Hello there,

I'd like to activate a touch switch electronically. It's a capacitive type switch.

There can be no wires connected to the capacitive switch, it must come from something completely isolated from the touch switch circuit.
I know I can use a piece of metal and a linear actuator (cheap type) but that requires mechanical movement and I'd like to avoid that if possible.

So basically we might have an external push button that we press, and that sends a signal to something mounted right top of the original touch switch, and that activates it (presses it in a manner of speaking). If it was a light for example, the light would turn on.
It does not have to be wireless (RF), but I would say an IR signal might work too. An IR sensor detects the IR light then does something to activate the touch switch. Purely electrical would probably be fine though, where a signal is sent to the actuator and that activates the touch switch.

 

The attached 20 year old patent explains how Apple's first touch switch worked. The first named inventor (Chris Krah) went on to design Apple's iPod and iPhone touch interfaces among other things.

The user's finger was sensed through a translucent plastic "plug" in the switch assembly.

 

The attached 20 year old patent explains how Apple's first touch switch worked. The first named inventor (Chris Krah) went on to design Apple's iPod and iPhone touch interfaces among other things.

The user's finger was sensed through a translucent plastic "plug" in the switch assembly.

Hi,

You seem to be providing some information on how a capacitive touch sensor works, and one circuit that will do that.

That's not what I was asking though. I was asking how to activate it with an electronic circuit that is completely isolated from the touch switch circuit, and without having to actually touch it.

In other words, maybe we have two small metal plates in front of the touch switch maybe glued to the surface. We have a circuit that connects to the two small plates. When we provide some sort of signal to the two plates, the touch switch thinks a human touched it and it activates whatever function it has like turn a light bulb on or something.
I don't know if two metal plates is part of the right solution though, that's just an example of what might be part of the solution.

I have a feeling I might end up doing this using a mechanical interface of some type, which I hate to do.

 

I am sorry that I misunderstood.

Perhaps obvious question: Why not use an optoisolator?

My experience in this are is limited, but if you can make a metal plate that has high enough capacitance to the sensing element, then you could use a low output capacitance transistor to ground the plate, thus letting it float.

Of course, this depends on exactly how the touch switch in question works.

 

Capacitive touch switches use a number of different schemes. The first ones used the touch to load an oscillator and change it's current draw. The first one like that used tubes, later ones use CMOS IC s.
Then some of them used the capacitively coupled in ambient electrical field to trigger a switch. I designed and built one of those in 1966. It used a type 2050 tube. It worked quite well.
Nowcapacitive touch switches may use coupling a signal to the sensor or capacitive loading. That kind can often be triggered by an external electric field. That is what I suggest for trying to trigger your capacitive touch switch.

In order to externally trigger one of those switches it is a requirement to know how it works, which there are several different ways that would use several different methods.
So we really need to know what sort of switch it is and what it is part of in order to be able to come up with better than random guesses that are not even close..
If, as an example, you can trigger the switch using a metal coin attached to an insulating object like a plastic straw, that is one type. But if that has no effect, while triggering the switch using the coin attached to a wire you are holding, that is a totally different kind of switch.

 

Maybe you can use IR, BJT, some RC, 74HC14, and CD4013?

 

https://youtube.com/shorts/tdOOKPsMTTE?feature=shared
https://forum.allaboutcircuits.com/...g-capacitive-touch-screen.199272/post-1890189

With capacitive touch you need 'plate' that can be charged (by the touch sensor circuit E-field) and discharged (to touch circuit ground/common) to simulate the finger/body part of the circuit.

https://www.youtube.com/shorts/d7-Un3Nu1rw

 

We do not even know if this switch is on a battery powered device such as a cell phone or a music machine, or if it is on an appliance such as a refrigerator or a video console. And I was certainly correct about the random guesses.
So please provide us with a lot more details.

 

Substitute the touch sensor with a photocell to be actuated by a laser shining on it.

 

It sounds like MrAl (TS) has something with a touch switch on it that he wants to active electrically without an electrical connection or modification to the device. If this is correct, then LEDs and such won't help.

If it is the kind of sensor that watches frequency shift of an oscillator to sense an approaching finger then is should be possible to sense the oscillator frequency by placing a scope probe very close to the sensor. Taking that as a basis, a starting point to find the frequency to be injected by electric field coupling can estimated.

 

Hi Al,
As there seems to be a shortage of ideas on a high-tech solution, perhaps a more eco-friendly, low carbon footprint solution may be a good alternative.

It is a very early precursor of Wi-Fi, it is called Hi-Five, the device can be use used in many different ways, in addition to remote pressing a non-contact push-button.

Scratching one's own back, or clearing the wax from your ears, or perhaps saying Good Morning to an irritating neighbour.

Eric
A little OT

 

Yeah, that would work!

 

Hi Al,
As there seems to be a shortage of ideas on a high-tech solution, perhaps a more eco-friendly, low carbon footprint solution may be a good alternative.

It is a very early precursor of Wi-Fi, it is called Hi-Five, the device can be use used in many different ways, in addition to remote pressing a non-contact push-button.

Scratching one's own back, or clearing the wax from your ears, or perhaps saying Good Morning to an irritating neighbour.

Eric
A little OT

But Eric, aren’t you up on the latest IoT developments? That’s been replaced by the fingerbot. You can get them with BT, Zigbee, and Wi-Fi interfaces!

​

 

I am sorry that I misunderstood.

Perhaps obvious question: Why not use an optoisolator?

My experience in this are is limited, but if you can make a metal plate that has high enough capacitance to the sensing element, then you could use a low output capacitance transistor to ground the plate, thus letting it float.

Of course, this depends on exactly how the touch switch in question works.

Hi,

No problem it was still an interesting read.

 

Capacitive touch switches use a number of different schemes. The first ones used the touch to load an oscillator and change it's current draw. The first one like that used tubes, later ones use CMOS IC s.
Then some of them used the capacitively coupled in ambient electrical field to trigger a switch. I designed and built one of those in 1966. It used a type 2050 tube. It worked quite well.
Nowcapacitive touch switches may use coupling a signal to the sensor or capacitive loading. That kind can often be triggered by an external electric field. That is what I suggest for trying to trigger your capacitive touch switch.

In order to externally trigger one of those switches it is a requirement to know how it works, which there are several different ways that would use several different methods.
So we really need to know what sort of switch it is and what it is part of in order to be able to come up with better than random guesses that are not even close..
If, as an example, you can trigger the switch using a metal coin attached to an insulating object like a plastic straw, that is one type. But if that has no effect, while triggering the switch using the coin attached to a wire you are holding, that is a totally different kind of switch.

Hi,

I'll try that experiment and see what happens.

 

Maybe you can use IR, BJT, some RC, 74HC14, and CD4013?

Hi,

Thanks for the reply. I can not modify the circuit or change anything or even get inside the unit to see the touch plate so I am very limited here.

 

We do not even know if this switch is on a battery powered device such as a cell phone or a music machine, or if it is on an appliance such as a refrigerator or a video console. And I was certainly correct about the random guesses.
So please provide us with a lot more details.

It's something that plugs into the wall outlet. Not battery operated.
The touch switch is covered by a plastic plate which is probably thin. I cannot get inside the unit to connect or modify anything. I'm sure it is capacitive.

 

Hi Al,
As there seems to be a shortage of ideas on a high-tech solution, perhaps a more eco-friendly, low carbon footprint solution may be a good alternative.

It is a very early precursor of Wi-Fi, it is called Hi-Five, the device can be use used in many different ways, in addition to remote pressing a non-contact push-button.

Scratching one's own back, or clearing the wax from your ears, or perhaps saying Good Morning to an irritating neighbour.

Eric
A little OT

Hi Eric,

Yes that's a good idea

Did you ever check out Futurama?
Fing-Longer | Futurama Wiki | Fandom

 

This little circuit simulates a variable capacitance that can trigger touch switches with no moving parts.

When the input is high, the electrode becomes a floating node between two reverse biased diode junctions, a very low capacitance.
Set it low and it looks like a touch, a substantial capacitance to ground. (like a finger)
Stick this circuit as close as possible to the electrode, which can be a finger-sized piece of copper foil stuck over the button.
Depending on the type of sensor, you might need to fiddle with the electrode size and placement.

 

I can get get inside the unit to connect or modify anything.

I can not modify the circuit or change anything or even get inside the unit to see the touch plate

Which one above ?
What about some foil on top of the capacitive sensor plate that will act-move with a compressed air nozzle ?

A remote controlled actuator touching the capacitive target ?