Capacitive touch trigger as an isolated system

Thread Starter

esahakian

Joined Jun 2, 2019
5
I need a way to trigger a capacitive touch screen device (like a tablet) using an isolated system without touching it. For example a kid places a small gadget on a tablet screen and the gadget will trigger a capacitive touch on the tablet without requiring anyone to be holding the gadget or the gadget being tethered to anything. So as long as the gadget is sitting on the tablet screen it is continuously emitting a touch trigger to the tablet. Basically a stylus that is an isolated (independent) system.

Do anyone know of such a circuit using an Arduino or something else? Or perhaps you can point me to the right resource for building such a circuit?
 

DickCappels

Joined Aug 21, 2008
5,944
If this is an electric field touch screen, it senses the finger by capacitive loading. The device assumes that you are holding the device and your finger completes a circuit from the back of the device to the screen through the series capacitances. It also works if the device has large capacitance to ground and so does the user. Some devices sense coupling between adjacent conductors on the touch surface.

You can probably simulate the touch of a finger (perhaps insulates so as to not scratch the screen by (for example) putting a sheet of meal foil on the back of the device and then running a wire around to the front, with a electromechanical finger model is connected. The finger model is placed over the area to be "tapped".

In theory if you are very careful you can switch the connection to the finger model with a very low capacitance transistor.

In the case of touch surfaces that sense coupling between adjacent conductors your only option may be to have the finger model touch the surface or not -a mechanical design problem.
 

Thread Starter

esahakian

Joined Jun 2, 2019
5
Need more information...

How large is the object?
no larger than 3x3 (inches) flat like a disc (half inch thick)
What is it made of?
wood or plastic + the circuit
What material is the actual surface?
again wood or plastic or anything that is needed..
The point is to have the item work isolated without needing any grounding.
 

Thread Starter

esahakian

Joined Jun 2, 2019
5
If this is an electric field touch screen, it senses the finger by capacitive loading. The device assumes that you are holding the device and your finger completes a circuit from the back of the device to the screen through the series capacitances. It also works if the device has large capacitance to ground and so does the user. Some devices sense coupling between adjacent conductors on the touch surface.

You can probably simulate the touch of a finger (perhaps insulates so as to not scratch the screen by (for example) putting a sheet of meal foil on the back of the device and then running a wire around to the front, with a electromechanical finger model is connected. The finger model is placed over the area to be "tapped".

In theory if you are very careful you can switch the connection to the finger model with a very low capacitance transistor.

In the case of touch surfaces that sense coupling between adjacent conductors your only option may be to have the finger model touch the surface or not -a mechanical design problem.
It must be an isolated system... You can not use a finger... If the item placed on the touch screen of a tablet, then it can simulate a touch at will...
 

Sensacell

Joined Jun 19, 2012
2,452
What sort position resolution is required?

Could you not use some magnets embedded into the objects?>
Detect them with a reed switch or hall sensor?
 

Raymond Genovese

Joined Mar 5, 2016
1,658
It is difficult for me to fully understand your task. It sounds like you simply want to detect if an object has been placed on a surface. Specifically, you want a tablet computer to detect when the object has been placed on a (the) screen). You state, however, that it has to be an " be an isolated system" system and "The point is to have the item work isolated without needing any grounding."

Can you run us through an example? Does the object have to be placed on the surface of the tablet and only the tablet? Can it be placed on a surface connected to the tablet?

You could, conceivably, use a resistive touch screen...

https://www.adafruit.com/product/333?gclid=EAIaIQobChMIxPPM8L_N4gIVCZ2zCh0bTgnWEAQYASABEgJLHvD_BwE

The object is placed on the resistive touch screen which is interfaced to the tablet. If the screen is large enough, you could determine where on the surface it is placed.

In a similar manner, you could use a force sensing resistor...

https://www.interlinkelectronics.com/fsr-406

In this case, the tablet could detect the presence or absence of an object.
 

Thread Starter

esahakian

Joined Jun 2, 2019
5
It is difficult for me to fully understand your task. It sounds like you simply want to detect if an object has been placed on a surface. Specifically, you want a tablet computer to detect when the object has been placed on a (the) screen). You state, however, that it has to be an " be an isolated system" system and "The point is to have the item work isolated without needing any grounding."

Can you run us through an example? Does the object have to be placed on the surface of the tablet and only the tablet? Can it be placed on a surface connected to the tablet?

You could, conceivably, use a resistive touch screen...

https://www.adafruit.com/product/333?gclid=EAIaIQobChMIxPPM8L_N4gIVCZ2zCh0bTgnWEAQYASABEgJLHvD_BwE

The object is placed on the resistive touch screen which is interfaced to the tablet. If the screen is large enough, you could determine where on the surface it is placed.

In a similar manner, you could use a force sensing resistor...

https://www.interlinkelectronics.com/fsr-406

In this case, the tablet could detect the presence or absence of an object.
Here is a practical example... Ideally I want this to work with existing tablets but if I have to use a resistive surface connected to a (phone or tablet) then I can consider it...

I am building a learning system for young kids. A child picks up a shape (Star, Circle, Triangle) and places it into a wooden board. We have all seen these toys before. Now I want to build a system using a tablet which shows the picture of the shape (Example a CIRCLE with a voice prompt) now the child will pick up the "Gadget" (in this case a CIRCLE) and places it on the tablet and then lets go... While the "gadget" is sitting on the tablet surface without anyone further touching it, I want the "gadget" to continue to emit a touch trigger to the tablet so the tablet knows the CIRCLE shape is still sitting there.

I know that it is very easy to build for when you are touching the "Gadget", then the tablet can be triggered because you are touching the "Gadget" and the gadget is on the tablet... This is basically a standard stylus. But an "Isolated" system sitting on a tablet all by itself should continue to trigger a touch signal. If this works then I can use multiple points on the "Gadget" (like 3 points) for the tablet to actually understand what shape is on the tablet.

But first things first... The "Gadget" can be powered internally to run any circuit it wants and it will be switched on by the parent and then kid will be told to place X on the tablet. And as long as the "Gadget" is sitting on the tablet then it will continue to register as "present"

I DON"T want the "Gadget" to have any wires coming out of it or connected to the tablet or any other source... It must be "Isolated"

I think this should make it clear enough to what I am trying to build... I was told that maybe an oscillating inductor circuit can do the trick but I have to run some experiments.
 

Raymond Genovese

Joined Mar 5, 2016
1,658
Here is a practical example... Ideally I want this to work with existing tablets but if I have to use a resistive surface connected to a (phone or tablet) then I can consider it...

I am building a learning system for young kids. A child picks up a shape (Star, Circle, Triangle) and places it into a wooden board. We have all seen these toys before. Now I want to build a system using a tablet which shows the picture of the shape (Example a CIRCLE with a voice prompt) now the child will pick up the "Gadget" (in this case a CIRCLE) and places it on the tablet and then lets go... While the "gadget" is sitting on the tablet surface without anyone further touching it, I want the "gadget" to continue to emit a touch trigger to the tablet so the tablet knows the CIRCLE shape is still sitting there.

I know that it is very easy to build for when you are touching the "Gadget", then the tablet can be triggered because you are touching the "Gadget" and the gadget is on the tablet... This is basically a standard stylus. But an "Isolated" system sitting on a tablet all by itself should continue to trigger a touch signal. If this works then I can use multiple points on the "Gadget" (like 3 points) for the tablet to actually understand what shape is on the tablet.

But first things first... The "Gadget" can be powered internally to run any circuit it wants and it will be switched on by the parent and then kid will be told to place X on the tablet. And as long as the "Gadget" is sitting on the tablet then it will continue to register as "present"

I DON"T want the "Gadget" to have any wires coming out of it or connected to the tablet or any other source... It must be "Isolated"

I think this should make it clear enough to what I am trying to build... I was told that maybe an oscillating inductor circuit can do the trick but I have to run some experiments.
Yes, that helps clarify what you are trying to do.

Your solution can certainly be accomplished with a camera and associated objection recognition software.

Lower tech ways might include embedding transmitters in the objects that can be detected - this might depend some on your ability to control the task such that the child can't take two objects out and place them both on the surface - that sort of thing.

Edited to add: You could, potentially, embed BLE beacons in/on the objects. This would be easy for the tablet to detect...again with the qualification that you have some control over having a bunch of objects very close by that would require you to determine signal strength and the like - but it could work. Cool project, keep us posted on how it goes please.
 
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Thread Starter

esahakian

Joined Jun 2, 2019
5
Yes, that helps clarify what you are trying to do.

Your solution can certainly be accomplished with a camera and associated objection recognition software.

Lower tech ways might include embedding transmitters in the objects that can be detected - this might depend some on your ability to control the task such that the child can't take two objects out and place them both on the surface - that sort of thing.
My current fallback option is passive RFID embedded in the "Gadget" and a surface for the tablet to sit on... Not the best solution but one way to go... Another idea was to build IR sensors like an electronic whiteboard that can sense where the pen coordinates are detected... Again needs hardware attached to tablet.
 
My current fallback option is passive RFID embedded in the "Gadget" and a surface for the tablet to sit on... Not the best solution but one way to go... Another idea was to build IR sensors like an electronic whiteboard that can sense where the pen coordinates are detected... Again needs hardware attached to tablet.
Yeah, RFID tags would seem to be worth a try.
 

djsfantasi

Joined Apr 11, 2010
5,704
It’s important to know what technology the tablet uses to detect touch. It it a resistive grid? Is it optical? Is it capacitive?

Knowing the technology behind the tablet touch screen, then you can pick an appropriate technology for the “Gadget”.

If it’s a resistive grid, then weight is important.

If it’s optical, then the surfaces of the “Gadget” are germane

Capacitive touch screens are where my confidence is low. I think that if the “Gadget” is composed of a conductive material, then they could be applied. Carbon-infused plastic or resin may work in this application. Or even if the “Gadget” is made from anti-static foam. While I cannot confidently provide an answer in this case, experimenting is not expensive.
 

BobaMosfet

Joined Jul 1, 2009
839
My current fallback option is passive RFID embedded in the "Gadget" and a surface for the tablet to sit on... Not the best solution but one way to go... Another idea was to build IR sensors like an electronic whiteboard that can sense where the pen coordinates are detected... Again needs hardware attached to tablet.
There's also NFC.
 
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