Looking for help with capacitive sensors

thebigb

Hi guys,

I'm not in any way up to speed with physics. Ive been doing some research online for a project I would like to do but I need someone who can answer laymans questions in layman terms?

My questions revolved around one main question. I have seen videos on youtube on how to make a capacitive sensor like this one

http://www.youtube.com/watch?v=0GmIkyEzHnk

But id like to know is it possible to make one of these on a larger scale, and with other metals.

If someone has any experience in this I would really appreciate the help.

Thanks

krd

thebigb wrote: »

But id like to know is it possible to make one of these on a larger scale, and with other metals.

For that kind of circuit, as the one in the video, the capacitor has to be small.

How it works. The human body carries an electrical charge. A certain amount will be on your finger tip. If your finger comes close enough to the capacitor - doesn't need to touch - the electrical field lines pass through the air - this changes the charge in the capacitor - and also changes the way the circuit works.

The bigger the capacitor, the more charge it can hold. In the video he uses a capacitor of ten pico Farad (pico is really small) he says the human body (he probably means the finger) can have a capacitance of up to 15 pico farad.

His capacitor looks hand made - I'm not sure how he made it, but it is possible to make one. A capacitor is basically two sheets of metal with something in between separating them. To get the same effect as the small capacitor, you'd need to cut your capacitor until it was in the range of 10 pico farads.

What are you trying to do? What are you trying to build? There may be other solutions for a similar effect.

thebigb

thanks a million for your reply, basically i am trying to see if it is possible to make a capactive touch sensor out of a large frame made with mild steel?

do you think this is possible?

krd

thebigb wrote: »

thanks a million for your reply, basically i am trying to see if it is possible to make a capactive touch sensor out of a large frame made with mild steel?

do you think this is possible?

No, I don't think it's possible. How much charge a capacitor holds is partly down to it's surface area - the larger the surface area the more charge . This makes it hard for the circuit to work - because it wouldn't notice the charge from your finger or hand as much.

To make a touch sensor that size, I think you need to think of something else instead of capacitance. Something like an infrared switch. I'm sure there are other options too.

thebigb

Ok, ive thrown that idea out of the window. Now what i am looking at is fixing long thin capacitive strips on the mild steel frame. Firstly is this possible?

the idea is that these strips will act as the sensor for the frame. So, im assuming something like alluminum would work ok?

where can i get the parts to build the circuit and what do i need?

krd

thebigb wrote: »

Ok, ive thrown that idea out of the window. Now what i am looking at is fixing long thin capacitive strips on the mild steel frame. Firstly is this possible?

It's kind of trick making a capacitor by yourself. It's two sheets of metal with a gap in the middle. The area of the sheets has to be precise, the gap has to be precise too.

I'll show you the equation - you don't have to worry about what all the numbers mean. Just to show you how important the size of the thing is.



C is the capacitance;
A is the area of overlap of the two plates;
εr is the relative static permittivity (sometimes called the dielectric constant) of the material between the plates (for a vacuum, εr = 1);
ε0 is the electric constant (ε0 ≈ 8.854×10−12 F m–1); and
d is the separation between the plates.

You'll notice, by looking at the equation, the greater the surface area(size of your aluminium strips), the greater the capacitance.

You're looking for a capacitor to be around 10 pico Farads - that is incredibly small, and you would need a great deal of precision to create one. I think the touch screens on I-phones use a huge array of these capacitors - they're incredibly small. They're not handmade.

the idea is that these strips will act as the sensor for the frame. So, im assuming something like alluminum would work ok?

I've been trying to think of alternatives. There are so many other kinds of switches that would be easier. The capacitance one really only works with a very small size.

krd

Actually..........I've had a rethink.

I think the project might be possible.


If you look at the formula. d , is the distance between the two plates. The greater that distance, the smaller the capacitance.

So. It might be possible, with a bit of experimentation, to hand cut a large capacitor, that could have the right capacitance.

It might just be possible..........I'm looking for a pen to do the calculation to see if it is.

=========================

I've tried some calculations - I don't really trust my calculating, I'm so rusty. One calculation, gives me a gap d of 8.5 cm between the two plates - for plates 10 cm by 10 cm. My calculations could be completely off. And there may be away to shorten the gap.

You need to do a few more calculations - pick the right materials. You need a mutlitmeter - which you clip or solder on to the plates to see if you're getting the right capacitance.