A member had a product out in the field, and its capacitive touch buttons started pressing themselves. Ghost presses, firing at random and switching equipment on with nobody near the panel.
And there was no consistency to it. A unit would run fine for a day or a week, then start triggering itself at the wrong times, and he couldn't pin down what set it off or when.
That's about the worst kind of failure there is: intermittent, and never there when you're watching for it.
One pattern did show up: the failures tracked the environment. When temperature and moisture changed, the ghost presses came back.
He'd built the product around a super-cheap, off-the-shelf touch-sensor module from Amazon.
So he shared the design and photos in my Hardware Academy, and one of the engineers read the module's datasheet line by line against what was actually on the board.
The module has a spot for a small capacitor that sets the touch sensitivity, and the datasheet is clear about it.
On the modules he'd bought, those pads were empty. The maker had simply left the part off.
So every unit shipped at maximum sensitivity. The switches were doing exactly what they were built to do, just tuned so hot that a film of moisture on the panel looked like a finger.
You've felt this effect yourself if you've ever tried to use a phone with wet hands.
Nothing in his own design put that trap there. It was installed at the module factory, and it only sprang when the environment shifted.
The fix the engineer prescribed: solder in the capacitor the maker skipped. A part that costs a couple of cents.
Sometimes the biggest problems come down to the simplest, cheapest part in the product. In this case, one that was never even on the board.
So there are two lessons here.
First, a module is a finished design, but it's finished to the maker's priorities. Whatever they left off or left at a default becomes your product's behavior, and a maker selling dirt-cheap modules will skip any part that saves them a penny.
Second, when a product works on the bench but misbehaves in the field, look for an environmental factor, like temperature or humidity, pushing on a setting you never consciously chose.
Inside the Hardware Academy, experienced engineers catch what a module maker left off, before your customers find it for you.
Talk soon,
John Teel
Predictable Designs
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