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I'm trying to make a design where the user can run his/her finger directly over a series of 10 LEDs (covered with 1 mm glass), and the LEDs should glow on/off based on the finger's position.

I wish to use capacitive touch (like on a phone, where the touchscreen is directly above the LCD) to accomplish sensing of the user's finger.

I am familiar with making copper pads on a standard PCB to make a capacitive touch sensor, but if this opaque PCB goes on top of the LEDs, then their light would be obscured.

Is there some alternative strategy I can try in order to let the LED light come through and still allow capacitive touch sensing of the user's finger directly above the LEDs?

EDIT: Perhaps below design should work? (incorporating reverse-mount LED suggestion of user @Some Hardware Guy)

Green = PCB; Brown = Copper pads (for capacitive-sense), and Yellow = Reverse-mounted LEDs

pcb

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2 Answers 2

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What about a reverse mount led meant to mount on the bottom of the board and to come through a hole in your PCB, and hopefully be flush with the top of the board (you could adjust your thickness to make if flush or just under. Then do your capsense traces on the top layer.

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  • \$\begingroup\$ This is creative; I'm concerned though that even in this solution, the user's finger won't be sensed when placed in areas directly above the LED -- and will only be sensed when placed directly above the nearby cap-sense pads. Or perhaps I'm misunderstanding the active zone of capacitive sensing -- is it tolerant enough to allow sensing "at an angle", i.e. even if the finger is slightly off the pad? \$\endgroup\$
    – boardbite
    Commented Dec 22, 2014 at 4:40
  • \$\begingroup\$ It's been a long time since I played with cap sense but it's basically trying to detect a change in capacitance between two traces (or trace and plane). I seem to remember you can make the sense traces all different shapes. If you could make the traces surround the led hole I don't think you would be able to find a spot where it wouldn't detect your finger. I don't think angle is involved so much as distance. Maybe someone with more experience with these would know more. \$\endgroup\$
    – Some Hardware Guy
    Commented Dec 22, 2014 at 4:45
  • \$\begingroup\$ Noted. Added a figure to question based on your recent suggestion. Empirical testing needed to see if cap-sense-pads surrounding the LED (which is only 1 cm X 1 cm) should do the trick! \$\endgroup\$
    – boardbite
    Commented Dec 22, 2014 at 4:54
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    \$\begingroup\$ See youtube.com/watch?v=KX0s55I7qyg and libstock.com/projects/view/851/capsense-click-example \$\endgroup\$
    – Passerby
    Commented Dec 22, 2014 at 5:30
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Maybe you could use FTO or ITO covered conductive transparent sheets to make your touch sensitive layer. Just an idea, I never used them myself, but it may be usable for your application. Although it might be a bit complicated to "draw" and cut your circuit on it.

Here is a link to Adafruit's relevant product with a video showing how it works.

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