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

[rsocor01]

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.

[Darrel Taylor]

Ummmm .....

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

Then ask for Byte_Butcher's finished code.

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

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

Butcher's in the "Hot seat" now.

[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

[Byte_Butcher]

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

[Darrel Taylor]

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.

Sweet!
<br>

[rsocor01]

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.

[Byte_Butcher]

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

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

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...

[Amoque]

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...

[Normnet]

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