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AT42QT1070 noisy buttons
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AT42QT1070 noisy buttons

by Chiefston on Tue Mar 25, 2014 2:09 am

Hello all. I am working on building an aquarium controller with an Arduino Mega, and using the AT42QT1070 breakout board for the buttons. Everything was fine when I had the project spread out on a breadboard. But after moving everything to perf board and placing it in my project enclosure, I am having a lot of issues with unwanted button presses. Is there some sort of recommendation for shielded wires, or maybe using some sort of decoupling caps, or anything that may solve the problem? It could be just noise in the box? Here is a link to my build thus far.
http://www.plantedtank.net/forums/showt ... p?t=557978

Thanks for any help!
Chiefston
 
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Re: AT42QT1070 noisy buttons

by adafruit_support_bill on Tue Mar 25, 2014 6:15 am

Very nice looking build! I can't see much detail on how your touch sensor pads are wired, but keep in mind that the wire running from the touch- pad to the breakout board are also part of the sensor. You can minimize interference on those by shielding them.

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Re: AT42QT1070 noisy buttons

by Chiefston on Tue Mar 25, 2014 10:41 pm

Thank you for the compliment. Today I tried using shielded cable. I have some 2 pair, shielded 22awg wire laying around. I used one conductor from each pair, and grounded the drain wire to GND. When I use this setup, when I touch one of the conductors, the output light stays on for just about 30 seconds, then goes off. During this time, I can continue to use the lit button, however the other button is locked out as it should be. Could this be some form of capacitance of the conductors?


Here you can see I have two wires going to digital inputs on the arduino, and there is a 3 plug polarized connector on the input side. The red and green wires are connected to the input pins, and the left-most wire is the drain wire, which is connected to gnd.
Image


Here is the other end of the wire, I have it soldered to a 5pin DIN connector. I have the drain wire soldered to the outside ground pin, and then the two sensor wires are soldered to the pins. The other three wires of the plug are +5v, GND, and a signal wire from a ds18b20 temp probe.
Image


And another view of the DIN plug.
Image
Chiefston
 
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Re: AT42QT1070 noisy buttons

by Chiefston on Tue Mar 25, 2014 10:46 pm

*Edit*
Also, if I unplug the polarized connector and touch the pins on the perf board directly, the sensor board acts normally. i.e. It only has an output when I'm touching the pin. As soon as I plug the wire connector in however, the output light then does the 30 seconds on thing
Chiefston
 
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Re: AT42QT1070 noisy buttons

by Chiefston on Wed Mar 26, 2014 1:56 am

Ok, so I attempted to troubleshoot the problem by moving the AT42QT1070 to its own perf board, on long standoffs to keep it somewhat away from the rest of the components.

Image

I also ran separate power straight from the Arduino 5V and GND pins, and moved which keys I was using to 1 & 4, which are spaced far apart from eachother. I also tried both the shielded cable and also just a twisted pair of wires. What I've found is that the shielded cable will ALWAYS cause the switch to activate for 30 seconds, no matter what. With the twisted pair, the board works, but only up to a certain length of wire. Any ideas on how to correct this? Maybe better shielded wires? Maybe a resistor between the wires and GND to dissipate any capacitance between the wires?
Chiefston
 
Posts: 7
Joined: Mon Oct 08, 2012 2:15 am

Re: AT42QT1070 noisy buttons

by Chiefston on Wed Mar 26, 2014 1:56 am

Ok, so I attempted to troubleshoot the problem by moving the AT42QT1070 to its own perf board, on long standoffs to keep it somewhat away from the rest of the components.

Image

I also ran separate power straight from the Arduino 5V and GND pins, and moved which keys I was using to 1 & 4, which are spaced far apart from eachother. I also tried both the shielded cable and also just a twisted pair of wires. What I've found is that the shielded cable will ALWAYS cause the switch to activate for 30 seconds, no matter what. With the twisted pair, the board works, but only up to a certain length of wire. Any ideas on how to correct this? Maybe better shielded wires? Maybe a resistor between the wires and GND to dissipate any capacitance between the wires?
Chiefston
 
Posts: 7
Joined: Mon Oct 08, 2012 2:15 am

Re: AT42QT1070 noisy buttons

by adafruit_support_mike on Wed Mar 26, 2014 2:12 am

Hmm.. shielded cable does have some capacitance between the wires and the shield. That may have been enough to make the sensor think it was seeing a touch.

From the photos, it looks like the wiring for the touch panel leaves the box and goes somewhere else. Where is the touch sensor in relation to the box?

If you can make it work in your design, it would probably be less painful to move the AT42Q1070 breakout out of the box and put it as close to the touch pad as possible. Let the cable carry power to the breakout, and data back to the logic inside the box.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.

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Re: AT42QT1070 noisy buttons

by Chiefston on Wed Mar 26, 2014 2:29 am

Thank you for the reply. That is a great suggestion, and I should be able to remotely locate the capacitive board by the buttons. It should be very easy to just route power and two signal cables back to the project enclosure. Your help is much appreciated!
Chiefston
 
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Re: AT42QT1070 noisy buttons

by adafruit_support_bill on Wed Mar 26, 2014 6:43 am

I agree with Mike's suggestion. You want to keep the sensor wires as short as you can. The longer the wires are, the more sensitive they becomes and the more likely that any capacitance along its length can trigger a false reading.

Shielding is a double-edged sword. Although it can shield parts of the wire from your body capacitance, it adds a constant capacitance of its own.

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Re: AT42QT1070 noisy buttons

by Chiefston on Thu Mar 27, 2014 2:19 am

Just as an FYI, I took Mike's advice and mounted the breakout board outside the project enclosure, and right next to the sensors. This solved all my problems. I am using roughly 1.5" x 1.5" single sided coper clad board as my sensors, and they are able to reliably sense through 3/4" plywood! Thank you again for your help!

Image
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Re: AT42QT1070 noisy buttons

by adafruit_support_mike on Thu Mar 27, 2014 5:49 pm

Awesome!
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.

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Please be positive and constructive with your questions and comments.