Mystery sink

August 16, 2007 10:09 AM

Why would a sink in a public bathroom which had an automatic water sensor suddenly turn on even though there was no one nearby?

So my friend was in the bathroom in the university, it is a modern one and so there she was washing her hands and as she goes to dry them, a sink some distance away suddenly turned on. Weird! She asked me why this would happen. I thought maybe because a fly flew by or something, but this isn't really such a great answer. Plus, there aren't many flies in the 11th floor public bathroom. Maybe some of you have an interesting theory?

posted by mateuslee to grab bag (9 answers total)

possibly light reflecting causing the sensor to turn on - just a guess...
posted by doorsfan at 10:15 AM on August 16, 2007

I know that when I was doing my Senior Internship the building I worked in was wicked old, but the bathrooms were all brand-spankin' new remodels. Because the building was so old it was apparently still "settling" and if someone was running down a hallway over the bathrooms the vibrations, despite not being felt by humans, were great enough to set off the brand new faucets. It made me very nervous every time it happened while I was in the bathroom by myself.
posted by banannafish at 10:23 AM on August 16, 2007

It's a water-saving device, don'cha know.

Seriously, it's likely an errant reflection. I had "ghost" cars all the time when setting up car washes, which use the same sort of eyes.
posted by notsnot at 10:28 AM on August 16, 2007

Banannafish, in your cases all the faucets turned on, right? But this is just one... I suppose the reflection theory is the best one so far...
posted by mateuslee at 10:38 AM on August 16, 2007

Most faucet controllers send out infrared light that gets reflected back. It can reflect off jewelry, a belt buckle, a tooth surface, or the metal snap on a purse, even halfway across the room, as long as the reflection returns to the proper spot.

If the controller is capacitive (i.e., senses the absorption an electric field by a nearby body), there will be spots further away that resonate. It's like a "whispering gallery."
posted by KRS at 10:39 AM on August 16, 2007

at my office a couple of the bathroom sinks have slow drips, so the water slides down the faucet until it gets in front of the sensor, and the water turns on, which causes another slow drip, ad infinitum.
posted by muddylemon at 12:02 PM on August 16, 2007

Because these things don't work very well.

Or Moaning Myrtle.
posted by sageleaf at 1:15 PM on August 16, 2007

The automatic plumbing controllers are fairly sophisticated electronic devices. They each have an 8-bit microcomputer inside of them. Most use an infra-red transmitter and receiver that work like your TV remote control. There is an infra-red LED that emits light and there is an infra-red detector that looks for the reflection. Most have lenses that focus the light so that the detector is most sensitive to a narrow range of distances. When your hand or body is within the designed range, enough infra-red light will be reflected to trigger the device.

When the device is first powered up, it can spend 5 or 10 minutes training itself to the background ambient light and patterns, like a shiny wall or stainless steel stall door, to eliminate false triggering. The accuracy of the detection algorithm will vary with the complexity of the software operating the microcomputer. The software has timers to determine how frequently it turns on the LEDs and takes a measurement, how long it needs to see an object before it determines it is a real trigger rather than a passerby, how long to ignore a momentary reflection, the frequency of the pulses it expects to see, how bright of a reflection is required, the maximum time it will leave the water on to prevent vandals from rigging an object in an attempt to keep the water continuously on. They will also automatically trigger at some predetermined interval if there has been no use for a long time, such as one flush per day to keep the bowl or sink clean. All of this is done with a single chip microcomputer that costs maybe 10 cents and 4K to 16K of software.

The light signals they are trying to detect are very faint and it is no easy task to design a system that is accurate, low-power, and cheap. The battery operated types can run for several years on a set of 4 AA batteries. They do this by shutting down the microprocessor 99% of the time, only turning it and the LEDs on for a millisecond at a time to take a measurement a few times per second. The electrical solenoids that control the water are assisted by the water pressure and a system of springs and diaphragms that actually do most of the work of moving the water valve. This reduces battery usage.

Since these are electronic devices, they are subject to failure the same as any other you are familiar with. Some are battery operated so the batteries could be failing. The lenses may have been bent out of adjustment during installation. The photo diode detector could be failing. So out of hundreds of installations, it would not be surprising to have one that that is not operating properly or turning on unexpectedly.
posted by JackFlash at 2:15 PM on August 16, 2007 [1 favorite]

Sensors do go bad.
posted by jeffamaphone at 4:41 PM on August 16, 2007