AT42QT1070 Random Triggering

[geoffpeters]

I have been testing an AT42QT1070 breakout for a few days. I have a 4" diameter by 3mm thick acrylic circle with a piece of single sided PCB, about 0.5" x 0.5", attached to the center on the underside. There is a solid conductor wire soldered to the PCB. It extends roughly perpendicular to the PCB. It is about 4" long and connects directly the the breakout board. When I approach or touch near the center of the acrylic disk the circuit triggers normally. However, sometimes the device triggers on its own, sometimes once and sometimes many times in succession. I am nowhere near the circuit when this occurs.

Does anyone have any idea of how to fix this or what might be the cause?

[adafruit_support_mike]

It's probably static charge generated by ambient temperature changes and air motion.

Acrylic is pretty far toward the negative end of the triboelectric series (http://www.siliconfareast.com/tribo_series.htm) so it has a strong tendency to pull electrons away from other materials. You can probably generate a charge on its surface by putting it in the same room with a cat, or showing it a picture of a rabbit.

Air is at the far positive end of the triboelectric series, so it has a strong tendency to give up electrons to other materials. Blowing dry air across a sheet of acrylic will eventually generate a significant charge.

Try rubbing the surface of the disc with one of those anti-static sheets you throw in the clothes dryer.

[geoffpeters]

Hi Mike,

Thanks for your reply. I'll do a bit of experimenting.

Perhaps I could use an anti-static spray or a ground ring of some sort.

[geoffpeters]

I have tried several ways to try and force the false triggering. I have placed the sensor assembly close to a halogen lamp for a few minutes to warm it up. It does not trigger. I have blown across it and aimed a heat gun on low temp setting a few inches from the acrylic. Still it does not trigger falsely. Just now as I am typing it is triggering in a seemingly random fashion. It has triggered about 20 times at different intervals in the last 30 seconds. Now it has stopped. I have tried 2 different power supplies.

Has anyone else had this problem?

[geoffpeters]

More info.

I attached a solid conductor wire to a different channel on the board. It is insulated, about 5" long, and has no electrode attached. It exhibits the same false/random triggering.

Any clues?

[adafruit_support_mike]

Hmm.. that sounds like antenna effects.

See if you can find a chunk of scrap coaxial video cable. Connect the center conductor to one of the input pins and the outer shield to GND. That should reduce any EMF problems in the area.

[geoffpeters]

I am testing this in a residential area not in a city and the background RF level is very low here.

I'll give it a try; it was one the things on my list.

[adafruit_support_mike]

If you have wall power and/or a flourescent light in the room, you have plenty of EMF.

My nearest neighbor is a quarter mile away, and I have to take special measures to avoid EMF when I work on high impedance circuits.