5-pad capacitive touch sensor unreasonably sensitive

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alexta69
 
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5-pad capacitive touch sensor unreasonably sensitive

Post by alexta69 »

Hello!

I've purchased the 5-pad capacitive touch sensor (https://www.adafruit.com/product/1362) and connected it to a Raspberry Pi. I wanted to connect pads via wires to the input contacts, but even with just the wire connected, touch is constantly detected. I'm attaching a photo to show it. The effect is not constant - sometimes it happens, sometimes it doesn't. Sometimes it flickers. If I tape some aluminum foil to the end of the wire, the effect is even stronger -- it almost never goes off at all.

Have I received a faulty unit? Or am I doing something wrong?

Thanks for your help!
Alex

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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by adafruit_support_mike »

Quick question: is the wire already connected to the breakout's pin when you connect power to the board?

Capacitive sensors take a baseline measurement when they boot up, and measure all further input relative to that. A couple of other people have seen similar behavior when they connected the sense wire after the board powered up.

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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by alexta69 »

Thanks for the quick reply. I don't remember now whether I tried powering up with the wire alone, I'll try it later today and update. But I am sure that I tried powering up with larger things connected to the wire (I started with tin can bottoms soldered to the end of it), and it exhibited this behavior then. Is there an upper limit to the amount of stuff, or mass of metal or whatever, that can be connected to this sensor before the capacitance becomes too big and it stays "always-on" (like above) even directly after boot-up?

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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by alexta69 »

Hi again,

I checked the behavior as you suggested, and indeed, it works fine after power-up with the wire.

I then taped a piece of aluminum foil approx. 10x10 cm (4x4 in) to the end of the wire and repeated the tests. Here the results depended on the pin I connected it to. Pin 4 worked well, 3 almost always went directly to "on" after power-on and stayed there, and the others usually also went directly to "on", sometimes flickered, sometimes worked as expected.

Is this behavior correct? Am I connecting too large a surface? Is there a known limit to the size of the surface? Or is the component problematic? Different behavior on different pins feels wrong...

Thanks again,
Alex

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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by adafruit_support_mike »

The sensors are designed for pads about the size of a finger pad (maybe 1 square centimeter), so 10cm x 10cm pads would make the system about a hundred times more sensitive to everything in the environment.

If you're using the sensor in a room with flourescent lights, I'd bet a quarter that the sensor is picking up a lot of radiated EM noise from them.

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alexta69
 
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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by alexta69 »

Got it. So if I want a larger touch surface, perhaps I can extend the wire and make a spiral out of it to cover the surface -- should work if I space it at 1 cm or so, right? Or is the long wire expected to create the same problem?

Is it better to use thin wire so that there's less capacitance in it?

Maybe it will also be less susceptible to interference from the fluorescent lights (which I can't avoid..) Do you figure the spacing of the spiral matters in regards to picking up interference? (Wavelength-wise and so on. Unfortunately I'm a bit lost in this field.)

Thanks a lot again for your help.

Alex

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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by adafruit_support_mike »

The connection between size and sensitivity is unavoidable.

Taking a step back, what do you want a sensor that large to do?

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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by alexta69 »

It's supposed to be a touch button, but the project itself is pretty large (the size of a person or so) so I wanted the button to be in proportion, something you touch casually with your hand, and not a finger-sized pad you have to look for and be precise with...

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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by adafruit_support_mike »

I see.

You can get the effect you want with a bit of misdirection.. go ahead and make the button large, but use a smaller pad underneath it. If the button is 10cm square, try making the pad beneath it 5cm square, centered to the button. Any whole-hand or multi-finger touch more or less centered on the button should be detected.

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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by alexta69 »

Got you. I will try this.
Thanks again for all your help!

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Re: 5-pad capacitive touch sensor unreasonably sensitive

Post by alexta69 »

Hey. I tried again with a small disk of aluminum foil 1 cm in diameter, and also with a little steel mesh, as in the photo below. I always powered on the board only after connecting the thing to the board. It still behaves very erratically, sometimes working, sometimes flickering spontaneously, sometimes staying on for a while.

I'm really starting to think I got a faulty board, because it can't be so unreliable, can it?

Thanks,
Alex

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