Any Ideas for a cheap touch sensor??????

Hi all,

Currently, I'm working on a project where it would be nice to have a cheap touch sensitive switch. So, I looked at mister_e's entry about a touch sensor.

http://www.picbasic.co.uk/forum/showthread.php?t=2671

I think it's a very clever idea, but there is a problem, I can't make it to work. I'm using the same set up as Mister_e with a different pic. I'm using a pic16f84 at 4 MHz, the same frequency that uses mister_e's pic. I have tried different combinations of resistors and I have added some capacitors but still it doesn't work. I can't find anything in the specs that would help.

When using an oscilloscope I can see the input signal at the input ports goes lower when touched by a finger but apparently not low enough to read a zero.

Does anybody have any ideas on how to make this work? Probably adding an op-amp or a pnp transistor would do the job.

Any ideas or comments?? Thanks.

Robert.

Ummmm .....

Buy a 16F726 with a "Capacitive Sensing Module".

Then ask for Byte_Butcher's finished code. :D

16F726 Capacitive sensing module
http://www.picbasic.co.uk/forum/showthread.php?t=10578

Or maybe, see if you can get the code first. :eek: then buy the chip.

Butcher's in the "Hot seat" now.

Hi Robert. Here's a link to the datasheet for the QT102 touch switch.

http://www.qprox.com/assets/Download...3.05-15437.pdf

OK Darrel, I posted my code. :)
http://www.picbasic.co.uk/forum/show...1611#post71611

I'm sure it needs a LOT of fixing to be "good", but it's functional enough now to make a touch sensor work. Enjoy!


Steve

Quote:

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Originally Posted by peterdeco1

Hi Robert. Here's a link to the datasheet for the QT102 touch switch.

http://www.qprox.com/assets/Download...3.05-15437.pdf

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I've used those QT102's before (and QT150, which is a 5 channel version). They work pretty well, but they aren't as cheap as using a 16F727 and it adds more parts to your circuit.

You might want to check the Atmel website... I think the QT102 has been superseded by a newer part and isn't recommended for "new designs".
Maybe I'm wrong...

Quote:

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Originally Posted by Byte_Butcher

OK Darrel, I posted my code. :)
http://www.picbasic.co.uk/forum/show...1611#post71611

---

Awwww, you weren't even in the "Hot Seat" long enough for the seat to get hot. :D

Sweet!
<br>

Thank you all for your prompt responses.

Well, I'm still struggling trying to make this touch circuit to work. I have tried different circuits with op-amps and ne555 chips but the main problem with these circuits is that your body must be grounded for the switch to work and this is not what I'm looking for.

I read Peterdeco1 suggestion about using a QT102 but that chip is too expensive ($20+ at mouser.com) and it would add more components to the final design. I will give it a try to Darrel Taylor's suggestion of using a 16F726 with a "capacitive sensing module" but I'm afraid that for this technique to work is going to require that your body has to be grounded.

Any comments on this issue or any related topic?????

Robert.

May be then you need inductive and not capacitive sensor.

Look at Microchip for this new technology.

It has + and - as it is for the Capacitive sensor too.

Ioannis

I remember, about 35 years ago working on an autopilot for my Dads boat, using a simple 2 transistor switching circuit, it was too sensitive and responded to touch, cured it with proper biasing resistor. BTW I was not grounded. You could put a groundplane and sense wire together coaxially or using spiral technique.

Quote:

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Originally Posted by rsocor01

... suggestion of using a 16F726 with a "capacitive sensing module" but I'm afraid that for this technique to work is going to require that your body has to be grounded.

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This statement in the datasheet seems to indicate a ground isn't required ...

Quote:

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Originally Posted by 16F726 datasheet

The capacitive sensing module allows for an interaction
with an end user without a mechanical interface. In a
typical application, the capacitive sensing module is
attached to a pad on a printed circuit board (PCB), which
is electrically isolated from the end user.

---

Quote:

---

Originally Posted by rsocor01

I will give it a try to Darrel Taylor's suggestion of using a 16F726 with a "capacitive sensing module" but I'm afraid that for this technique to work is going to require that your body has to be grounded.

Any comments on this issue or any related topic?????

---

No way! You shouldn't need your body grounded at all!

I'm testing the 16F726 touch sensor right now, using it to toggle a solid state relay on and off.

I'm sitting here in a carpeted room, on an insulated chair, wearing rubber soled sandals and the sensor works great!
My wife came in a little bit ago and it worked for her too.

I've got a simple 6 button "keypad" with bits of copper foil on the backside (inside) of a plastic box. When I touch the outside of the box where the sensors are... PRESTO... just like magic!

Try it. You'll like it. :)

Check out the Microchip M-touch pages. Seems to be a lot of implementation info there.

Yes, the PIC16F726 with a capacitive sensing module sounds like the best option.

What I meant by grounded is that in all the touch-button circuits I've tried they would only work if with one hand you touch the circuit's ground (negative lead of the power supply part of the circuit) and with your other hand you touch the touch-button or metal pad. It's like your body acting like a capacitor.

Robert

No, there should be no need for any part of your body to be grounded or connected to circuit Vss or any of that. You should be able to just walk up to it on insulated feet and poke it with one finger, just like a "real button".

My buttons have a ground ring *around* them also, which helps a lot I think.

FWIW, here's photos of my "test bed" for the touch sensors. On the front you can see it's just a plastic box. The black dots are at the centers of the touch pads. I'll come up with some nice stick on labels for the buttons later.
http://sweetwatergems.weirdstuffwema...touch_0710.jpg

On the back you can see the six touch sensors... just little squares of sticky backed copper tape with a wire on each one. Just temporary to learn how it all works. The glue isn't all that good so I'll need something more permanent later.

Around the pads, you see the "ground ring" that surrounds it all. it's connected to system ground (Vss) and helps limit the range of the sensor so it doesn't overlap others nearby, and also seems to make individual pads more sensitive.
http://sweetwatergems.weirdstuffwema...touch_0711.jpg

Dunno if that helps in your quest for a cheap touch sensor, but my tests so far with the 16F727 are good. A finger touch on any of the six sensor pads activates my routines real nice.

I like 'em a lot so far! :)


steve

Wow, I'm impressed with your project Byte_Butcher.

The ground around the pads prevents overlapping false alarms but also decreases sensitivity according to the next paper.

http://ww1.microchip.com/downloads/e...tes/01102a.pdf

There might be a tradeoff in there. Anyway, good job!!!!

You might also continue to look for the Atmel Qtouches. I've used them many times even recently and never paid more than ~$1 per chip. Checked Digi-Key... $.78 - $1.01 single price. Biggest issue is I don't think the come in DIP package anymore...

See the other basic site Touch interface for a proven design.
The code is simply loading an array with AD readings, sorting high to low, retaining all but the highest and lowest and then averaging to determine the final reading.
The hardware is two leads the sensor and the driver.
See Microchiphip AN1286 as the sensor and driver code is almost identical with above post.

Norm