Capacitive PCB Switches

krypton_john

Senior Member
Hi All,

I'm sure there was a therad on these a while back but I can't find it.

Does anyone have some into on this technique, where you can stick a few pads on both sides of the PCB, attached to some kind of decoder, and sticking your finger in proximity to the pad activates a momentary switch. Something to do with the pads forming a capacitor and the finger affecting the electric field between them?

Could any kind person point me in the right direction here?

Cheers
JohnO
 

Tom2000

Senior Member
Here's an article that discusses the theory of capacitive touch switch technology, should you want to roll your own: http://www.imagineeringezine.com/e-zine/capacitance-1.html

Also, don't forget about good ol' fashioned resistive touch switch technology. Your fingertip is a pretty nifty several-k-ohm resistor. You can use this to your benefit by connecting the base of a general-purpose transistor to ground through something like a 100 k resistor. Use your finger to short a couple of PCB pads, one connected to +V, the other to the base of the transistor. Shorting those pads with your fingertip will turn the transistor on.

Good luck!

Tom
 
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MartinM57

Moderator
Very interesting - quite cheap in terms of bang-per-buck at GBP3.62...considering the 28 pin DIP surface mount holder below it at GBP1.87 :)

Could you use a PCB as a front panel - no copper on the user side and square "pads" etched out on the internal side?
 

Dippy

Moderator
Yes, and you an even have your copper pads on the inside of an enclosure or behind a perspex panel. But to get the best read the Data Sheets concerning applications and limitations. You even use the single channel Qprox to detect people on steps.
 

krypton_john

Senior Member
Very interesting - quite cheap in terms of bang-per-buck at GBP3.62...considering the 28 pin DIP surface mount holder below it at GBP1.87 :)

Could you use a PCB as a front panel - no copper on the user side and square "pads" etched out on the internal side?
As Dippy said, they work over a distance so no need for the pcb to be visible - it can be behind the front casing of your non-metallic enclosure.

BTW I was incorrect in my original post about double sided pads. You only need one surface. The other side of the capacitor is *you*.

Cheers, and thanks for all the comments,
JohnO
 

Tom2000

Senior Member
The Qprox chips are very attractive, except that they come in tiny little SMD packages. A note on their site says that they're discontinuing the chips that incorporate Microchip silicon, due to a price increase. It looks like the Qprox chips that come in DIP packaging are all Microchip parts. So, save for those into microsurgery, Qprox is pretty well out of the question.

I was googling for an alternate approach that might not be heavy on parts count, and stumbled across pressure-sensitive conductive rubber.

This stuff is new to me, but looks very interesting. It's conductive rubber that radically drops its resistance as a pressure threshold is exceeded. It's designed for applications such as keyboards.

This stuff isn't terribly expensive, and appears to be readily available in small quantities direct from the manufacturer.

It might be an alternative to Qprox microsurgery.

Good luck!

Tom
 

Dippy

Moderator
I haven't tried that stuff but I have tried the Quantum stuff that come in little pills. I found it very fiddly.
I'd sooner use s/mount.
 

Tom2000

Senior Member
I haven't tried that stuff but I have tried the Quantum stuff that come in little pills. I found it very fiddly.
I'd sooner use s/mount.
Oh, well, so much for that idea. I was pretty excited, thinking I'd stumbled upon something new and cool.

Thanks, Dippy. You saved me from a waste of time. Good info.

Tom
 

Dippy

Moderator
Oh, I wasn't trying to put you off Tom. I was trying to make a pressure switch using it but gave up. You can also get QTC cable. Actually I gave up with that too. I'm sure it is useful for some things... though I don't want a list thanks.
 

MartinM57

Moderator
The Qprox chips are very attractive, except that they come in tiny little SMD packages. A note on their site says that they're discontinuing the chips that incorporate Microchip silicon, due to a price increase. It looks like the Qprox chips that come in DIP packaging are all Microchip parts. So, save for those into microsurgery, Qprox is pretty well out of the question....
That's not quite the whole story....the website actually says

<<
Some QTouch devices are based on Microchip silicon. Since the pending acquisition of Quantum by Atmel was announced Microchip has decided to raise its prices to us by as much as 280%. Accordingly we can no longer recommend these devices for new designs, and any new or unconfirmed orders of existing Microchip based devices will face substantially higher prices.

We are working to replace these Microchip devices with different, pin-compatible silicon at much lower prices (much lower than even the old prices) within the next few months. All devices in the following part series are affected:

QT100, QT102
QT110 through QT118H
QT140, QT150, QT160
QT411, QT511
Custom devices beginning with QT1C, QT2C, QT5C, QT9C, QT10C, QT15C

Some substitute devices are already sampling. Please contact your distributor or representative for further details and for samples of the newer devices.>>

So it suggests that there will be pin compatible DIP products (presumably based on Atmel AVRs :)) in the future
 

Tom2000

Senior Member
Hmmm... you might be right, Martin. Reading that note, and then the list of products showing the discontinued parts, it looked like all the DIP chips were going sayonara. On re-reading their advisory, maybe they will be re-introducing DIP parts. Not sure, though.

If they do, it will be a boon to us old-time hobbyists. Here's hoping...

Thanks,

Tom
 

MartinM57

Moderator
Not getting any calmer.....

QT1103 has a state change output whenever the key state changes and you can then interrogate it over an 1- or 2-wire RS232 interface at up to 38400 baud to get it to tell you the key state.

Nice :)
 

krypton_john

Senior Member
Martin, not wanting to raise your already highly aroused state but... they also have them in DIP packages that you can easily use in your own PCB's. I posted such a link earlier in this thread.

Now, go have a cold shower or something!
 

MartinM57

Moderator
OK I'm calm(er).

Just a frisson of excitement for something I'm doing that is currently plastered with toggle switches that I really wanted to be membrane switches but wasn't happy with the mechanical side to them/annotating the legends etc

Those DIPs would certainly be best for prototyping, but I would want the smaller packages for the production versions

...and breaaaaaaaaaaaaaaaaaaath ;)
 
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