Last night's LOST season finale made mention of a "Dead Man's Trigger," which is another term for a Dead Man's Switch. Although these devices are well known to some geeks, I thought I'd write a little explainer to introduce the notion more broadly. (Warning: very minor LOST spoilers follow.) While the LOST switch was in fact a "fail-deadly" (the opposite of a fail-safe), the concept is similar. In fact, this isn't the first time we've seen a Dead Man's Switch on LOST: Season 2 was all about Desmond's switch.
The Dead Man's Switch is generally a fail-safe device intended to take action if a human fails to routinely activate it -- in other words, if the human dies, the switch goes off, and something happens.
Historically, a Dead Man's Switch was installed in potentially dangerous machinery like locomotives, streetcars, and subways. The switch itself could take many forms, though typically it was a handle of some sort that needed to be operated by the user. By forcing the human operator of these machines to occasionally (or even continuously) activate the switch, you could (theoretically) ensure that the machine would shut down if the operator was dead, incapacitated, or missing. Common mechanical Dead Man's Switches exist in New York subways and even lawnmowers and tractors. Here's a snippet from Wikipedia:
In many modern New York Subway trains, for example, the dead man's switch is incorporated into the train's speed control. On the R142A the train operator must continually hold the lever in place. This was depicted in the movie and book The Taking of Pelham 123, in which a group of men hijack a New York City subway train for ransom, but because of the Dead-man's feature, cannot escape while the train is moving.
Every lawn mower sold in the US since 1982 has an "operator-presence" device, which by law must stop the blades within 3 seconds after the user lets go of the controls.
Many modern systems could benefit from a Dead Man's Switch. For example, typical automative Cruise Control systems don't have such a switch, which could allow a vehicle to effectively drive itself after the user falls asleep. Of course, this doesn't last long, as the vehicle hits something sooner or later -- but wouldn't a better failure mode be to slow down rather than keep going?
In high tech, ubergeeks have been using Dead Man's Switches to protect their data or implement disaster-containment protocols in case something goes terribly wrong. If you're a political dissident or involved in something else potentially dicey, a Dead Man's Switch can be implemented as software, a sort of "nuclear option" that will zap your data -- or contact trusted compatriots -- if you fail to activate it. Slashdot held a discussion entitled What Does Your Dead Man's Switch Do? in 2007, leading to several interesting stories (and tons of off-topic discussion). Here's a good example:
I often take part in political protests, and have on occasion been arrested and held for days.
So, I put together a quick routine using perl and chron that dispatches email to my workplace, the local legal rep contact, and some friends. The later includes directions to a hidden key and asks them to feed my cat until they hear from me. I only enable the system when I'm expecting a significant risk of arrest. Once it's started, if I don't either log into the machine or send myself an email containing a specific string once every 24 hours, the alarm goes off.
The Dead Man's Switch has also appeared in literature; one popular example I can recall is the nuclear trigger on Raven's motorcycle in Neal Stephenson's Snow Crash. But I'm sure there are more examples out there. Have you come across a Dead Man's Switch, in life or in fiction? Share in the comments!