Need a cheap touch sensor idea.. here it is

[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. I think it's a very clever idea, but there is a problem I can't make it work. 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.
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 would do the job. Thanks.

Robert.

[Archangel]

Hello Robert,
Well we know it will not work without changing a few things for the 16F84, so post what you have and let's look and see. Open a new thread in PBP so as not to hijack this example thread.

[Ioannis]

Do NOT put extra capacitance at the inputs beside the sensor itself.

Even the Osc. probe will load inputs.

Maybe play with the 22k resistors. Also good idea would be to compare spcs of the two PIC's.

Ioannis

P.S. If necessary, admins move this message to keep thread clean. No problem.

[George]

What a great idea, is it working reliably - I have a device I need this sort of thing for, pref very cheap with little extra in the way of components, did rsocor01 ever get it running? He didnt mention it on his other thread. Just wondering. would higher value resistors and a longer charge period help with the sensitivity?

I need this unit to be fairly reliable, is it likely to do this if set up right? Hopefully will be mass produced. Most commercial capacitive systems take a running average and are constantly resetting their zero point to allow for atmospheric conditions etc dont they?

[rsocor01]

George,

No, I never got it to work the way it's explained in this threat. The way to go is with the series PIC16F72x. They work great. Refer to the threats started by byte_butcher about his project. That should help you.

Robert

[George]

Hey I tried it and got it going really well, i changed some values to make it more sensitive (I don't have much space available for the pad) ie - changed the resistor to 100K and put a 1ms delay in on the charge, through a 3mm sheet of glass a sensor pad size of down to 6mm x 6mm worked reliably. This was using a 16F676 @ 4MHz. Why is rsocor using a 16F84? I cant even understand why they are still being made and so popular, they are just junk along side the more modern chips.

Thanks Mister E for such a great idea, I think as long as I tune the sensor pad size/resistor it should be pretty reliable as I've had no issues with it yet and it worked first time.

[Archangel]

Why is rsocor using a 16F84? I cant even understand why they are still being made and so popular, they are just junk along side the more modern chips.

There are dozens of books, hundreds of instructors who recommend them. They work out of the box with default configs, no A/Ds, No Comparators, perfect choice for that first PIC. Remember George, newbies often know absolutely nothing about PICs, and these chips offer the easiest way to get started without getting tripped. Move up to 16F628A / 648A, adds comparators and memory and timers, same footprint, 1 brick at a time, is how a wall gets built. But yeah, for the experienced programmer, to much money too little chip.