PCB touch pad

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.

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.

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?

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.

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.

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.

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.

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.

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.

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

http://focus.ti.com/lit/an/slaa379/slaa379.pdf

Nice app note, thank you David.

Silicon Labs has C8051F700 Capacitive Touch Sense.

This is the PDF AN447 - Design Notes For Capacitive Sensing.

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.