Door Bomb and Safety Catches

Johnny leaves the airport by taxi, ending up in a disreputable part of town. During his ride we see another video phone call with a different interface, and the first brief appearance of some high tech binoculars. I’ll return to these later, for the moment skipping ahead to the last of the relatively simple and single-use physical gadgets.

Johnny finds the people he is supposed to meet in a deserted building but, as events are not proceeding as planned, he attaches another black box with glowing red status light to the outside of the door as he enters. Although it looks like the motion detector we saw earlier, this is a bomb.


This is indeed a very bad neighbourhood of Newark. Inside are the same Yakuza from Beijing, who plan to remove Johnny’s head. There is a brief fight, which ends when Johnny uses his watch to detonate the bomb. It isn’t clear whether he pushes or rotates some control, but it is a single quick action.


This demonstrates an interesting difference between interface design for the physical world and for software systems. Inside a computer, actions are just flipping bits in storage and thus easy to undo. Even supposedly destructive actions such as erasing files can often be reversed. In the real world, the effects of, for example, explosions tend to be much more permanent.

We generally don’t want destructive actions to be too easy to perform, from guns and other things that go boom to formatting computer disks.

A widely used solution in the real world is the safety catch, as with guns, or arming switch, seen in countless thriller films with nuclear weapons. Another example are the two-hand safety switches used in high voltage electrical distribution panels. Activation of these requires two individual actions, separated in time and at least a short distance in space. Some systems, both real and in film, go even further and have covers on the arming switches, so even just preparing for activation requires two separate physical actions.

While the bomb is on his belt, Johnny doesn’t have to worry about accidentally pressing the “explode” button on his watch because the bomb is not active. Only after he has armed it and placed on the door can the watch activate the bomb, so he can take his time and verify whether or not it is necessary before doing so. And when it is active, he can do so very quickly even though he is in the middle of a fight.

But safety catches and arming switches introduce modes to an interaction, which have a bad reputation in interface design. Had the watch-bomb designers followed most conventional GUI design guidelines, there would be no arming switch on the bomb. Instead the watch would have popped up a “Do you really want to explode the bomb (Y/N)?” dialog, possibly with a short delay to ensure Johnny thought about his decision before answering. He would have been decapitated.

Compare to LoTek

Later on in the film we see an example of a poorly designed system without a safety catch. The LoTeks in their bridge home have a defensive “bug dropper”, so called because it drops ancient Volkswagens from a great height.


The bug dropper can be activated by pushing just a single handle. Because there is no safety switch, a guard accidentally drops a flaming VW Beetle onto the lead characters, nearly killing them.


From the description above it would seem that safety catches are the obvious solution. But of course it’s more complicated than that. Consider what would have happened if Johnny had met friends instead of enemies and settled down for a conversation. Thirty minutes later they’ve agreed on another meeting, and Johnny taps his watch to bring up the reminders app. Oops!

Should the bomb have disarmed itself after a given time period? If it did, how would Johnny be notified of this?

Most of us do not design interfaces for lethal hardware and life or death situations. There are however an increasing number of drones and other physical devices which are now remotely controlled from phone or tablet apps rather than dedicated hardware controllers as in the past. The “Internet of Things” will bring even more real world actions under computer interface control. In the future, we will most likely see more of these safety catches and arming switches in computer interfaces, and we need to figure out how to use them properly.

4 thoughts on “Door Bomb and Safety Catches

  1. Got to be careful that you don’t introduce another mode here for the user to keep track of. If the Johnny takes the watch off to disarm the bomb, does it re-arm when he puts the watch back on?

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