Eli5 why does the flush sensor on a touchless toilet go off and splash my butt at the slightest movement while I’m sitting down, but the touchless sensor on a sink won’t turn on unless I’m in JUST the right spot?

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Eli5 why does the flush sensor on a touchless toilet go off and splash my butt at the slightest movement while I’m sitting down, but the touchless sensor on a sink won’t turn on unless I’m in JUST the right spot?

In: Engineering
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The flush sensor on a toilet faces toward the front of the stall, and the sensor spreads out in a V. It expects to sense 3-4 feet away. It works when it senses a bounce back (which could be anywhere bouncing around the walls as you move).

The flush sensor on a sink is facing down into a curved reflective surface which is constantly bouncing back the signal. It works when it _doesn’t_ sense a bounce back, so you have to block the entire sensor.

The sensor is sending out a signal and track the time until it comes back. With that, it can tell if the signal bounced off of you, or the bathroom door (This is essentially how radars and sonar work). So the toilet uses this signal to know when the user sits down (start sending out the signal) and gets up (flushes the toilet).

However, if your bending over a bit far for whatever reason, that signals can bounce on your back and bounce away from the receptor on the toilet, hitting the wall behind you. The toilet is then like “oh shit (pun), this is taking a while, so the dude might have just left and we didn’t catch it.” So then it flushes, even when you’re sitting right there.

Engineer here, but I do not work in the industry that makes these. But my buddy owns a plumbing company and I asked him this one day over dinner.

He said mainly its plumbers that don’t want to take the time or read the directions how to adjust them properly. They usually just install and go, or leave it to the apprentices.