A lot of automatic sinks and such use an infrared proximity detector. Basically it shoots out an infrared beam and waits for something to get close enough to bounce it back. But that means there’s a relatively small threshold where your hands can be to set it off. If they’re in the wrong spot, it may not bounce back properly or even be in the beam.
Ever notice how there’s a sweet spot when using a sink?
Automatic toilets are similar and were the BANE of my childhood. Being small and afraid of loud noises… I was always setting off the toilets by wiggling, thus setting off the proximity sensors.
That’s reflective sensors for you, they don’t work much better in industrial settings where the specific setup is much better controlled. It’s just an inherently unreliable type of sensor. Cheap and simple to use though, that’s why you find it in so many places even though it’s just bad at it’s job.
What others have said about sensors as well, then add that sometimes the spot that activates the sensors is directly where the sink is. So basically the sink set the faucet off all the time. Some of them have adjusters to compensate, but sometimes the sweet spot is in a place you would never put your hands to wash them.
Also a big thing is how they are cleaned. If there’s a smudge or damage from using a harsh cleaner on the sensors screen (it’s eye) it’ll throw the sweet spot off by a lot.
Also they are battery powered. So the batteries could just be weak, or the holders could be worn out from changing batteries often.
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