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Capacitive touch sensor touching itself
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Capacitive touch sensor touching itself

by samuelguyer on Sat Dec 06, 2014 10:42 pm

OK, yes, sort of a joke subject, but...

I have a project that uses an Adafruit capacitive touch sensor. It mostly works great, but I've noticed flaky behavior with the sensor. Specifically, it starts "self activating" without any touch. The tiny red LED starts lighting up sporadically, and my program responds to the events as if the button was being pushed.

I have a feeling the problem has something to do with the way sensor is connected to the other parts. The project consists of the sensor, a neopixel strip, and an Arduino Micro. Each of these three components has direct 5V and ground wires to a battery. The sensor and neopixel strip each have a signal wire to one of the Arduino's digital pins. There are no other components -- no resistors, no capacitors, no diodes. I suspect that something weird is happening with the shared power source, but I'm not an electrical engineer, so I don't know how to check.

Should I be powering the capacitive touch sensor from the Arduino 5V source instead of directly from battery?

Any help would be greatly appreciated! I can provide more details if necessary.

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Re: Capacitive touch sensor touching itself

by adafruit_support_bill on Sun Dec 07, 2014 7:29 am

What kind of battery are you using? A photo or diagram of your complete circuit would be helpful.

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Re: Capacitive touch sensor touching itself

by samuelguyer on Sun Dec 07, 2014 9:43 am

I cannibalized one of those phone recharging battery packs -- they're cheap, and they include the circuity for charging and stepping the voltage up to 5V. It seems to work pretty reliably.

I'm including three photos of the project here. It is an electronic hourglass. You program it using the capacitive touch sensor, and then turn it over to start the timer (it includes an accelerometer to detect the orientation). Here is the back of the device showing all the circuitry:

IMG_8044.JPG
IMG_8044.JPG (665.89 KiB) Viewed 662 times


Here is a close-up of the Arduino board. The two connectors are for the neopixel strip (top connector) and capacitive touch sensor (bottom connector). The accelerometer is hidden under the connectors:

IMG_8045.JPG
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Here is a view from the top showing the touch pad. The neopixel strip runs under that piece of frosted plexiglass:

IMG_8046.JPG
IMG_8046.JPG (410.14 KiB) Viewed 662 times

samuelguyer
 
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Re: Capacitive touch sensor touching itself

by samuelguyer on Sun Dec 07, 2014 9:44 am

It looks like the picture were rotated 90 degrees, so the "top" and "bottom" references in my text won't make sense.

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Re: Capacitive touch sensor touching itself

by adafruit_support_bill on Sun Dec 07, 2014 12:15 pm

Nice looking project! I don't see anything obviously wrong there. The touch sensor is reasonably well isolated from other active circuitry and the power arrangement looks fine. One thing that you can try is to move the sensor ground so that it grounds directly to the Micro instead of via the ground rail on the proto-board.

Another thing to be aware of is that the sensor self-calibrates at power-up. If your hand is near the sensor as it is powering up, it can affect the calibration and act erratically

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Re: Capacitive touch sensor touching itself

by samuelguyer on Sun Dec 07, 2014 3:42 pm

That worked! Thank you! All I did was move the sensor's ground to the micro ground instead of the ground rail.

I've built of a few of these things as gifts for people, and there's another one that has a cap sensor problem. This problem is more clear-cut, but seems less fixable. There seems to be a short-circuit in the sensor board. My multi-meter shows zero resistance between vin and ground. I don't see an obvious soldering problem on the board. Could it be defective?

Here's a photo of the hourglass in action...

IMG_8051.JPG
IMG_8051.JPG (489.66 KiB) Viewed 626 times

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Re: Capacitive touch sensor touching itself

by adafruit_support_bill on Sun Dec 07, 2014 4:06 pm

If you keep the meter on it for a while, does the resistance start increasing? The apparent zero resistance could be just the bypass caps charging up. If it were truly a short, you would have overheating components somewhere and nothing would power-up.

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Re: Capacitive touch sensor touching itself

by samuelguyer on Sun Dec 07, 2014 4:48 pm

It stays zero and gets very hot. And yes, the rest of the system will not power up.

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Re: Capacitive touch sensor touching itself

by adafruit_support_bill on Sun Dec 07, 2014 6:31 pm

OK. That does indicate a real short. Please post some photos of the soldering and connections to that sensor.

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Re: Capacitive touch sensor touching itself

by samuelguyer on Sun Dec 07, 2014 11:02 pm

Here are pictures of the connector soldered to the cap sense breakout board. Even completely disconnected from anything else, the board appears to have a short.

IMG_8052.JPG
IMG_8052.JPG (200.79 KiB) Viewed 568 times

IMG_8053.JPG
IMG_8053.JPG (204.57 KiB) Viewed 568 times

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Re: Capacitive touch sensor touching itself

by samuelguyer on Sun Dec 07, 2014 11:04 pm

At one point I was worried that I had accidentally mixed up the vin and ground wires on the connector. If that happened, could it fry the board?

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Re: Capacitive touch sensor touching itself

by adafruit_support_bill on Mon Dec 08, 2014 8:27 am

If you contact support@adafruit.com with a link to this thread we can send a replacement.

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Re: Capacitive touch sensor touching itself

by samuelguyer on Mon Dec 08, 2014 8:55 am

OK. Thanks for all your help -- I really appreciate it.

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Please be positive and constructive with your questions and comments.