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I've asked this before, but knew less about the tech and didn't word the question properly. I know other people have asked similar questions and knew little, so let me be as specific as possible here:

What I want to do is make a fine-tipped stylus for my iPad so that I can write and draw without an awkward clunky stylus that may as well be another finger.

I know there are fine-tipped styli out there, but I dislike all of them as they either do not work as advertised, scratch the screen, or are to flimsy. But most importantly: I like making things.

The sensor array in the iPad is mutually capacitive. A grid of capacitors is laid out under the glass. A constant voltage is passed through one axis of the grid. The other axis waits and detects changes in the voltage. So to trip a sensor, an object such as a human finger or stubby capacitive stylus must be brought close enough to change the voltage of one of the capacitors with its local electric field. When that field covers a wide enough area, the device registers it as input and decides where the center point is. This is how it maintains accuracy while being so low-definition.

So to reiterate: how can I make a device that generates the same electric field as a human finger over a ~5mm radius from a much finer tip? It can be ugly, hooked up to wires, even grounded to human skin with a wrist strap. I don't care about form. Just function.

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  • \$\begingroup\$ So, if I'm understanding correctly, you want a stylus with a tip more like a pen than a finger, but you want it to have the same response (~5mm wide area) as a finger? \$\endgroup\$ – player3 May 19 '14 at 1:27
  • \$\begingroup\$ @player3 that is exactly it. \$\endgroup\$ – Raiden Dark May 19 '14 at 15:49
  • \$\begingroup\$ How about using a aluminum rod. .255 or. 500 of an inch. Shape the point u want polish it so it won't scratch the display? \$\endgroup\$ – user62165 Dec 26 '14 at 23:29
  • \$\begingroup\$ @Zerodark29 Have you tried this? Sounds like it would have the problem of not projecting the field over the 5mm area \$\endgroup\$ – Raiden Dark Dec 28 '14 at 0:53
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I've been wanting to find out info about this subject my self and also don't have much in the way of electronics background. I believe that Raiden wants to create his own DIY fine point stylus like the Nota stylus (http://hex3.co/products/nota) which is described as electronically activated. There are also others which are coming to market in the near future.

As far as I understand they work by increasing the capacitance of the stylus point. So I've looked into what can be done to increase capacitance of the stylus.

Formulas

C = Q/V OR Capacitance = Charge / Voltage

C= εS/D OR Capacitance = (Dielectric Constance X Surface Area) / Distance between plates

If i understand correctly when you touch the screen you make a sort of capacitor due to the layer of insulating film /glass and the change in voltage at that point is the detected. (May be completely wrong about this). So in the 2nd equation if you reduce the surface area you will reduce the capacitance. So in order to increase the capacitance we need to change the dielectric constant of the material being used.

Here is an example of a fine point stylus that uses foam and water https://www.youtube.com/watch?v=alrweIVt2RQ . The Constant of water is ~80 compared to rubber of ~3 and i believe that is why this example of a fine point stylus works. Unfortunately it is inconsistent and water doesn't play nice with touch screens. I would theorise that using a material with a high constant would help reduce the size of a stylus point. For example Graphite has a constant of ~ 36.

My other thoughts are to try using an electronic circuit to add voltage to the stylus point. But thats well out of my range of understanding.

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    \$\begingroup\$ Funny how someone with no professional knowledge on the subject gives me the best information. After watching that video, I'll have to give it a go and see how it works hands-on. If my suspicions are correct, the more dielectric a material is, the grater the field it propagates? Not sure if I worded that right, or if it's even correct. I'll have to get my hands on more materials and see what happens. \$\endgroup\$ – Raiden Dark May 25 '14 at 18:32
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Some data as what the excitation signals for the screens are would help. But even without that you could design a powered stylus with a fine tip.

The basic setup would be a simple metal wire connected with one end to a voltage source. The other end of voltage source should be insulated. Now you have a artificially generated electric field around the wire. The nice thing about it is that the tips always get the highest fields. You should put an insulation around the wire tip to prevent scratches. If you put a battery in, it won't even be drained because there would be no significant current path.

As for the needed voltage, I can't guess that well without any data. The higher the voltage the higher the field will be, what means that you can have a smaller tip.

The problem with this approach could be that the electric field projected from the tip is too weirdly distributed. I have no idea what the expected input signals for the processing system are. So it could be possible that the tip is not recognized because it has a too weird field distribution. If you knew the exact parameters of the system you could do something similar to a microatennae which gives the same inputs to the system as a fingertip.

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  • \$\begingroup\$ Now this is the kind of answer I was looking for. Finally, someone that can read! I'll do some experiments and get back to you here. \$\endgroup\$ – Raiden Dark May 25 '14 at 18:28
  • \$\begingroup\$ Good to know that I was on the right track. The fine tipped comercial products use this principle, as I learned now. If you feel like spending money you could buy one and do some reverse engineering. Or at least find someone who has dissasembled one of them. \$\endgroup\$ – WalyKu May 25 '14 at 19:51
  • \$\begingroup\$ Yeah, I would, but if you read the reviews, they all suffer from this wavy diagonal line effect. I don't want to pay that much for a broken product. I would upvote, but I'm new here, and five points of rep short of being allowed to. \$\endgroup\$ – Raiden Dark May 27 '14 at 17:57
  • \$\begingroup\$ The wavy artifact is a problem with the display and not the stylus. The display has a finite number of cells, and when they are rectangular you get that wavy effect(according to a video from the nota vendor, but it is plausible from an engineering standpoint). You won't solve it with a DIY stylus. Well you could create a new firmware for displays that takes measurements from multiple cells to compensate this effect when using thin tips. I think fingers(or classic stylii) are fat enought that the software actually uses inputs from multiple cells, so they don't have the problem. \$\endgroup\$ – WalyKu May 27 '14 at 18:03
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I would recommend trying to get ahold of a commercially available stylus with a metal shaft and either covering it's tip with a metallic material, or somehow molding a replacement tip out of a mixture of plastic and metallic material (maybe you can 3D a replacement?). Now for the fun part, now that you have a "conductive" tip, connect it to the pen's body via some sort of wire and voala! Every time you touch the metal body of the stylus, you increase the capacitance of the tip of the pen and thus, make it work with any display!

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  • \$\begingroup\$ That sounds exactly the same as just using a regular capacitive stylus. How would that get away with having a finer point like a pen or pencil? I've already coated a pen with a mixture of thermal paste and epoxy, and it's plenty conductive, but it only registers on much higher definition sensor arrays like a galaxy s4. Not an iPad. It's too thin. The field needs to be amplified over a 5mm diameter. Passive conductive materials don't accomplish this. \$\endgroup\$ – Raiden Dark May 19 '14 at 15:54
  • \$\begingroup\$ The key is to add your capacitance to the mix by connecting the conductive tip to your hand! \$\endgroup\$ – beersnob May 21 '14 at 15:33
  • \$\begingroup\$ I know that already. I do not want a traditional capacitive stylus. They do not work unless the tip is just as flat as a finger. I want a file tip. \$\endgroup\$ – Raiden Dark May 23 '14 at 4:02
  • \$\begingroup\$ @RaidenDark I'm not sure I agree with you.... I've made a capacitive stylus out of a pen and a wet sponge strip that worked just fine on the iPad (surface area ~ 1 sq. mm). \$\endgroup\$ – Shamtam Dec 27 '14 at 16:09
  • \$\begingroup\$ @Shamtam, I've done the same. I believe this is due to something about the dielectric properties of water + capacitance of water. Not sure though. Haven't been able to get any other material to behave the same. Keeping it wet is like using a quill and inkwell. Not my cup of tea. \$\endgroup\$ – Raiden Dark Dec 28 '14 at 0:56
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I saw this write-up on how to make a stylus with a tip roughly the size of a ballpoint pen, which I'm sure you can reduce further as your needs require.

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  • \$\begingroup\$ I'm well aware of this method. I do not like it. I want to project the field from a fine tip. Not use somewhat transparent material to physically widen the tip. That's fragile. I'd rather replace batteries than that tip. Which is why I'm asking how to project the field. Like, can I use a battery with wires and some other circuitry to make a small, weak electrostatic field? \$\endgroup\$ – Raiden Dark May 19 '14 at 16:01

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