicker in the lights or a glitch on the TV at the same time (although lightning causes TV and radio glitches by other means too). When we have a pure power glitch not caused by lightning, it is always the smoke detectors that exhibit the symptom first. Most appliances can take a cycle or two of missing power, but the smoke detectors seem to be the most sensitive. I don't know if this is deliberate or just a byproduct of their sensitive detection circuitry.

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Have you read Axeman's comment to my answer? (Posted after you answered) I can only explain that with the EMP. –  stevenvh Oct 2 '12 at 14:45
@Steven: The unconnected laptop is a mystery, but much of the other damage can be explained by ground bounce. If the lightning temporarily raises the house ground by a kV but not the power line, all kinds of bad stuff can happen. –  Olin Lathrop Oct 2 '12 at 15:12

I'm going to assume an ionisation detector, as those are the most common on the market today. Inside a smoke alarm there is a controller IC. This IC registers the current ionised by the americium-241, typically on the order of 100pA. If smoke ($CO_2$) enters the chamber the current stops, triggering the alarm. Bill Hammack explains it all.

Now what happens when say a nearby lightning strike introduces a high power RF blast to all electronics? That current goes all over the place. It probably swings between several microamps both positive and negative, for a brief period of time. The smoke alarm IC never expected to see this. It doesn't want to see this, as it isn't designed to handle it. So the internal current comparator probably goes a bit crazy and latches the alarm logic which resets every 15 seconds or so. This causes the alarm to trigger.

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you didn't have to explain how the ionization chamber works, because it doesn't seem to play a role; as I see it the optical variant would have the same problem. Right? –  stevenvh Jun 18 '11 at 14:29
@stevenh Not necessarily so. An optical detector would work on detecting a voltage and wouldn't require a high impedance input which is sensitive to RF noise. It would be puzzling if the detector was optical and it went off. –  Thomas O Jun 18 '11 at 14:46
at first I was focusing on the sensor, but the EMP from the lightning bolt may cause problems anywhere in a circuit (though high impedance parts are surely more vulnerable). –  stevenvh Jun 18 '11 at 16:57
Why the downvote? Is there something wrong with my answer? –  Thomas O Jun 20 '11 at 6:41
@Whoever downvoted - Downvoting without explaining why you do is a bit cowardly. –  stevenvh Jun 20 '11 at 7:33

Smoke detectors are not the only devices that can misbehave when lightning strikes. Like every spark a lightning is a strong transmitter on a broad range of frequencies that can induce wrong signals in electronic devices.

Because smoke detectors are designed to rather go off spuriously than stay silent wrongly, your smoke detector was triggered by the lightning's signal.

Often you can hear and see those signals on radios, TVs, oscilloscopes, etc., too.

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You don't explain how it triggers the smoke detector, and I think that's what @Freed wants to know. –  stevenvh Jun 18 '11 at 13:39
Then I don't get the question. It triggers it in the same way as ANY noise signal would trigger any (motion etc.) detector or even a simple electronic switch... rise the voltage over some comparator/schmitt trigger/input buffer level - bam! - triggered. Nothing fancy (lightning specific) is going on. –  AndreKR Jun 18 '11 at 14:17