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Original question:

Using capacitive sensor on the metal base of a lamp

MSP430FR2512: Using MSP430FR CapTIvate Technology to detect a touch on the metal base of a lamp

Piocky
Intellectual 290 points
Part Number: MSP430FR2512

Hello everyone,

I wrote a post three months ago to require some help for selecting a component that would fit our requirement: we are designing a connected bedside lamp, which contains an alarm too, and we would like to use a touch on the metal base to snooze it. We finally decided to select the MSP430FR2512 for our first test, as we only need one electrode in our case (as well PWMs and GPIO for other tasks).

Our first test with the EVM connected to the base was quite promising, as I was able to detect very clearly a touch on it, but lately I tried to redo the calibration in our system, and now the measure is not as sensitive as it was during the first test. I needed to increase the conversion count to at least 1000 (even tried 2000 for better detection), but this increases the noise quite a lot and the SNR measure is now pretty poor.

For now our PCB is connected to the base by a simple wire, taped on the base (that's what I did for the first test, but it's clearly not a long-term solution). We are now looking for a better solution to connect the MSP to the base. Our main idea at the moment is to use a connector like the one on this picture:

The only is that our base is a cylinder, and it won't be easy to fix this connector to it, except on the top, which is quite far from our PCB.

I was wondering if you guys had any advise for connecting a PCB to an external electrode properly without using soldering?

Thanks a lot for reading this.

Yoann

  • 0
    TI__Genius 17960 points

    Hi Yoann,

    the connector as such does not really matter. Basically you always have to consider the background of capacitive sensing. In reality you're dealing not with capacitance but with charge and thus with electric fields.

    So what is important is the difference in the potential between the electrode and the earth GND and GND of the application, the distance between everything carrying electrode potential and other potential is essential. Everything conductive close to the electrode signal, is short cutting the E-Field, increasing charge in that area and thus lowering sensitivity. For the same reasons, you should consider active shielding of the electrode connection.

    Best regards

    Peter

  • 0
    TI__Expert 7015 points

    Hi, Piocky, 

    I read your previous post 3 month ago. But I am not sure what is your current solution: touch on metal, normal touch sensing or proximity? 

    You mentioned the metal base is 2mm aluminum. It is hard to make a deformation. CapTIvate touch-on-metal also needs the deformation to sense the small capacitance change for trigger the touch. Another point is the metal base is needed to connect to GND for CapTIvate touch-on-metal application. That is to keep a stable electrical potential as a base. 

    Please let me know what sensing mode you are using then I can provide comments. 

    Thanks, 

    Lixin 

  • 0 in reply to Lixin Chen1
    Intellectual 290 points

    Hi Peter and Lixin,

    Thanks for your quick answers! I took a look at touch on metal, but as Peter said, a deformation is not possible to detect in our case I think, so it's more a proximity sensor with a calibrated threshold that would let us detect the touch on the metal. From what I read on this section, I expected that not having a GND would increase the sensitivity (as it increases the detection range). But as our PCBs are in the base, I guess that's our GND reference, which make a difference between my first and my second test.

    You don't think that the way our base is connected to the MSP could have any influence on the measure? Because when I'm touching the cable directly I can see a lot of capacitive variation, and not when I touch the base (still see something though).

    An engineer friend of mine also advised me to increase a bit the serial resistor of 470ohm to decrease the noise, do you agree on that?

    Thanks again for your help!

    Yoann

  • 0 in reply to Lixin Chen1
    Intellectual 290 points

    (I didn't want to change this thread as resolved, sorry for the missclick)

  • 0 in reply to Piocky
    TI__Genius 17960 points

    Hi Piocky,

    no problem. I am closing it from our side.

    Best regards

    Peter

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