Capacitive Sensing Continued

Hello readers from MAKE: as well as all other readers 🙂

My prototype touch sensor worked so well, that it hasn’t needed much changing. I sent the design off to Custom PCB, and less than a week later, I had a pile of circuit boards waiting for me.

I changed the layout around a little, mostly adding a 2×8 header for accepting a ribbon cable style connection. The header combines power, ground and outputs into a single connection, making it easier to connect to the main board of my larger project (sprinkler controller). Each touch output is paired with a ground wire, which I suppose makes it more resistant to interference. The caps I used this time are polyester film 220 nF, doubling the amount of capacitance compared to what was used on the prototype.

Yes, the ugly piece of plexi is still ugly. Don’t worry, it will be hidden from view. In the final configuration, this board and its plexiglas spacer will be inside a plastic project box. I’ll have a laminated “keypad” overlay affixed to the outside of the box so I can see where the buttons are. The spacer will be flipped around, going on the solder side, giving me enough clearance to flush-mount the sensor with the wall of the box. Flush mount is very important, as even the slightest air-gap will ruin the proximity sensing effect.

Nothing much to see solder side… a few smt passives set options on the chip, as well as decouple and filter the incoming power. The big resistor limits current for the meager power led which no one will ever see once the board is in use.

I’m very close to finishing the larger irrigation control project, hopefully sometime this week! Thanks for reading!

14 Replies to “Capacitive Sensing Continued”

  1. I’ve had a few problems with the 160.
    not much reliability or stability.
    I had to switch to the qt113.
    basically my design had 2 qt160 driven by one oscillator.
    I guess I did something wrong…
    good job.


  2. Looking good.
    One thing I’d suggest is seeing if LEDs under or near the capacitive plates will work. That way you can have some sort of feedback that something has happened, which is sort of important for cap sensors. Even better would be a bunch of RGB LEDs…..but that is just for coolness factorization.

  3. Phil, Quantum claims LEDs are possible, but can interfere with the reliability of the sensors. Their older datasheets say you can put leds right in the sensor area, the newer datasheets claim the PN junction of the led will create some sort of feedback in the electrostatic field. My touch pad is sealed approximately behind a total of 1/4″ plastic, the outer 1/8″ being opaque black, and its designed to be a weather tight enclosure, so leds poking through are not desirable.

    I like leds too, and i had actually designed a daughterboard for the touchpad, which carries a pic and six bicolor leds to provide visual feedback.

  4. I’m thinking the electrical traces embedded in the lcd would interfere with the sensors. The datasheet goes on and on about how a simple led can interfere with the sensing.

  5. Hi,
    I´m working on a 9button-touch-pannel with a qt160 DIP-version. I used the basic circuit from qprox. The output pins have always 0VDC.-I don´t get it running. Could you put your circuit online?

  6. Hi, please can you tell me, what exactly is the “polyester film 220 nF” ? You inspired me so I’m working now on something like this, but with QT1081.
    Thanks rochne

  7. oh shit I’m stupid, bad translation only… I thinked that you used some kind of polyester layer (film) over the PCB, where keys are … sorry 😉

  8. Hi, there

    Is polyester 220nF a WIMA or else?
    Is a interface paper sheet being insert between plexiglass and PCB and the pad on PCB is laid on bottom side?

    and the last question is
    Would you mind if I translate this to Thai and republish on my site , as well, with your credits?

  9. i’m a mtech student in embedded system engineering.i’m doing my mtech main project in capacitive seat occupancy sensing.i would like to get a simple sensor configuration for the same.can anyone help me

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