Types and Designs of PCB Integrated Touch Pads for SPST Switch Functionality

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#1 21659364 03 Aug 2011 07:27

Bob Casiano Bob Casiano

Anonymous

Post #1
21659364 03 Aug 2011 07:27

I have a project that needs a SPST switch or button switch. But, I'd like to have to add another component to the BOM. I'm looking into a touch type pad built into PCB's.

What are the types and designs of touch pads designed into pcb's? Any references would be greatly appreciated. Thank you.
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#2 21659365 03 Aug 2011 08:18

Olin Lathrop Olin Lathrop

Anonymous

Post #2
21659365 03 Aug 2011 08:18

Microchip has some app notes on this, but I don't know the numbers off the top of my head. They tend to call this "capacitive sense" or sometimes "cap sense". They also have several PICs with cap sense modules built in. Dig around on the Microchip site and you should be able to find some good information.
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#3 21659366 03 Aug 2011 09:32

Alec Alec

Anonymous

Post #3
21659366 03 Aug 2011 09:32

Hi Bob,

There are a lot of semiconductor manufacturers that have been pushing into capacitive touch controllers.

Were you thinking of something where you have a microcontroller with capacitive sense capabilities built directly into the chip, or would you prefer to use your own microcontroller and interface to an external capacitive touch controller chip?
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#4 21659367 03 Aug 2011 09:53

Khaba Bulu Khaba Bulu

Anonymous

Post #4
21659367 03 Aug 2011 09:53

Hi Mr Bob, checkout this link from Mikroelektronica, they've got a nice set of touch pads and their products are fiable, http://www.mikroe.com/eng/products/view/443/smartglcd-240x128-board/ that is if u are not cost conscious, but if ur switch is to be a simple one why not use transistors connected in darlington pair as a switch? it is very sensitive to touch and is cheaper and simpler.
#5 21659368 03 Aug 2011 11:27

Bob Casiano Bob Casiano

Anonymous

Post #5
21659368 03 Aug 2011 11:27

Excellent Olin, appreciate the info. I will peruse their site, as least for the app notes. For this project, I'll be using a TI part MSP430G2102. TI has a Development kit, MSP430 LaunchPad, that I'll be purchasing today for $4.30. Can't beat the price.
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#6 21659369 03 Aug 2011 11:29

Bob Casiano Bob Casiano

Anonymous

Post #6
21659369 03 Aug 2011 11:29

I would prefer the microcontroller have the touch sensor capability and not add additional parts if possible. Trying to keep the part count low and design simple.
#7 21659370 03 Aug 2011 11:36

Bob Casiano Bob Casiano

Anonymous

Post #7
21659370 03 Aug 2011 11:36

Hello Mr Khaba, thank you for the reference. I'll have to consider the Smart GLCD for another project. As far as your recommendation for transistors is compelling. I just looked up darlington pairs, since it has been a very long time, and they would indeed be a viable option. But, I am trying to keep my part count down. If I can design a pattern into a pcb that the microprocessor can monitor, then I don't have to put parts on to do the same function.I will consider the darlington pair for another project though. Thank you for your advice and recommendations.
#8 21659371 03 Aug 2011 12:05

Alec Alec

Anonymous

Post #8
21659371 03 Aug 2011 12:05

For the MSP430 Launchpad, there is an extension board you can look at "CAP TOUCH BOOSTER FOR LAUNCH PAD":http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=US&KeyWords=430BOOST-SENSE1&x=0&y=0 for a cap touch reference.

To learn how to do your own design, TI offers a couple documents dedicated to capacitive touch with the MSP430.

"MSP430 Capacitive Single-Touch Sensor Design Guide":http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slaa379
"PCB-Based Capacitive Touch Sensing With MSP430 (Rev. A)":http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slaa363a

Good choice of microcontroller for the application.
#9 21659372 03 Aug 2011 12:18

Alec Alec

Anonymous

Post #9
21659372 03 Aug 2011 12:18

I was skeptical about the Darlington pair idea, but I breadboarded it on my desk quick and sure enough it works like a charm. I wouldn't have thought enough conduction could occur through a persons skin for the base of the first NPN in the pair.

It might not be practical in a real world application to keep the bare conductive PCB pads exposed though (corrosion, false positives due to oversensitivity, etc.).

Still, it's kind of cool something so simple works.
#10 21659373 04 Aug 2011 13:32

David Figueroa David Figueroa

Anonymous

Post #10
21659373 04 Aug 2011 13:32

Hello Bob. check into this website..it may interest you on touch pads.

http://focus.ti.com/lit/an/slaa379/slaa379.pdf
#11 21659374 04 Aug 2011 21:05

Bob Casiano Bob Casiano

Anonymous

Post #11
21659374 04 Aug 2011 21:05

Nice app note, thank you David.
#12 21659375 05 Aug 2011 01:15

David Vera David Vera

Anonymous

Post #12
21659375 05 Aug 2011 01:15

Silicon Labs has C8051F700 Capacitive Touch Sense.
#13 21659376 05 Aug 2011 01:17

David Vera David Vera

Anonymous

Post #13
21659376 05 Aug 2011 01:17

This is the PDF AN447 - Design Notes For Capacitive Sensing.
#14 21659377 05 Aug 2011 07:20

Khaba Bulu Khaba Bulu

Anonymous

Post #14
21659377 05 Aug 2011 07:20

Alec, to avoid false positives, try putting the darlington in a small case. Well the first time I used it for an electronic for a friends door(he loves strange things), I housed it in my sisters lip gloss case which I truncated to make it short, then i covered the two probes i extended with a paper. The probes were held together with a small piece of bed foam.
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Topic summary

✨ The discussion focuses on implementing SPST switch functionality using PCB-integrated touch pads, primarily through capacitive sensing techniques. Capacitive touch sensing, often referred to as "cap sense," can be integrated directly into microcontrollers or achieved via external capacitive touch controller chips. Microchip and Texas Instruments (TI) provide application notes and microcontrollers with built-in capacitive sensing modules, such as the TI MSP430G2102, which is supported by development tools like the MSP430 LaunchPad and the CAP TOUCH BOOSTER extension board. Design references include TI's "MSP430 Capacitive Single-Touch Sensor Design Guide" and "PCB-Based Capacitive Touch Sensing With MSP430." Alternative low-part-count solutions include using transistor-based Darlington pairs as touch-sensitive switches, though practical issues like corrosion and false triggering may arise if conductive PCB pads are exposed. Silicon Labs also offers capacitive touch sensing solutions, exemplified by the C8051F700. Additional resources and application notes, such as AN447 for capacitive sensing design, provide further guidance on PCB touch pad design and implementation.
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FAQ

TL;DR: For a PCB “touch button” acting like an SPST, use capacitive pads read by an MCU; TI’s MSP430 LaunchPad cost $4.30, and “Can’t beat the price.” [Elektroda, Bob Casiano, post #21659368]

Why it matters: This FAQ helps embedded designers pick proven PCB touch-pad patterns and parts to minimize BOM and debug time.

Quick Facts

Low-cost entry: MSP430 LaunchPad cited at $4.30 for prototyping capacitive touch. [Elektroda, Bob Casiano, post #21659368]
TI’s 430BOOST-SENSE1 “Cap Touch Booster” provides a ready reference for pad layout and firmware on MSP430. [Elektroda, Alec, post #21659371]
MCUs with built-in capacitive sensing (e.g., select PIC families) reduce external parts and routing. [Elektroda, Olin Lathrop, post #21659365]
External-controller path: Silicon Labs C8051F700 family supports capacitive touch sensing. [Elektroda, David Vera, post #21659375]
Edge case: Exposed copper pads can corrode and cause false touches; protect or coat them. [Elektroda, Alec, post #21659372]

What PCB touch-pad types can emulate an SPST switch?

Common options include single solid pads, interdigitated “comb” pads for higher sensitivity, and pads with guard rings to reduce noise. For an SPST action, read one pad and apply thresholding and debounce in firmware. TI’s MSP430 capacitive-sensing references illustrate these patterns and settings. [Elektroda, Alec, post #21659371]

Do I need a dedicated capacitive-sense chip or can my MCU handle it?

Many MCUs integrate capacitive-sense peripherals, so you can avoid extra ICs. Microchip PIC devices and TI MSP430 variants are cited as examples. This keeps BOM and routing simple for a single-pad SPST input while maintaining responsiveness. “They tend to call this ‘capacitive sense.’” [Elektroda, Olin Lathrop, post #21659365]

Is the MSP430 a good fit for touch pads, and what starter gear helps?

Yes. The MSP430 LaunchPad plus the 430BOOST-SENSE1 “Cap Touch Booster” offers a proven path to evaluate pad shapes and firmware. TI’s app notes linked with MSP430 examples cover layout and signal processing, making it an efficient SPST solution. [Elektroda, Alec, post #21659371]

I want the lowest part count—what’s the best approach?

Select a microcontroller with built-in capacitive sensing and pattern the pad directly on the PCB. This avoids external switches, transistors, or companion touch ICs. The thread author explicitly chose this path to keep parts minimal. [Elektroda, Bob Casiano, post #21659369]

How much does it cost to get started with an MSP430 touch button?

The MSP430 LaunchPad was purchased at $4.30 in the thread, enabling quick prototyping of capacitive touch without extra ICs. That entry price helps validate layouts before scaling. [Elektroda, Bob Casiano, post #21659368]

Are there ready-made boards with touch pads I can study or reuse?

Yes. MikroElektronika’s SmartGLCD board was recommended as a reference; its 240×128 module showcases integrated touch-capable interfaces and build quality. Use it to benchmark responsiveness and mechanical integration before custom PCB spins. [Elektroda, Khaba Bulu, post #21659367]

Can a simple Darlington pair act as a touch switch?

Yes. A user breadboarded two NPNs in Darlington and confirmed it “works like a charm,” leveraging skin conduction to trigger. It’s sensitive and cheap, but lacks the robustness and tuning of capacitive sensing in MCUs. [Elektroda, Alec, post #21659372]

What are the drawbacks of the Darlington touch trick?

Exposed copper pads invite corrosion and false positives from humidity or contamination. Encapsulating or coating the interface helps, but MCU-based capacitive sensing offers better filtering and environmental tuning. [Elektroda, Alec, post #21659372]

Where can I find a solid design guide for PCB touch buttons?

Use TI’s MSP430 capacitive-sensing notes, including the “MSP430 Capacitive Single‑Touch Sensor Design Guide,” as linked in the discussion. They cover pad geometry, ground strategies, and firmware thresholds. [Elektroda, David Figueroa, post #21659373]

What external touch-controller resources are suggested?

Silicon Labs’ AN447 “Design Notes for Capacitive Sensing” was cited for external-controller design tactics and noise handling. Pair with the C8051F700 line when you need a dedicated front end. [Elektroda, David Vera, post #21659376]

How do I lay out a simple one-pad capacitive sensor for SPST behavior?

Create a circular or interdigitated pad on top layer; add a ground guard ring if space allows.
Route a short, shielded or guarded trace to the MCU’s capacitive-sense pin.
Follow TI’s MSP430 reference settings for baseline, threshold, and debounce. [Elektroda, Alec, post #21659371]

What firmware strategy makes a touch pad behave like a clean SPST?

Continuously measure capacitance, subtract a running baseline, and compare to a threshold. Add time-based debounce and optional hold-to-toggle logic. TI’s MSP430 reference materials and booster-pack code demonstrate these steps clearly. [Elektroda, Alec, post #21659371]

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