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The truth about bathroom sensors (and why they fail so often)

A motion sensor at work.
A motion sensor at work.
Sloan Valve
Phil Edwards is a senior producer for the Vox video team.

This is your bathroom life at pretty much any public place: at the toilet, urinal, faucet, soap dispenser, and air dryer, waving your hands, praying it will work. Sometimes it does. Sometimes you're left supplicating yourself to a motion sensor you don't understand.

Motion sensors like these dominate our bathrooms, and they're only going to become more common. But how do they work? Why do they break? And what's next?

To find out, I got in touch with Peter Jahrling, the director of design engineering at Sloan Valve Company, which makes a lot of the motion sensors and plumbing found in offices, malls, and other places around the world.

Why motion sensors have trouble

How a toilet's motion sensor works

How a toilet's motion sensor works.

Vox/Phil Edwards/Illustrations via Sloan Valve

Every motion sensor is different, but most use the same basic principles. "In most of the products on the marketplace, including ours," Jahrling says, "we all use active infrared sensing." It's cheap and reliable, and that makes it the common choice.

But these sensors don't always work perfectly. Possible trouble includes:

  • Cleanliness. The biggest obstacle is when a motion sensor isn't cleaned or properly maintained. "The biggest problem we have is maintenance," Jahrling says. "They scale up with lime and calcium." Those minerals typically come from hard water and often cause problems in daily use.
  • Distance. Most sensors have a range of 3 to 5 inches, but it varies. You have to be close enough to set it off.
  • Urinating while standing up. If a man uses a toilet as a urinal, the sensor might not catch him (though Jahrling says some systems have fail-safes against this).
  • Battery life. To pick one example, look at the urinal battery requirements for one Sloan product: Four AA batteries will last about three years at 4,000 flushes a month. That's a long time, but it's not forever. Every bathroom requires maintenance.

Your bathroom experience is timed to the millisecond

A public bathroom is precisely timed

A public bathroom is precisely timed.


Ideally, motion sensors recognize you very quickly. "Something in the 100 to 200 milliseconds is an acceptable delay time," Jahrling says. "That's what you experience when you put your hand in a valid detection zone."

If it works, that means a motion sensor is detecting your hands faster than the blink of an eye.

But owners control when the air dryer cuts off, leaving your hands soaking wet

The hand drier is out of your hands.

The hand dryer is out of your hands.


Most motion sensors allow owners to tweak how long their air dryer runs, their faucet flows, or their toilet flushes. For faucets, timing can be anywhere from one second to as long as the user has his hands under the tap.

Practically, it means that you don't get enough soap or your hands end up wet. Ideally, owners use it to help speed up bathroom use in high-traffic areas. For example, Sloan Valve had to increase the speed on its motion sensors in a casino bathroom because there was such high foot traffic near the slot machines. High-traffic optimization is necessary in stadiums, too.

4) The next bathroom innovation will probably come from a prison

Prisons are surprisingly important to plumbing improvements.

Prisons are surprisingly important to plumbing improvements.


The future of bathrooms will come from some industrial lab — but it will also be inspired by work in the prison, including remote control of bathrooms.

"It's been proven out many years in the prison market, in controlling it so they can't do any abuse," Jahrling says. Wardens need to be able to flush and monitor plumbing to control for prisoner abuse or other malfunctions.

The more durable flushing mechanisms required in prisons are also useful in commercial markets. And soon, commercially available apps may know what every toilet, faucet, soap dispenser, and hand dryer is doing.

"It's literally micromanaging the particular plumbing fitting," Jahrling says. "People supplying the public service can anticipate replenishment or repair prior to it going down."

Sanitation keeps motion sensors popular

We've come a long way from early automated plumbing, like a 1960s switch on the bathroom door that flushed every toilet in the facility at once. But the motivating factor is the same — a more sanitary environment.

At least one thing hasn't changed since then. If people see a toilet is unflushed, they're unlikely to flush it themselves. "If they don't get rid of their stuff," Jahrling says, "Nine times out of 10, you're out of there." So now we have a fleet of machines to do it for us.