How to activate capacitive touchscreens without bio-electricity? - Page 1
Forum section: electronics / projects

author: peter
author_rank: Newbie
timestamp: October 04, 2011, 11:36:37 pm
content: Hey does anybody have suggestions on activating capacitive touchscreens without bio-electricity? i would like to have a 'touch node' sitting on the touchscreen and only activate the screen when a circuit is completed. Any ideas what the circuit/material would look like?Peter

author: amspire
author_rank: Super Contributor
timestamp: October 04, 2011, 11:58:15 pm
content: I don't think touch screens are anything to do with bio-electricity. I don't think those mobile phones can read our minds yet.The capacitive screens just work on capacitance, and for that, you basically need a body with a significant self capacitance which is dependant on the size of the conductive body - like a human body, a car body, or a wire to ground (mains earth).I would imagine that a lightweight electrode on the screen connected to a lead to ground via a switch or relay near the electrode would probably work. Give the electrode a diameter of, say, 5 -10mm.If you have access to the touch screen circuitry, use a switched wire from the 0 volts.If it is something completely encased - like a mobile phone or tablet, you can use a flat conductive sheet in contact with the back of the unit as the ground.Worth a try.Richard

author: peter
author_rank: Newbie
timestamp: October 05, 2011, 12:29:53 am
content: Thanks for the quick response Richard! I think most capacitive touchscreen styluses transfer the capacitance of the body-- what I meant by bio-electricity (maybe it's bio-capacitance?).Is an electrode just a metal surface with current flowing through it? like this:https://skitch.com/shacklebolt/f6ek3/circuitIf I have a 5V battery, would I need a resistor?

author: amspire
author_rank: Super Contributor
timestamp: October 05, 2011, 12:36:04 am
content: I had just added to my previous reply before I saw your response.You don't need any battery.Just try different size electrodes until you find the size that works the best. The closer the relay (if that is what you want to use) is to the electrode, the better it will probably work. It is probably possible to use electronic switches, but a small relay is probably the easiest thing to use.Richard

author: amspire
author_rank: Super Contributor
timestamp: October 05, 2011, 12:53:13 am
content: Just to explain a bit more, the capacitive touch screens have a grid with AC voltages applied under the top plastic layer.When you touch the screen, you are conducting away a tiny current at the touch point. The screen is detecting that. It is not detecting any voltages coming from you.So all you have to do is to provide an alternate way for the AC volts to flow from the screen.Aluminium Foil would be excellent for making electrodes for initial experiments. You just have to make sure that when you are doing tests, you are not close to the screen or foil. Your fingers have to be a few centimeters away at least.Richard.

author: peter
author_rank: Newbie
timestamp: October 05, 2011, 01:04:01 am
content: cool! makes sense-- so a battery, aluminum foil, a switch and do i need a resistor? if so what size?

author: amspire
author_rank: Super Contributor
timestamp: October 05, 2011, 01:13:39 am
content: No battery.What type of device is the screen on?

author: AscendedDaniel
author_rank: Newbie
timestamp: October 05, 2011, 06:31:44 pm
content: I work with capacitive touchscreens daily. When we need to control when a finger is applied to the screen, we use copper tape. The tape has a conductive adhesive that keeps it on the screen. Then, a short wire to a NO relay. When the relay is activated, it connects the copper tape to ground. This increases the response to the sensor and usually appears as a finger.I've also been looking at using a series capacitor to isolate the connecting wire from the proxy finger.

author: logictom
author_rank: Unknown
timestamp: October 06, 2011, 11:03:47 pm
content: How about an elaborate system of pulleys and a hotdog?http://www.joop.in/Archive/korean-sausages-work-well-as-iphone-stylus-video/